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 Page 1 of 2 12
Username Post: 1990 V3500 7.4 TBI stalls in drive        (Topic#240772)
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-25-10 01:17 PM - Post#1904520    

Truck (77,000 miles) was running just fine then all of a sudden when you put it in gear it the RPM drops, stumbles, then stalls, I have to 2 foot it to keep it running. It needed a tune up anyway so I replaced the cap & rotor, plug wires, plugs, and fuel filter. Hooked the scanner up and the it is requesting 25 - 30 steps of IAC when in drive but it will still stall so I replaced the IAC but that didn't fix it. I then disconnected the MAP sensor and it would keep running in gear but barely. So I replaced the MAP sensor but the stalling still continues when put in gear. TPS seems to be good, voltage is 0.95 at idle and smooth increase to WOT. I also checked for vacuum leaks around the TBI by spraying carb cleaner around to base, no RPM change. O2 voltage seems Ok is is changing when running. I'm kind of out of ideas. Anyone have any ideas???



 
Don M 
Contributor
Posts: 152

Loc: Tampa, Fl. USA
Reg: 09-22-07
04-25-10 03:24 PM - Post#1904580    
    In response to jrlichina

Check your fuel pressure. Should be between 9 and 12 PSI. The easiest way to check the pressure is to disconnect one of the rubber lines at the fuel filter and insert a “tee” fitting with a gauge made-up of parts you can pick up at the local home improvement store.

Low pressure will cause idle issues. My theory is the injectors don’t atomize the fuel properly under low pressure causing the poor idle. At higher RPMs more air is flowing through the system and the higher air flow helps overcome the poorly atomized fuel issue.

Don




 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-25-10 04:38 PM - Post#1904615    
    In response to Don M

Yes I guess I do need to check the fuel pressure just in case but since this is TBI there are no rubber lines to the TBI. Also the truck has twin tanks and the symptom is the same when running on either tank which at least means the both pumps are working. I would think fuel pressure isn't the problem since it runs fine off idle, no starving issues. But I guess I should check the regulator, that could be the issue. Thanks for the input. Also more info, the Block Learn is running at 129 and the Integrator is at 130 which are right where they should be.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-25-10 05:23 PM - Post#1904652    
    In response to jrlichina

Forgot to say that to check the pressure I need to use a special adapter that connects to the fuel line going to the TBI which has a provision for a gauge (I'm sure that piece is cheap )



 
Bill K.b 
Senior Member
Posts: 4437

Loc: upstate NY
Reg: 10-24-05
04-25-10 06:45 PM - Post#1904702    
    In response to jrlichina

Trans type?

Just throwing it out there, if a 700R4 or other locking converter trans, could the converter be not unlocking? (seems unlikely a 454 3500 would have a 700R4 trans though). A friend a few years ago had a car that would die when put in gear and it was something to do with the trans, but I don't remember exactly what.

Those who can, do. Those who can't, criticize it on the internet.

Driving 2002 Express 2500
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& yes, I once tried a frame swap on a 51 Chevy.


 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-25-10 07:44 PM - Post#1904728    
    In response to Bill K.b

Ahh yes I should have added that info as well. It is a turbo 400 so the lockup issue wouldn't apply. I wish I had a lockup converter( sure could use a few more MPG )



 
gchemist 
Senior Chevytalk Moderator
Posts: 24561
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-26-10 04:29 AM - Post#1904815    
    In response to jrlichina

Welcome to the site. Bookmark this site for TBI tuning http://www.cfm-tech.com/GM_tbi_tuning_tips.htm
The 454 is not listed, but you can tune it the same way.

Connect a vacuum gauge. Connect it on a TBI port. What does the needle do? Use this to interpret needle movement.
http://www.centuryperformance.com/tuning-with-a-va...

Stall converts are availble for the 400th. You want a stall below your cruise RPM and high enoug to move the heavy truck.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
04-26-10 04:55 PM - Post#1905233    
    In response to gchemist

http://www.summitracing.com/parts/SUN-CP7838/

Has the adapter you need.

You can temporarily raise fuel pressure by blocking the rubber return line going back to the tank.

TPS voltage is WAY TOO HIGH. Should be .5 volts with throttle closed, 4.5 volts at WOT. Not any adjustment on the TBI TPS, so you will have to slot the plastic with a dremel in order to adjust the closed throttle voltage.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-26-10 06:42 PM - Post#1905334    
    In response to Vaughn

Thanks for the tip on the fuel pressure gauge setup, I think I may buy that one. My GM manual gives two different voltage values which is kind of confusing. One place in the diagnostic section says it should be 0.95V or less, and then in another place says less than 1.25V(which seems way high) and I thought 0.5 was where it should be at. Do you know for sure it should be 0.5V?



 
gchemist 
Senior Chevytalk Moderator
Posts: 24561
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-26-10 07:22 PM - Post#1905375    
    In response to jrlichina

Yes, the volts should be at 0.5V.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
04-27-10 06:55 PM - Post#1906032    
    In response to gchemist

It is DEFINITELY .5 volts closed throttle. This is true for ALL tbi motors.

