94 Suburban 5.7L periodic searching for idle while stopped at stop lights

[Bnixon]

I have a 94 C1500 Suburban with a 5.7L TBI engine. I have been having issues with the engine randomly searching for idle when I stop at a traffic light. It doesn't happen all the time but does seem to be worse when it is hot. I have already rebuilt the throttle body and checked for vacuum leaks, and the spark plugs look okay. The voltage gauge also seems to fluctuate with the idle when it does it. I suspect this is due to the changing rpm, but not sure which came first, the voltage drop or the engine idle? I am looking for the next steps to troubleshoot this issue. Any ideas?

Thanks for the help
Brandon

 

[1997]

how many miles?
has it had a "tune up", plugs, wires, cap, rotor, fuel filter etc recently?
checked for codes? OBD1 can set codes without check engine light coming on.
O2 sensor original?

You may want to invest in a scanner before replacing any parts.

 

[Schurkey]

Connect scan tool, look at the data stream. Verify EVERY sensor, and the computer outputs like IAC and EGR.

Connect timing light, verify electronic timing advance.

Connect fuel pressure gauge, verify fuel pressure at idle and under load.

Are you sick of hearing "Let's Go Brandon" yet?

 

[Sabinoerc]

+1 on checking fuel pressure. I had surging idle once which was result of marginally low fuel pressure.
Does it also occasionally shift late and hard? Can also be a sign of marginal fuel pressure

 

[Bnixon]

how many miles?
has it had a "tune up", plugs, wires, cap, rotor, fuel filter etc recently?
checked for codes? OBD1 can set codes without check engine light coming on.
O2 sensor original?

You may want to invest in a scanner before replacing any parts.

160K miles has a fresh tuneup, including all that you said, plus new EGR, IAC, vacuum lines, O2, etc... We bought it in Feb of 2021 from the original owner and wrenched on it till august to be ready for the HotRod Power Tour. So we have gone through almost everything, which is why I am a little stumped on where to go next. I currently have no codes in the ECU.

 

[Bnixon]

Connect scan tool, look at the data stream. Verify EVERY sensor, and the computer outputs like IAC and EGR.

Connect timing light, verify electronic timing advance.

Connect fuel pressure gauge, verify fuel pressure at idle and under load.

Are you sick of hearing "Let's Go Brandon" yet?

"Are you sick of hearing "Let's Go Brandon" yet?" Lol

I haven't checked the base timing so I will do that next. All I have for OBDI systems is a simple GM scanner that just jumps the pins to flash the light for ECU and ABS systems. My Nice level 2 scanner only works on OBDII systems. I also replaced the ignition control module as the one in it failed and wouldn't pulse the injectors. This failure is not a super common issue on the HEI systems from what I understand.

 

[Bnixon]

+1 on checking fuel pressure. I had surging idle once which was result of marginally low fuel pressure.
Does it also occasionally shift late and hard? Can also be a sign of marginal fuel pressure

I will re-check that. The biggest issue I am having is it is intermittent. And of course, doesn't do it when I have the tools to check stuff

 

[Bnixon]

OK finally got a chance to troubleshoot the Burb. Base timing was about 4 Deg advanced so reset that to 0Deg. No change in idle condition. The base idle was off so I reset that to ~650-700 Rpm and during the process of setting the base idle, I noticed the IAC was not closing. It was a new part but it was one of the cheap local Advanced Auto brands... that was in stock when I needed it last year. It would work to bring the idle down when started but would not close during the base idle set procedure. I replaced it with a good one and that seemed to solve 90% of the problem. I still have a slight miss I think but it is way better at stop lights now. I will be checking the fuel pressure next week and pulling the plugs to examine and try and determine the cause for the slight miss. I think I am going to order an AC Delco ignition module as the one that is in there is a cheap one from the local auto parts store...again it was what they had in stock when the original one died. Just needed to get up and running again. Thanks for all the suggestions and help in troubleshooting this issue.

 

[PlayingWithTBI]

I think I am going to order an AC Delco ignition module as the one that is in there is a cheap one from the local auto parts store...again it was what they had in stock when the original one died.

Good idea. How about your ignition coil? A bad one can/will take out the ICM. You can spray water on the coil, with the engine idling, to see if it stumbles. I had a Pertronix Flame Thrower fail in about 2000 miles. It was occasionally shorting from the windings to the iron frame causing a little stumble at idle. When I sprayed water on it, bot the wires, the engine almost died.

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[DerekTheGreat]

It's been my experience when they hunt for idle like that, yeah, the base idle needs to be adjusted. My '89 TBI 350 truck also has an occasional miss at idle. It's not every day and it isn't a dead miss. After watching how the computer messes with my timing while idling with a timing light, I blame whatever is responsible for making all the timing requests at idle. Seems the Standard Motor Products ICM I've got in there now has yielded the smoothest idle of all the Chinesium that's been in there.