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Username Post: Cylinder Head Ground        (Topic#359868)
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-19-20 11:18 AM - Post#2786857    

1990 C1500 5.7 TBI. I was just checking over all my grounds on the truck. On the back of the passenger side cylinder head is a ground location. I cannot see it, but I can snake my hand in there. The bolt there has one braided ground strap that runs down to the frame. I read somewhere that there was a PCM ground that went there as well. I cannot feel anything missing, like a broken lug, etc. Does anyone know for sure, or have a factory manual for the 1990 showing physical grounds? Thanks.



 
CowboyTrukr 
"9th Year" Silver Supporting Member
Posts: 4385
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
02-19-20 01:10 PM - Post#2786863    
    In response to go_hercules

Negative. The head ground is supposed to go to the firewall and then to the frame just above the starter. The computer ground is at the thermostat.

The head ground is one. The thermostat ground is another. The battery negative should hit the fender and the engine block. On the intake, there’s probably another right behind the alternator. Driver side of the core support should have a lighting ground.

Right where the fuel filler hose lives, there’s one on top of the frame. Back of the truck, driver side frame rail has another one for the tail lights.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-19-20 03:52 PM - Post#2786873    
    In response to CowboyTrukr

You're right. I went and double checked the strap from the head and it does go to the firewall and then down to the frame. All others are just like you listed. So, just so I understand, that strap from the back of the head is the ONLY one at that ground location? Thanks a lot for the help.



 
CowboyTrukr 
"9th Year" Silver Supporting Member
Posts: 4385
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
02-19-20 10:16 PM - Post#2786896    
    In response to go_hercules

That’s right, Herc. Just the one.

Now some have gone as far as beefing that up with a piece of fine stranded #4 to both locations. Makes a better connection for the cab accessories.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-19-20 10:48 PM - Post#2786899    
    In response to CowboyTrukr

Thanks for that. Luckily, all my grounds look good. Chasing down some gremlins so wanted to rule the grounds out anyway.



 
CowboyTrukr 
"9th Year" Silver Supporting Member
Posts: 4385
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
02-20-20 12:48 AM - Post#2786902    
    In response to go_hercules

What’s going on?

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-20-20 09:11 AM - Post#2786921    
    In response to CowboyTrukr

Starts great cold, starts great after shutting off and restarting, starts great after sitting a couple of hours. Runs great all the time. The ONLY issue is that after driving, shutting it off and waiting about 30-45 minutes, it starts right up but surges up and down until it dies. I can keep it running if I play with the gas pedal. Once it cleans out, which is maybe a minute later, it runs perfect. At that point I can kill it, and it restarts perfect. Good fuel pressure (11psi), no vacuum leaks (21" steady), new coolant temp sensor (both new and old checked good when resistance tested in hot water), scan tool shows good map readings, fuel trims in closed loop good, coil and ICM resistance tests good, etc. Checked all grounds (voltage drop less than 50mv at each ground KOEO), cleaned them anyway. EGR (negative backpressure) works as it should and stays closed at idle (EGR solenoid works good). Replaced IAC (both old and new show about 20 counts at idle in closed loop). Injectors do NOT drip at all after shutdown. Throttle position sensor consistent across range (0.6v at idle). Even suspected voltage sag while cranking might trigger the PCM to go into low voltage mode. Jump started it to test that and no change. Scan tool shows voltage drops to about 10-10.5v while cranking. Connected a potentiometer to the ECT circuit to simulate high and low temps to trick the PCM into richening or leaning, no change. Swapped a known good MAP sensor in, no change. Timing at 0 degrees in bypass. Advances normally when not in bypass. Watched spark with a spark tester when acting up, jumps 40KV gap no problem. Replaced the coil, even though resistance hot and cold on the original were good. Reluctor in distributor looks good (no cracks I can see). I have tried everything I can think of. That's why I was just double checking that I had not missed a ground somewhere. Even took the PCM apart to look for burnt components or traces. Looked like new, and connectors looked like new. I am stumped.



Edited by go_hercules on 02-20-20 09:16 AM. Reason for edit: No reason given.

 
CowboyTrukr 
"9th Year" Silver Supporting Member
Posts: 4385
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
02-20-20 09:30 PM - Post#2786966    
    In response to go_hercules

Very thorough.

@Chevytech may be one who can offer some insight.

