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1991 GMC 5.7L TBI - Rough Idle/Stalling

Apr 10, 2018 12:56PM PDT

(Let me try again. Data disappeared!)
As some of you may recall, original problem was a Code 42 that was finally tracked down to a faulty Ignition Control Module that had less than 4500 miles on it. Replaced the module The code 42 was removed and no other Codes have become apparent.

The new engine has new spark plugs m(just changed again.), plug wires, distributor and coil (Just changed again.). All sensors have less than 4500 miles on them.

The engine starts easily but idles very rough and can only be kept running by blipping the throttle; when first started. It frequently stalls when shifting from park to drive. You can hear the misfiring in the exhaust. No spitting back through the TBI. Once the engine reaches operating temperature, the engine still idles rough but doesn't stall as much. The engine seems to smooth out above 1600rpm. Transmission shifts normally once underway.

After installing the new Ignition Control Module, timing was adjusted as described in Chilton. "...disengage the timing connector which comes out of the harness conduit...This will put the system in the bypass mode."
After the timing was set, the Code 42 reappeared because the Direct Ignition System (bypass circuit) was "open", normal and expected. Reconnected the
"timing connector" and reset the Code 42. No other error codes have shown up.

Now comes the interesting part, with the "bypass circuit" open (disconnected) the ENGINE RUNS SMOOTHLY, with no stalling!
Reconnect the "timing connector", start the engine and the ENGINE RUNS ROUGH, misfiring, etc.

No error codes are being displayed.
What do I check next?
Your assistance/recommendations are much appreciated.
AK up North

Discussion is locked

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Clarification Request
1991 GMC 5.7L TBI - Rough Idle/Stalling
Apr 11, 2018 9:16AM PDT

Further research indicates it may be the Idle Air Control Valve acting up.
Will check it next.
Any advice?
Best Wishes,

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Apr 11, 2018 11:09AM PDT

Hi, Gus; I know that I've run into this same scenario, but can't remember for sure the outcome? According to my data, the 5.7 uses a vacuum advance unit on the distributor. Try blocking off the vacuum source temporarily to see if the roughness goes away. (could be over-advancing). If that doesn't seem to help, try the same with the EGR valve. (too much air, not enough fuel when cold).
It doesn't sound like an IAC issue, but you can spray the IAC with carb cleaner through the top of the TBI where it 'sucks' in the air (with the engine running). Get back to me on the results and we can go from there.

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Apr 11, 2018 9:37PM PDT

Thank you kind Sir.
Don't believe our particular distributor has the vacuum advance, but will double check tomorrow.
Will check the EGR valve next.
Sure do appreciate the advice. Thank you.
Will keep you posted.
Best Wishes,

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Clarification Request
EGR Valve - Test
Apr 12, 2018 3:56PM PDT

Just my luck, can't find our vacuum gauge. It is probably sitting right beside our missing box of drill bits. Will pick up a new one tomorrow, in town.
Did run tests on the EGR solenoid. It checks out in spec.

Interesting thing, disconnected the factory (new) coil and wiring. Wired in the MSD 6 Capacitive Discharge Ignition System and their High Vibration Blaster Coil to the factory distributor and wiring harness.
The engine runs smoother at low rpm, idles well (engine cold or hot) and no longer stalls when moving the gearshift lever from Park to Drive. Took it out for a quick test drive resulting in no noticeable difficulties. Transmission shifts as it should.
There are no error codes to be read. No engine trouble light. (But there wasn't with the factory coil either.)

Could it be possible the new factory coil was bad, or quickly went bad?

Will purchase a vacuum gauge tomorrow, just the same. Want to follow up, testing the EGR valve.
Updates to follow.
Advice or comments are much appreciated.
Best Wishes,

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Apr 13, 2018 8:28AM PDT

Great job, Gus! The coil never crossed my mind. I wouldn't fret over the EGR valve since everything seems okay, now. Have a great weekend.

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EGR Valve
Apr 17, 2018 1:28PM PDT

Let me say right up front, the truck seems to be running fine!
Drove it to town last Friday. The engine was running rough (misfiring) between idle and 1600rpm when cold. Did better once warm but stalled a couple of times and not running as smoothly as it should. Picked up a vacuum gauge and new EGR valve just in case.

Yesterday, started the engine and let it come up to operating temperature. It ran rough during the warmup period, requiring an occasional blip of the throttle to keep it running. Attached the vacuum gauge per instructions and verified the EGR solenoid is functioning correctly. This left the EGR valve itself as a possible culprit.

Removed the EGR valve, 1/2 turn at a time with a box end wrench; no room for a ratchet or full swing of the tool. Took a while. Blew into the pintle orifice to ascertain if it was stuck in an open position. Could not force any air past the pintle seat, indicating the EGR valve should be working properly.

HOWEVER, when removing the bolts holding the EGR valve in place, noticed they seemed to be loose; very easy to turn. The valve was not firmly affixed to the intake manifold! Could have been allowing an air leak to occur? Same effect as the pintle being stuck in an open position?

So, as long as had the new EGR valve in hand, replaced the old one (keeping it for backup). Firmly bolted it to the manifold with a new gasket in place. Reattached wiring, vacuum hoses and engine air intake assembly and filter.

Started the engine and it immediately idled smoothly, with no misfiring evident. Took the truck out for a short test drive. Made almost 30 stop and goes, without the engine missing a beat; much less stalling. Transmission shifted as it should.

Loren, THANK YOU for pointing me in the right direction and saving me a lot of heartburn; not to mention money!
Best Wishes,

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Apr 18, 2018 11:29AM PDT

Wow, Gus! Never even considered it would be that simple! Good for you, my friend. Supposed to be good (dry) weather for the next few days. Back to some yard work for me. LOL.

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Power steering
Mar 11, 2019 9:32PM PDT

My 92 GMC was doing this too. I tried several fixes with no luck. I also had a minor power steering leak. I bought some Lucas power steering leak stop that also takes care of hard steering. The hesitation went away. Apparently there were hydraulic loading issues possibly caused by a sticky spool valve in the steering gear. Engine power is better and steering is improved. The felt misfire seems to be completely gone.