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Username Post: Horrible Idle When Hot        (Topic#355580)
djackson210 
Poster
Posts: 42

Age: 36
Loc: San Antonio, Texas
Reg: 01-31-19
03-11-19 02:47 PM - Post#2761856    

'91 C1500 5.7

I've got myself a horrible idle when the truck gets up to operating temperature. Most noticeable when I come up to a red light, or get stuck in traffic. Trucks sits there & acts as though it wants to die. RPM's get low & fluctuate up & down. Oil pressure follows the RPM's. However when going down the road or on the highway, truck runs like a champ. No detectable engine miss.

None of the above happens with the engine "cold".

Best I know I've covered all the usual suspects....

All the below listed parts were bought new about 6-12 months ago. They have less than 5K miles on them. Also, any others maintenance listed below has less than 5K miles since they were performed. However, this engine is a rebuilt 350 I purchased back in July of last year. So some of the parts below were transferred from the old motor to the rebuilt.

EGR Valve
TPS Sensor
IAC Valve
ECT Sensor
PCV Valve
Fuel Filter

Best I can tell all vacuum lines are good. No dry rotting or cracking & all snuggly attached as they should be.

Throttle body base gasket "should" be good. A new one was installed when I installed the junkyard throttle body spacer back in January.

I also did a complete TBI tear down, clean & rebuild back in May of last year. Best I can tell fuel injectors have a good conical spray pattern to them. I don't see any sputtering or anything of the sort. I believe I've run 2 cans of Berryman Injector cleaner through the tank to "help" "clean" the injectors if possible.

New distributor, spark plug wires & spark plugs.

Any & all input is greatly appreciated. I've got my hunches on what I believe is transpiring but I'd love to hear some other input.

My hunch(es) is either the timing is wrong, there's a vacuum leak somewhere that I'm missing or possibly the EGR port on the intake is dirty. Hell it could be all 3. Or none of them, I'm at a loss here!


'91 Chevy C1500. Single Cab LWB.


Edited by djackson210 on 03-11-19 02:51 PM. Reason for edit: No reason given.

 
Sabino 
Member
Posts: 51

Reg: 07-16-05
03-11-19 09:33 PM - Post#2761899    
    In response to djackson210

I have a 93 c1500 5.7 which had a lot of drivability problems. I don’t know if a 91 has same/similar ECU. Regardless, one of my problems was an idle just like you described. I replaced an EGR, knock sensor fuel filter, checked TPS sensor voltage, watched injectors spray, and finally got TunerPro working to look at log data. I saw Idle Enrichment kicking in during idle hunting up and down but couldn’t tell if it was the a symptom or cause - and if cause, no clue what to do about it.
I finally read someotherguy’s bit in the FAQ on checking fuel pressure and bought a meter, adapter to check fuel pressure. That was the solution for me.
My problem was a marginal fuel pump. I had 8-9 psi and it would vary when reving engine. It should be higher. I had replaced pump a few years ago with an Autozone pump and discounted possibility it could be pump because truck was ok otherwise. But I was wrong. Bought an ACDelco pump and that fixed the problem. No more bad idle + much more power. Had a solid 12 psi with new pump.

I assume the problem was with lower pressure the spray was not visually different but not enough to hold desired idle rpm - so it would drop and the idle enrichment would kick in and dump some more fuel and it was run up, turn off enrichment and repeat. I guess I didn’t notice the gradual degradation of fuel pressure during normal driving as it just resulted in more throttle needed than before. Which, btw had messed up shift points and when I replaced the fuel pump the shifting and downshifting also greatly improved.

I’d recommend reading that FAQ and checking the fuel pressure and timing.



65 Belair 350
68 Camaro 327
85 Caprice 5.0L
93 Suburban 5.7L TBI


 
djackson210 
Poster
Posts: 42

Age: 36
Loc: San Antonio, Texas
Reg: 01-31-19
03-12-19 07:18 AM - Post#2761917    
    In response to Sabino

I forgot to mention in my list that I did add a fuel pressure gauge under the hood. Mounted right at the back of the TBI. Fuel pressure looks good (11-12psi) at idle when cold last time I checked. But I'll definitely be popping the hood to check the fuel pressure gauge next time it gets hot.

It could very well be a fuel pump. I have literally no idea if my truck has ever had a new fuel pump put into. Guy I bought it from kept no records whatsoever. I've never understood that mentality of keeping zero records. I wish we lived in a world where vehicles were so good that we could get by with not keeping any records whatsoever.

But I digress....

I appreciate the input Sabino. I'll definitely report back in the next day or so with fuel pressure readings with engine at operating temp. Thanks!

'91 Chevy C1500. Single Cab LWB.


 
djackson210 
Poster
Posts: 42

Age: 36
Loc: San Antonio, Texas
Reg: 01-31-19
03-14-19 02:12 PM - Post#2762079    
    In response to djackson210

I got her up to operating temp & checked the fuel pressure. It holds solid at 12psi. So best I can tell it's not a fuel pressure issue.