Greg

'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-21-20 07:03 AM - Post#2786990    
    In response to go_hercules

Wow, a lot of good work & effort - it’s a bummer when it doesn’t pay off.

At this point, unless someone comes along which has been there/done that, (which is possible, a lot of experience on the site) I would put some effort into getting data logging capability so you can capture the behavior in the act.

I had problem on my 93 once where it hunted up/down on idle all the time and I messed around a long time looking at things without success. Finally spent some time getting a TunerPro up & running and captured the trace of act. I saw that idle enrichment was kicking in as idle rev’d up and off when it went down. Led me to fuel pump and even though it was ~11psi at idle I noticed it dipped a bit if I blipped throttle so I replaced it and problem went away. (Along with other drivability problems I was also suffering)

I’m not suggesting fuel pump is your problem, I have no explanation on why it would be fine cold and marginal in that scenario you described. Just saying that something in data trace correlated with problem may point you towards the issue.

Eg. Maybe something with open loop / closed loop control? Cold start is open loop unit some conditions are met. After driving and restart, not sure, it may go straight into closed loop (someone here can correct if wrong) and you have a lazy o2 sensor that doesn’t respond quickly enough until it warms up or something. Too hard to know unless you are looking at the data.

Btw - I think there are pins you can ground on ALDL after starting which will tell you if you are in open or closed loop. I think it’s called “field service mode”. An easy thing to check would be whether the bad idling to good idling transition is at same time of a change from open to closed loop or dropping out of closed loop. A real long shot but easy to check.
Good luck - hopefully someone with experience will come along and give you some ideas.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-21-20 09:09 AM - Post#2786996    
    In response to Sabino

Thanks for those suggestions. I am actually using an OTC 4000 Scanner, and can read real time data on OBD1. It does start in open loop consistently, and switches to closed in a few minutes of driving. As for the fuel pump thought, I just can't figure how fuel pressure could do that. After starting and surging, moments later it clears up and runs perfect. Not ruling it out, just thinking it through. I am planning to change the ignition control module today. Hate throwing parts at it, but it's the only thing I can think of to do at this point. I have read that the module when hot can act up, but usually gives a no-start. I can watch the spark and it looks good, but maybe on a scope I could see drop outs. And the injectors I think depend on the pulse from the ICM...



 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-21-20 12:53 PM - Post#2787008    
    In response to go_hercules

Well the new ICM made no difference. I don't know what to check next. I keep thinking about the fuel pump, but with 11 psi and the fact that it only acts up after sitting 45 minutes after driving doesn't make sense to me. Trying to think of a way to verify it. Maybe putting a potentiometer in place of the TPS and tricking the PCM into thinking the throttle is open more than it really is which should enrichen it. At least maybe I could confirm if it's a rich or lean condition causing this behavior. The scanner can't tell me because it's in open loop, so no fuel trims yet.



 
rockfangd 
"7th Year" Gold Supporting Member
Posts: 2870
rockfangd
Age: 34
Loc: Utica ny
Reg: 04-13-10
02-21-20 07:03 PM - Post#2787023    
    In response to go_hercules

I may have missed something but have you checked the injectors?
Almost sounds like a stuck open one.
Would expain the immediate restart, the start after a few hours, and the no restart when sitting for a short while.
Fuel leaks down into cylinders and until the plugs dry it will run like crap or not at all

Old School GM fan FOREVER


Edited by rockfangd on 02-21-20 07:05 PM. Reason for edit: No reason given.

 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-21-20 11:13 PM - Post#2787028    
    In response to rockfangd

Yeah, I can watch the injectors under any condition and they spray nice and evenly both sides. I even took blue paper towels and stuffed them under the injectors after shutdown a few times to see if there were any drips. None at all, high and dry. For the life of me, I just cannot even figure out if it's fuel or ignition. Maybe neither, I have considered that maybe the PCM is bad. But I haven't went down that road because I really think it's just something weird I haven't recognized.



 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-22-20 09:25 AM - Post#2787045    
    In response to go_hercules

I don't know the capability of the scan tool you have, but if you can capture data during the event you could check some basic stuff to try to sort if either timing or fuel is intentionally being changed by ECM during the event. The problem can sometimes be it's hard to tell chicken from egg - whether the ECM is getting bad info from somewhere and thinking it has to change fuel/timing or something is causing a problem and ECM is attempting to recover by changing fuel/timing. You may be able to tell if ECM is getting bad info by checking sensor inputs to ECM which is pretty easy. You can log TPS, for example, during event @ idle with foot off throttle. If it's changing you have a problem. You can look at knock counts during event - if you get a burst of counts around same time - it could be your problem.