This is probably why your block learn is too high (126 is best, 129 isn't that great). Block learn adapts the ECU to faulty sensors in order to make even the worst fuel injection motors run, but it is not the best way to keep it running at peak performance. If you have the motor tuned up and all your sensors are working right, block learn will be 126 - which means that there is no "adapting" done to the ECU.

If you want to check the performance of both the ECU and the 02 sensor, look at the O2 Cross Counts. The higher the number, the better - this means that the ECU is sitting at the perfect Oxygen content all the time (or in other words the fuel mixture is perfect).



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-28-10 11:02 AM - Post#1906370    
    In response to Vaughn

So I modified the TPS as much as I could to get it close to 0.5V and it does now stay running in gear. Every once in while it will drop 100 RPMs then bounce back up to a steady idle when in gear. I could only get it down to about 0.64V so I'll try a new TPS and see what it shows out of the box. On a side note about the GM manual, the two sections covering TPS codes did state 0.5V so I don't know why in 2 other places it says 0.95V or lower or 1.25V or lower which I knew sounded way off. I'll also check the cross counts again, I think that count was relatively low. Thanks for the input all!!



Edited by jrlichina on 04-28-10 11:05 AM. Reason for edit: No reason given.

 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
04-28-10 01:55 PM - Post#1906425    
    In response to jrlichina

GM says that because of the Block Learn Function. When they incorporated that into the OBD II ECUs, it was supposed to take care of the all the ranges of values on the sensors, so they didn't have to be as accurate. They found out later it wasn't the panacea it was supposed to be, which is why most tbi tune-up articles will tell you to set it specifically to .5 volts for best throttle performance.

Get an AC Delco TPS sensor (Napa), because these were stringently built to OEM specs.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-29-10 07:29 PM - Post#1907224    
    In response to Vaughn

I have a new TPS coming on Saturday so hopefully that one will allow me to get to 0.5V then I'll post my results. Thanks for all the help!!



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-02-10 05:26 PM - Post#1908752    
    In response to jrlichina

I replaced the TPS and adjusted it to 0.5V at idle (throttle closed) and it idles great: BL 126 & 0 IAC steps but when I put it in gear it dies. If I put it in gear and touch the gas a tad it stays running but then the BL slowly climbs, maxed out at 156 at one point. Anything just off idle and the trucks runs absolutely perfect. If I drive it around a bit and come to a stop it will idle fine for about 30 seconds then it starts that BL climb and sometimes it just stalls. The 02 cross counts to rise to 256 then starts over. I am stumped....



 
tony1963 
Frequent Contributor
Posts: 1206

Loc: Birmingham AL
Reg: 10-11-08
05-02-10 05:32 PM - Post#1908758    
    In response to jrlichina

Sounds like a vacuum leak.

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jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-02-10 07:40 PM - Post#1908838    
    In response to tony1963

I have checked for vacuum leaks earlier and found none.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-03-10 02:35 AM - Post#1908910    
    In response to jrlichina

Do I need to reset/power off the ECU for it to re-learn anything?



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
05-03-10 09:30 PM - Post#1909561    
    In response to jrlichina

You can reset power if you like (take one battery cable off and touch it to the other, this will short out the capacitor inside the ECU and wipe the memory). This will also wipe trouble codes and reset the IAC. However, it won't fix the problem.

Your Block Learn should only climb a point or two and then stop. If it doesn't stop, there is another problem in the system.

O2 crosscounts shouldn't climb and then reset. It should stay about in the same range all the time.

O2 crosscounts changing makes me think there is a fuel pressure problem, a bad O2 sensor, or a vacuum leak. Vacuum leaks on a TBI motor can be difficult to pin down, a lot more so than on a carb motor. You may have to replace a fair bit of vacuum hose to get rid of it, or you may have to rent a smoke machine and hook it up to the intake to find it.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-04-10 05:14 AM - Post#1909643    
    In response to Vaughn

I'll look hard again for a vacuum leak there are only 3 lines going to the TBI and I'll check the intake surfaces again. I guess I'll have to break down and buy the fuel pressure test kit even though it runs the same on either tank (could be the regulator). I do get a good smooth cone out of the injectors at idle. I'll check out the O2 sensor. Thanks again.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-04-10 05:51 AM - Post#1909658    
    In response to jrlichina

As I understand it the O2 cross counts should climb as this indicates that the O2 is crossing that 0.450V threshold meaning the ECM is trying to keep the fuel mixture at 14.7:1 which is good right? (Providing the BL isn't running away at the same time.) And it resets since the max value for that variable in the ECM is 255 (an unsigned char 0xFF).



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
05-04-10 11:39 AM - Post#1909810    
    In response to jrlichina

Crosscounts are when the o2 sensor goes above and below that threshold voltage that the O2 sensor puts out (basically rich/lean/rich/lean...,etc). The higher the number, the closer that the fuel mixture stays at stoichometric 14.7 to 1.