Anyone else got any pointers to throw my way?

'91 Chevy C1500. Single Cab LWB.


 
Sabino 
Member
Posts: 51

Reg: 07-16-05
03-16-19 08:54 AM - Post#2762224    
    In response to djackson210

Well, that was best idea I had. Did you rev engine while checking pressure and it still hold steady?
There are people here that have a ton of experience on these engines and maybe one of them can help. There is also a FAQ here on checking the basics which you might want to check out for ideas.

Unless someone has advice it seems you are at the point of continuing to replace things or getting into the data from the ECM.
I assume a 91 is obd1 like the 93. You could log data during idle and look for something occurring in sync with the rpm fluctuations and maybe that will provide a clue. Difficulty is, as I mentioned, it can be hard to tell if something is changing in reaction to some other fault or if it is actually the problem. I got a cable from Moates and used TunerPro. It’s not too hard to set up and I figured I’d be using it again on a truck this old.

65 Belair 350
68 Camaro 327
85 Caprice 5.0L
93 Suburban 5.7L TBI


 
rockfangd 
"6th Year" Gold Supporting Member
Posts: 2626
rockfangd
Age: 33
Loc: Utica ny
Reg: 04-13-10
03-16-19 12:48 PM - Post#2762240    
    In response to Sabino

need to know what the ecm engine coolant temp is when warmed up.
I would replace the ECT sensor on the intake by the thermostat with a good quality new one and see if that resolves your problem

Old School GM fan FOREVER


 
djackson210 
Poster
Posts: 42

Age: 36
Loc: San Antonio, Texas
Reg: 01-31-19
03-18-19 07:24 AM - Post#2762352    
    In response to rockfangd

  • rockfangd Said:
need to know what the ecm engine coolant temp is when warmed up.
I would replace the ECT sensor on the intake by the thermostat with a good quality new one and see if that resolves your problem



Your suggestion leads me to an interesting concern...

I'm in the process of installing some aftermarket gauges in the truck. Is the computer in the truck going to run into a problem when I remove the factory ECT sensor to install the aftermarket temp gauge?


'91 Chevy C1500. Single Cab LWB.


 
Shepherd 
Frequent Contributor
Posts: 1688

Loc: Lake George, NY
Reg: 11-11-15
03-18-19 07:48 AM - Post#2762354    
    In response to djackson210

you may get a check engine lite and the pcm must read coolant temps of course.



 
Sabino 
Member
Posts: 51

Reg: 07-16-05
03-18-19 08:35 AM - Post#2762361    
    In response to djackson210

ECM uses coolant temp. I think one of the things it uses it for is to decide when it can go into closed loop. If you don’t get into closed loop after warming up that could be causing a bad idle. If you found you were never getting into closed loop, temp sensor and grounds would be things to check. (There are something like 3 grounds in various places - a bad or degraded ground can mess things up also. )

There is a thread on coolant sensor, how to check sensor with problem similar to yours. https://www.chevytalk.org/fusionbb/showtopic.php?t...
I didn’t read it so don’t know if directly applicable to you.

65 Belair 350
68 Camaro 327
85 Caprice 5.0L
93 Suburban 5.7L TBI


 
CowboyTrukr 
"8th Year" Silver Supporting Member
Posts: 4180
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
03-18-19 09:25 AM - Post#2762367    
    In response to Sabino

There are two temp sensors. Computer one is at the stat. The gauge one is in the head. This computer isn’t smart enough to know if the temp sensor is working or not.

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


 
rockfangd 
"6th Year" Gold Supporting Member
Posts: 2626
rockfangd
Age: 33
Loc: Utica ny
Reg: 04-13-10
03-18-19 08:23 PM - Post#2762432    
    In response to CowboyTrukr

The one by the thermostat does not go to the cluster.
Only to the ECM. So If you put in aftermarket gauges only use the one on the side of the head


Old School GM fan FOREVER


 
djackson210 
Poster
Posts: 42

Age: 36
Loc: San Antonio, Texas
Reg: 01-31-19
03-19-19 08:50 AM - Post#2762474    
    In response to rockfangd

Thanks for all the input folks. It'll definitely be keeping me busy. Along with the aftermarket gauge install of course.

I know right off hand where 2 of the 3 ground straps are. Those 2 are in good shape. But I'll do my research & find that 3rd one & check it out.

Thanks also for the heads up about the temp sensors. I'll also g'head & throw in a new ECT sensor to rule that potential problem out. I'll be draining some coolant anyway to install the aftermarket coolant temp gauge so I might as well throw a new ECT sensor in while I'm at it.

I'll definitely keep this thread updated with what I find out after the new ECT install.

'91 Chevy C1500. Single Cab LWB.


 
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