Some thoughts on what I would try with disclaimer that my knowledge on this stuff is pretty limited. There are people who know so much more and how to modify/tune, etc. and I may be wrong on some of this stuff.
Anyway - I'd log RPM, TPS, Knock count, BLM & Int (I think also called long/short term fuel trim?),Block Learn cell, spark advance, Open/Closed Loop flag. I'd think any datalogger would have that stuff. There is also a Accel Enrichment flag (it's what kicked in @ idle on mine which clued me to fact my fuel pump was marginal) and a Stall Saver flag in the datastream - whether all dataloggers pull that out, I don't know.

If you can capture that stuff during the event - go thru and see if something is changing at same time rpm is dropping. eg
- TPS is ~0.5V and not changing
- no big bursts of knock count
- should stay in the idle BLM cell, not sure what cell that is but regardless, cell # shouldn't be changing
- INT/short term fuel trim is running up/maxed (or down/min) indicating ECM is trying to adjust fuel. If maxed out - could indicate low fuel pressure.
- timing is changing

I'm sure I forgot something but you get idea and - all this easy to say, harder to do

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-22-20 09:57 AM - Post#2787048    
    In response to Sabino

Awesome help and suggestions. A few things I can mention: my scan tool does not have capture I think, but will check if I can, I have watched TPS voltage, MAP pressure, knock signal, etc. while it's surging and all are steady. You mentioned fuel trims, but I don't think they change in open loop, since my problem is in open loop I read BLM and INT at 128 during this event. My tool does not read timing. I have watched the timing with a light while it's acting up and the timing stays around the 10 degress or so where it normally is at idle - it moves a little with RPM changes, but like you said who knows if the advance is moving in response to RPM or vice versa. And one more thing, my tool does not show things like fuel enrichment, do any OBD1 programs capture this? As a side note, I was thinking of putting a quarter under the fuel pressure regulator spring just to see if it helps - which might prove it's running lean during this. Any thoughts on that? Thanks again, I will check on any ideas you can come up with, believe me.



 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-22-20 12:38 PM - Post#2787057    
    In response to go_hercules

UPDATE. In my earlier post I mentioned that I had tricked the PCM into thinking it was colder to force more fuel at startup. I also mentioned it did not help the problem. Well today I tried it again but instead of making the temp super cold, like 0 degrees F, this time I made it about 50 degrees F. This is after a good drive, and letting it sit 45 minutes. I checked the scan tool, and it said the coolant temp was 180, so I put the potentiometer in place of the coolant sensor and the scan tool said 50 degrees. Fired it up and it ran smooth with no surging. The idle was very high, I guess as expected. So does this maybe confirm that I have low fuel pressure when hot soaked? After I did this, I re-connected the coolant sensor and it fired up and ran perfectly as I described before. So why would it need the extra fuel, then once cranked, it doesn't need it 1 minute later? Was even thinking a fuel relay problem, in that when hot and oil pressure low, the oil pressure switch is not taking over a faulty relay, whereas it would when it had better oil pressure. Is that a stretch? Could I jumper the relay and start and run it that way to rule out a relay problem?



 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-22-20 12:49 PM - Post#2787059    
    In response to go_hercules

ah, I didn't read carefully. Let me make sure I understand
The ONLY issue is that after driving, shutting it off and waiting about 30-45 minutes, it starts right up but surges up and down until it dies. I can keep it running if I play with the gas pedal. Once it cleans out, which is maybe a minute later, it runs perfect.

When surging @ warm start @ idle it's initially in open loop.
When it starts running OK is it then in closed loop?
Does it go quickly to closed loop on subsequent hot starts?

If it only has the problem in open loop, perhaps I was making it too complicated.

as you say, when in open loop there is no adjustment of trims. (Int should be 128 - but I think the BLM should be at whatever it has adjusted to for the particular cell - not sure.) Either way - the fuel pressure has to be good in open loop or you have problems. Since you checked that timing isn't changing, it's fuel or air.

