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

[mjordan]

When I crank engine up runs fine, drive it 2-3 miles and as I come to a stop light it stumbles and stutters and wants to die. I checked the temp sensor and voltage is good and comes down as engine warms up. I have no vacuum leaks that I can tell and engine runs fine off idle. Runs 80 down the freeway without a hitch. Pulling my old grey hair out. Help!!!!

 

[Baighn92]

I have a very similar problem. But still haven't found the cause.

My mechanic did the following and the stumble at idle still has not gone away.
-changed coolant temperature sensor, and verified proper resistance, good.
-changed fuel filter.
-checked fuel pressure, good (12psi).
-Rebuilt TBI with kit.
-new gaskets for TBI.
-new air filter.
-checked distributor bushing play, good.
-checked Idle Air Controller, good.
-checked operation of EGR valve, good.
-Installed new NGK plug wires.
-New spark plugs.
-checked engine vacuum, excellent.

My mechanic is quite stumped at the cause. He increased the idle up a bit and it still has a stumble but at least it doesn't nearly stall like it did before.

Hope you find the fix for this....

 

[smdk2500]

I had a 94 that would do similar things and there was a issue with the MAP sensor. I cannot remember exactly what the issue was that was 10 years ago lol. I do know it wasn't throwing a SES light. Just my 2 cents but something to look at.

 

[pro17]

Baighn92, your mechanic did all that, and charged you for it? Thats like a perfect case study in parts cannon.
mjordan needs to check fuel pressure. If its less than 10 psi what you are experiencing is going to happen. I've experienced it personally when my truck was TBI, as have a couple other people I know with TBI engines. I may even have a youtube video on it somewhere. Dont be fooled just because you can go highways speeds doesnt mean you have great pressure.

 

[DerekTheGreat]

I had an '88 454 TBI that sputtered & stumbled immediately when started cold and sometimes hot.. Turned out the base idle was too low. Someone punched the cap out and adjusted it for whatever reason and the computer wasn't happy with it. May or may not be your issue but it's something to look into. There is a procedure on how to adjust it, I'm too lazy to link it.

My '89 350 TBI truck would fire up and then immediately die when the idle went any lower than 1000 rpm's after changing the plugs, wires, rotor & cap. If I unplugged that tan wire w/black stripe, it wouldn't die. Replaced distributor and all has been well since. There was quite a bit of play in that Dizzy's shaft though, guessing it didn't like the new rotor or me touching it much.

 

[Baighn92]

Baighn92, your mechanic did all that, and charged you for it? Thats like a perfect case study in parts cannon.
mjordan needs to check fuel pressure. If its less than 10 psi what you are experiencing is going to happen. I've experienced it personally when my truck was TBI, as have a couple other people I know with TBI engines. I may even have a youtube video on it somewhere. Dont be fooled just because you can go highways speeds doesnt mean you have great pressure.

Hahaha nono...all those things that were replaced were done one-at-a-time and then ran for a bit to see if it worked.
Didn't do all that stuff at once.

Engine still stutters at idle to this day. Did notice better fuel mileage though.

 

[JMiller]

Idle Air Control Valve? They are relatively cheap. I dont know of anyway to test them.

 

[JMiller]

Also I just recently had the problem of clogged injectors. I cant remember what it cost but I traded mine for a cleaned set. Runs much better now. I had no power past half throttle before.

As for the stumbling idle. The IAC solved that problem for me. I've also experienced idle too high (1500) with bad IAC. I have a 91 s10 4.4 and 92 k1500 5.7. In my case IAC seems to be a common issue with idle problems.

 

[mjordan]

Thanks everybody for you input.... I am going to check fuel pressure this weekend and update ya'll. I cleaned the IAC and reset it but problem is still there. Once again thanks.

 

[mjordan]

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?