'91 5.7l TBI random interval misfire

Back again with my profile picture truck. It recently developed a misfire at random intervals that only happens at idle or just barely above idle. Parts I know have been replaced that other forums I found suggested are the TPS(which solved an older issue with surging) and an EGR valve. I've seen it discussed that a burned up temp sensor could cause it as well as an old/dirty IAC valve. Asking around before I try and throw any funds at it since I just got the truck back from having all 4 ball joints replaced and don't have much money to attempt trial and error.

When was the last time you did plugs?

Sent from my SAMSUNG-SM-G891A using Tapatalk

EGR valve would have almost nothing to do with random misfires at idle. To test-- disconnect and plug the vacuum line going into it, problem goes away, EGR replacement, problem doesnt, break out the book and keep researching.
Theres about a thousand and one different things that could cause this.
Have you-
1) Done a complete tune up? (Cap/Rotors/Plugs/Wires/PCV?)
2) Cleaned and tested your IAC valve?
3) Tested the new TPS sensor you replaced? (Had a brand new one go bad within a week of purchase... Autobone garbage)

Misfires are hard to diagnose without supporting information. Year make model engine size etc.

I was having a bad misfire when the truck got hot, turned out the OEM ignition module had no thermal paste underneath it and was overheating. Might be your problem, might not. Get a manual, and a multimeter and get to testing the sensors on the top end. And yes, a bad coolant temp sensor will cause some wierd issues, they cost about 12$ and are right next to the thermostat housing.

Breakdown said:

Misfires are hard to diagnose without supporting information. Year make model engine size etc.

Click to expand...

Look at the title of the thread......pretty simple really...lol.... says it's a 91 5.7 tbi.... model (1500, 2500, 3500, k or c) don't matter to much and neither does make (gmc or chevy) we all know the motor, and that's what's misfiring...


Sent from my SAMSUNG-SM-G891A using Tapatalk

The engine got a rebuild back in Feb. of 2015 with new plugs thrown in.

Sorry jeez I was on duty last night and had been up for twenty straight hours so I overlooked the obvious.

Maldred are you getting any other odd indications or just the random misfire? I.e. Does it do it when it's hot or cold? Mine would misfire when it was hot only that's why I'm asking. When it's misfiring is it popping out of the exhaust or backfiring through the intake or just having a shudder/rpm loss?

Plugs in 2015? Pull them out one at a time and visually inspect them, if good, then re-install. Cost = zero.

Spray the plug wires with water when the engine is running. Cost = zero.

Pull distributor cap, visually inspect terminals inside and rotor. Cost = zero.

Hot and cold and a shudder that shakes the truck and drops the voltmeter bit by bit as it happens. Sometimes it's a one off thing for about 10 minutes or sometimes it will happen about 2 or 3 times in a few minutes. Would material of the plugs matter at all?

Maldred said:

Back again with my profile picture truck. It recently developed a misfire at random intervals that only happens at idle or just barely above idle. Parts I know have been replaced that other forums I found suggested are the TPS(which solved an older issue with surging) and an EGR valve. I've seen it discussed that a burned up temp sensor could cause it as well as an old/dirty IAC valve. Asking around before I try and throw any funds at it since I just got the truck back from having all 4 ball joints replaced and don't have much money to attempt trial and error.

Click to expand...

My truck did this. Your plug wires may be bad. Check all your plug wires where they connect to the plug and look and see if any are touching the exhaust manifold heat sheild. If so thats most likely your problem. Simply spin the boot just a little bit so it isnt on the heat shield. My truck did the same thing. Under load it would misfire and run like ****.