I'd bet on that 11 psi isn't actually good enough or it's not always 11 psi. I assume you checked pressure cold? or do you have fuel gage installed/modified TBI for adjusting the fuel pressure?
putting a quarter under the fuel pressure regulator spring If so - yes, easy to try to increase fuel pressure with adjustment but if fuel pump isn't delivering pressure needed it won't make any difference trying to change spring.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-22-20 01:10 PM - Post#2787060    
    In response to go_hercules

I have hard time believing it's a flakey fuel relay. Doesn't hurt to jump it to rule it out. I think there is actually a pin on the ALDL connector which will jump the fuel pump always on.

I think you're right that the A/F ratio used in open loop is richer the colder the engine/CTS reads. So interesting idea to fake it and then that it ran OK. Does seem to suggest it was too lean initially when it was not running well.

Why it might run OK with CTS plugged back in soon after - not sure. Maybe the pressure delivery from the pump increases a bit after a few minutes - also not sure why that would be so.
Another possibility is it jumped into closed loop on the restart. 180F is right at temp required to go into closed loop. maybe running it a few min while faking it cold it warmed up enough to put it into closed loop on the restart.


65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
highway star 
Newbie
Posts: 21

Loc: North Mississippi
Reg: 10-09-18
02-22-20 01:11 PM - Post#2787061    
    In response to Sabino

Have you pulled the plugs & read them. One fouled?



 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-22-20 01:13 PM - Post#2787062    
    In response to Sabino

It starts in open loop, and while surging stays in open loop. Does the same thing cold. Only goes into closed loop after driving it a few minutes like you would expect. I only checked fuel pressure cold. Did not try it hot. Even if fuel pressure from pump was low, wouldn't the quarter boost at least some? If not, that would mean that basically the regulator is doing all it can, which would mean dead heading it. That doesn't seem likely to me, but possible? I might just try it before rechecking pressure hot since I have to borrow the Tee and gauge from Autozone to do it.



 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-22-20 03:53 PM - Post#2787069    
    In response to go_hercules

As I understand fuel pressure regulator - the spring opens when the desired pressure is reached in cavity that feeds the injectors. Theoretically, if fuel pump was not delivering enough pressure to open the spring (deadheaded) making the spring stronger does nothing. I think it's a bit more complicated in real life as pressure & flow rate are related. eg. if fuel filter was clogged and restricting flow but fuel pump was strong you could have high pressure at low flows but lower pressure at higher flow rates. These intank fuel pumps are not positive displacement - they have an impeller and pressure/flow depend on rate of impeller turning and tolerances. I've also heard of pumps having problems due to rips/small holes in the plastic tube connecting pump to sending unit in tank.

I would do what you plan - measure fuel pressure after filter again to be sure. Watch it for a while and while blipping the throttle to see if pressure dips when more fuel is demanded.

I guess if you did have low fuel pressure it might make some sense it would idle better/OK cold than hot when in open loop. When cold, the ECM is shooting for a richer A/F than warmer and will hold injector pulse longer @ cold. Even with low pressure you'd get more fuel @ cold than in warmer conditions. But since theoretically the A/F targets @ open loop idle are supposed to be good enough for the different temperatures / conditions it doesn't really say why low pressure would be more of a problem @ warm than cold. maybe older engines/lower compression have different thresholds of pain vs temp than new. don't know.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-22-20 04:06 PM - Post#2787070    
    In response to Sabino

I agree with what you're saying. By tricking the PCM into a colder temp, I am forcing it to increase the on-time of the injectors, at whatever pressure they're seeing. Forgetting the potentiometer trick, in normal operation when hot starting and getting the surging, did you have a thought about why once it cleared out and started running good, which is literally only a minute or less, then I can kill it and it restarts and runs perfectly? I mean the fuel pressure hasn't changed, temp has not had long enough to change, etc. But it's only that first hot start which makes it even weirder. That's why early on I did not even suspect ignition because something hot soaked could not cool down that fast.



 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-22-20 05:59 PM - Post#2787075    
    In response to go_hercules

I agree it’s weird and not sure it’s fuel pressure but no other ideas at present.

