1991 GMC 350 TBI Engine stumbles and dies at idle after warm up

[94burbk1500]

Well I have tyried dam near evry sensor on the truck and still stalls..... Replaced Dizzy, TPS, IAC, MAP, CAp & Rotor, almost acts like converter is locking up... Wonder how I test that? I have put it on the scanner several time and have had code 43 pop up twice. So that is when I replaced the Dizzy but now I had a code for Map Sensor, replaced it. No light has popped up since, but still stall and sputters around 1500 rpm and when i come to a slow stop or when I go to give it a little gas in the parking lot... I'm lost. Could a EGR valve cause this?

I would expect it to also have problems at idle if it were the egr, but it's possible. Luckily that's easy to test, just disconnect the vaccuum line coming off of the egr and plug it with a screw or bolt. You'll throw a code after cruising for a bit, but it would not have an issue if the egr is at fault.

 

[Baighn92]

Recently my truck was running very strangely. After all the work done to correct the stumbles and misses, it started to misfire bad when a quick load was put on it and then a couple weeks later it lost all power and misfired terribly...actually back-fired a couple times (back-fired....not after-fired). Later it wouldn't start after multiple attempts. Turned out to be a bad ignition coil. I put a new one on and it fired up immediately on first attempt and idled smooth as butter for 15mins. I literally just did this, so I'll see how the drive in to work tomorrow will be. Hopefully this corrects my sporadic stumble at idle.

 

[Baighn92]

Just an update....truck is running perfect. Absolutely no stumble or miss. Over a year long problem finally fixed. Thought an ignition coil either worked or didn't, learned it can give you some false flags leading you to think it's other issues.

 

[c1500deathtrap]

Just a tip, if you are misfiring usually it is ignition. If you are backfiring or "late firing" it IS ignition. For op, i would put my money on ignition timing. get a light on it and see what the hell it is doing. Or view live data with winaldl or simmilar to see what the computer is trying to do. I have had bad ignition modules that would go on the fritz when the truck warmed up max out the ignition advance and retard alternately, looked like you were watching a damned am radio signal on the live data stream. Had all sorts of issues, high idle, idle stumble, stalls, and reduced power. as a redneck test / get me home fix, unplug the ESC wire and using a light set about 18 degrees advance at idle and it will run ok ish. but if unplugging esc fixes the idle i would take a peek at the ICM.

 

[c1500deathtrap]

Also check all of your grounds. That should always be your start point on electrical gremlins.

 

[Baighn92]

Update again: Even after the ignition coil was replaced, I noticed the odd miss here and there. I finally said f*ck it and bought new Delco platinum plugs for it. Upon removing the old plugs (that my former mechanic had installed) I checked their gap and was embarrassed to find that he did not gap them. I couldn't believe it. I was under the impression he was a thorough mechanic but they were definitely NOT gapped as they were all sitting at .050" which is the gap the plugs are in out of the box. Shameful.
So I have now been running the truck for over 6months and no stumble, miss or rough idle. It is finally running awesome.

 

[JWS6196]

Have you checked the knock sensor? I had a '91 and the knock sensor connector broke. Every time I would stop I would have to throttle the gas and hope it didn't die.
The ignition control module , or 'candy bar', also goes out often. I usually carry a spare ICM and a small tube of the white grease the dissipates the heat it produces.