The crosscounts go up to a limiting factor and then stays there, basically limited by the sampling rate of the processor. Once it gets to this limit, it stays there almost all the time. The better the O2 sensor, the higher the crosscounts - within limits.

It should not start out at a low number then max out the buffer - it should do this once when going into open loop, then the number will stay in a constant range.

By the way, do you know whether you are in closed loop when you are taking this reading? The motor will stay in open loop until the CTS (coolant temperature switch) gets hot enough to tell the ECM to go into closed loop. If the motor is in open loop, you will get strange readings. Check to make sure the CTS is working. It very probably will max out the BL and o2 crosscount memory on a continual basis, until it goes into closed loop.

What tool are you using to read off these values? If it is a program on your laptop and you connect to the ALDL via a serial port, this would explain a lot.

Your CHAR 0xFF is probably a memory location, not the value that is registered in that location. 0xFF is a hexadecimal representation, usually denoting the memory location (instead of a binary or BCD code of that value inside, like 0101000101, etc). There are ways of accessing the value inside that memory location, but it normally means porting a snapshot of the memory to a PC, where you can dissect what is in there.

If it holds 255, the memory location is 8 bits long. If it exceeds this, the reader that you are using probably has a fit, because it tries to overwrite a memory location that is not allocated for it to use.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-05-10 04:48 AM - Post#1910188    
    In response to Vaughn

All of my values that I look at are during closed loop. I use my AutoXray scanner for getting the data stream information. I only mentioned what I am assuming the the data size to be for the cross counts since it was resetting at 255. Years back I did a lot of embedded and PC software development for Bear Automotive a company that made scan tools, engine analyers, and gas analyzers and I specifically worked on the scan tools and engine analyzer systems. I hope to work on it this weekend and I plan on just replacing ALL my vacuum lines to take that completely out of play which I should have done when I bought the truck 2 years ago. Thanks again for the input, I love it!



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-20-10 08:54 AM - Post#1917881    
    In response to jrlichina

Ok I've been busy and finally had a chance to get back to the truck. I did find one tiny vacuum leak in the line that feeds the cruise control. I can get it to idle better but it is still not right. I also noticed that the idle adjustment screw on the TBI has been messed with (the tamper cap is off). Does anyone know the correct procedure to set the correct throttle plate opening??? I'll need to get that correct and start over. Thanks again for all the help!



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
05-25-10 11:51 AM - Post#1920427    
    In response to jrlichina

I found a great site that has some awesome tips on TBI mods and adjustments they also supply many hard to find parts. Like the fuel pressure adapter I was looking all over for! TbiParts Had some handy docs on how to set base RPM and some other diagnostic procedures. I'm going to try and reset my throttle plate opening and report back if that helped.



Edited by jrlichina on 05-25-10 11:52 AM. Reason for edit: No reason given.

 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-18-11 03:58 PM - Post#2078515    
    In response to jrlichina

Ok spring is here so it was time to get back to the truck. Well turned out the stalling was due to 4 of the magnets on the distributor shaft were broken off and just floating around in there. I can't believe it ran at all much less as good as it did. So I put in a new distributor and it runs fine in gear. But I can't find the damn EST bypass connector to set the timing!! I looked everywhere and don't see it. Does anyone know where it is hiding? Again this is one of those odd year trucks 1990 7.4 TBI in the old square body style since it's a crew cab. Any help on it's whereabouts would be greatly appreciated!!



 
gchemist 
Senior Chevytalk Moderator
Posts: 24561
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-18-11 05:33 PM - Post#2078560    
    In response to jrlichina

For full size trucks , this timing wire is typically a tan wire with black stripe located near to or just to the right of the relay cover on the firewall.

http://www.cfm-tech.com/GMtbituneup.htm

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
04-18-11 06:01 PM - Post#2078574    
    In response to gchemist

If it is there, it is the only single wire that comes out of the distributor base.

Some models have a EST wire that isn't part of the distributor. I don't have a picture of that connector that it is part of, maybe someone else does.



 
jrlichina 
Forum Newbie
Posts: 20

Reg: 04-25-10
04-23-11 05:19 PM - Post#2080962    
    In response to Vaughn

I found it taped up to the wiring harness right near the master cylinder. So now here's something odd. I got the engine warmed up with the EST disconnected and set the timing to 0 degrees. Plugged the EST back in and it runs smooth in gear or idling. When I went to start it up in the morning it was running real ruff, rich, seemed to backfire a bit but as soon as it warms up it runs smooth. It never had a cold running issue ever. I've replaced pretty much everything, plugs, wires, cap, rotor, distributor, IAC, TPS, Coolant temp, MAP. So I'm kind of stumped why it is running so poorly now when cold. Any ideas?



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
04-23-11 07:26 PM - Post#2081024    
    In response to jrlichina

You should really set it to 4 BTDC (or even 8 degrees if it doesn't knock), even if the manual or timing sticker says to put it at 0 degrees. You can put it back to 0 degrees during emissions tests. It will run much better at those advanced settings than at 0 BTDC.

Find out what the CTS reads during cold idle, and see if it indicates whether it is in closed loop or not.



 
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