There are two conditions, I think, for it to go into closed loop. Reaching a certain temp - on my 93 C1500 it’s 79C/174F based on watching it in datalog - but I’ve read other temps online so different engines/ECM may have different criteria. The other criteria is an engine on time - I’m not sure of the time criteria but I think it’s on order of a minute or two. (There is another criteria to stay in closed loop which would not be the issue here. That criteria is that the ECM has to be able to change the short term trim and see the O2 sensor move up & down- on mine I think it has to be able to drive the O2 above about 700mv and below about 400mv.)

So, in the case you mentioned where it was about 180F when you faked the ECM colder, if that’s the “warm” condition you may be be at temp where it will drop into closed loop whenever the engine on time criteria is met which may only be a minute or two. If you drop into closed loop than the ECM will drive the fuel trim to vary the O2 and even with marginal fuel pressure it would end up increasing inj pulse on time.
Maybe it only runs bad in open loop and the confusion why/how it gets into closed loop so quickly. If you could confirm that it’s only bad in open loop and hopefully runs well in closed loop that would be interesting.

It might be interesting to look at INT & BLM after it gets thru bad running @ idle and when you know it’s in closed loop. I think you’d expect to see INT to climb up from 128 when it goes into closed loop if fuel pressure is marginally low.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


Edited by Sabino on 02-22-20 06:03 PM. Reason for edit: No reason given.

 
65_Impala 
Very Senior Member
Posts: 4440

Reg: 12-29-02
02-22-20 07:22 PM - Post#2787081    
    In response to Sabino

It does sound like it's in open loop when surging and then goes closed loop and trims out which gets it running right. What do the fuel trims do once it goes closed loop?

The thoughts about the regulator are correct. If the pump can only produce 11psi then the regulator can't make it produce more.

There is a pump prime as a terminal by the relay or an ALDL terminal (G), but it only works when the relay is off, so only when the engine isn't turning over. The fuel pump wire should be a tan/white so you could simply jump power to it.


Sabino- I've not found a tune where the closed loop temp was that high yet. It's possible though. Typically it's in the 100-130*F range but I've seen newer stuff even lower. The running delay before closed loop varies depending on the start. On this TBI stuff, if it's considered a cold start then it's minutes of time and if it's considered a hot start then it's typically something like 20-30 seconds.




 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-22-20 09:16 PM - Post#2787092    
    In response to 65_Impala

Guys, according to my scan tool, it stays in open loop during this whole ordeal. Once it clears out and I stop and restart, say within a minute or so, it is still in open loop. Only after I drive it a few minutes does it go into closed loop. Interesting what you said about the 174F threshold. I will watch my temp next time to see if that is where mine switches to closed loop. After this few minutes and I'm driving, the INT usually runs around 135, so it's adding fuel at that point. On a long steady drive, the INT usually averages around 130 or so. BLM always a little higher, maybe 135 or so. Impala, the thought about the fuel relay was that it could be dropping out and not enough oil pressure to keep it alive in that hot start - which is why I was interested in jumping it. Does that make any sense, should I try that? I don't think that is happening but I am grasping at straws. I really think I will try shimming that FPR spring since it is pretty easy to do. If not enough pressure to make a difference, at least I will know. Seems like my problem is tough because it's on the hairy edge and only shows up in this one situation. It would be a lot easier if it did it all the time. Every test takes over an hour - a 30 minute drive, 45 minute cooldown, a 1 minute test. Then I can't test anything else unless I do the whole cycle over !!!

Oh, by the way 65-Impala, I saw a beautiful 65 the other day in Evening Orchid color. Didn't get a close enough look to see if it was a 409 though.



Edited by go_hercules on 02-22-20 09:17 PM. Reason for edit: No reason given.

 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-23-20 08:03 AM - Post#2787106    
    In response to 65_Impala

Typically it's in the 100-130*F range but I've seen newer stuff even lower. The running delay before closed loop varies depending on the start. On this TBI stuff, if it's considered a cold start then it's minutes of time and if it's considered a hot start then it's typically something like 20-30 seconds.

Thanks - I've wondered about the temp range I saw. When I've seen someone mention a temp criteria it was lower. I didn't do an exhaustive check - just watched it in the datastream once or twice. It very well could be that was just the temp when the time criteria was met.
I was also wondering if the time criteria varied with temp or other conditions. nice to know.
I've also wondered if there is a O2 mV criteria to go into closed loop. I once had a lazy O2 where it would change enough driving to go into Closed loop but would not vary enough to enter closed loop @ idle after cold start or, if driving and then stopping, would drop out of closed loop. Only found it because I failed emissions @ idle. Tracking that down, it made me think O2 had to get over 700mV before it'd go into closed loop. However, I couldn't find anyone who knew.

according to my scan tool, it stays in open loop during this whole ordeal. Once it clears out and I stop and restart, say within a minute or so, it is still in open loop. Only after I drive it a few minutes does it go into closed loop.

I think important part is verifying it only struggles when it's in open loop. I don't think the fact that it's not always bad in open loop - even separated by a few minutes is necessarily that surprising. eg. fuel pressure could change slightly, temp could be slightly higher, ECM in different A/F map for some reason,

You've been so thorough on your checks/troubleshooting there's not a lot left except something assumed good is not actually good. As I had problems ~11psi (1yr old Autozone) which went away when I replaced with an ACDelco pump that gave me 13psi - that's where my mind is stuck. These engines are sensitive to fuel pressure and I've read many posts which were resolved with a new pump & filter - though admittedly never one like yours in which the problem was only during such a limited time/condition.

btw - though this would have no relation to a problem in open loop - I think it should go into closed loop @ idle if you sit long enough. Also should stay in closed loop @ idle after driving. If it doesn't you may want to replace O2. I think only about $12 and easy to replace.

65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


Edited by Sabino on 02-23-20 08:06 AM. Reason for edit: No reason given.

 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-23-20 09:26 AM - Post#2787111    
    In response to Sabino

It does stay in closed loop at idle once there. And it will go into closed loop if allowed to idle long enough. The O2 varies nicely, from around 0.1 to above 0.9. And you can see it varying before it goes closed loop. With all you said, I am really thinking fuel related. I mentioned I was going to shim the regulator spring, buy like one of you guys mentioned I, and the more I thought about it, I don't think that would work if the pump is outputting all it can. So if I am seeing 11 at the gauge, the only way shimming would work is if the regulator was initially set to something lower, like say 9 or 10. Or if the spring weakened over time. I don't think either of those is likely. I was even thinking of pinching the rubber section of return line, but with old hoses that doesn't sound too wise. More pondering today.



 
65_Impala 
Very Senior Member
Posts: 4440

Reg: 12-29-02
02-23-20 07:21 PM - Post#2787156    
    In response to go_hercules

The oil pressure switch comes on at 4psi so it's not likely that the switch isn't on if the engine is running unless the engine was shot.

You just posted that the O2 swings when in open loop. Does it really or does it just go high or low at times? In open loop, it should not oscillate between rich and lean like it does in closed loop.

The INT and BLM being in the 130-135 range is saying it's adding fuel but that's not very much and wouldn't make me suspect the pump or a fuel delivery problem. New vehicles with low miles can be found 5-10 counts either high or low of 128. The fuel and weather conditions easily make the fuel trims vary that much.

I'd probably look at the EGR again. Possibly block it off as a test.



 
go_hercules 
Contributor
Posts: 170

Reg: 10-27-16
02-23-20 08:23 PM - Post#2787165    
    In response to 65_Impala

I'll look again at the scan tool, but I am pretty sure I recall the O2 swinging in open loop, but you've got me wondering. Funny you should mention the EGR. That is something I had thought about doing at one point - blocking it off. I have checked it by applying a vacuum, then watching to see that vacuum bled off when the engine started, just what a negative backpressure type is supposed to do. And I have pushed up on the diaphragm while idling to see that it gets rough, which it does. And I took it off to see that nothing was blocking the passage, like a chunk of carbon that might get stuck in the EGR - all clean. So it seems that it is okay, but I will add that to my test list.



 
Sabino 
Member
Posts: 135

Reg: 07-16-05
02-24-20 07:30 AM - Post#2787183    
    In response to 65_Impala

Good idea on blocking off EGR as test. Much easier than replacing a fuel pump on a guess- I’d do that first.

Also agree INT doesn’t seem high, though while driving you may have more throttle than normal for a given speed/condition and could be in a BLM cell for acceleration? which would confuse things, at least for me as not alot of experience. Seems INT @ closed loop idle where you should be in idle BLM cell would be more interesting.

go_herc - when you measured fuel pressure where and how did you do it? I read one thread where the adapter they borrowed replaced the fuel filter which isn’t a great method. Just checking


65 Belair 350
68 Camaro 327
93 Suburban 5.7L TBI


 
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