Misfire when warm.

[grouch]

Short days and long work times is making this difficult to track down. I had my '93 C2500 GMC pickup singing a song but it's started misfiring when warm. When you first start it, it fires right up and runs smooth with plenty of power. As it warms up, it begins to run rough and stutters under load. The stutter is getting worse.

When I got it, I replaced the cap, rotor, plugs, plug wires along with the coolant sensor. This isn't like the bad sensor issue. It was off time by a couple of teeth so it has a new timing set. It's definitely a heat soak problem. I'm wondering if the coil is giving me trouble. I replaced the old one due to rodent damage and I may have gotten a defective one. I get home after dark so it's getting it off to bench test it that's the problem. Can anybody think of anything else that could cause this? 350 engine and automatic.

 

[drakon543]

intake manifold gasket ignition coil or the tbi unit needs a rebuild would be a couple things id look into. vacuum leaks can get worse when things heat up. you already seem to be aware of the ignition coil issues. the tbi unit has a couple spots that can start leaking. also the fuel pressure regulator can start causing issues. so a tbi rebuild kit might help.

 

[OR VietVet]

Any codes? Remove the plugs to see if they all look like the same and firing fine with no fouling. You can use a mechanic's stethoscope and remove the probe and listen for vacuum leaks. Sounds like when it goes to closed loop that is when the problems start. How old are the oxygen sensor(s)?

 

[exp500]

Is the reluctor clean and not rusted heavily? ( inside dist)?

 

[OR VietVet]

May be the module that acts up after building heat. Maybe the heat sink film is burned away. If so, get a new module and the squeeze film for heat sink.

 

[grouch]

Is the reluctor clean and not rusted heavily? ( inside dist)?

The one that was in it was pretty bad. I ordered a new distributor but didn't put it in until this all started. I got home a little early today and drove it until it started to miss under load. At idle, it's smooth. I was wondering if a plug wire was failing as the headers are VERY close to the #6 and 8 plugs.Hooking a timing light to each plug wire got a consistent pulse. I'm going to pull the plugs next time I can work on it and see what they look like. They are fairly new but I have gotten faulty plugs before. Especially if the parts guy gets cute and slaps them on the counter.

 

[Fless]

The one that was in it was pretty bad. I ordered a new distributor but didn't put it in until this all started. I got home a little early today and drove it until it started to miss under load. At idle, it's smooth. I was wondering if a plug wire was failing as the headers are VERY close to the #6 and 8 plugs.Hooking a timing light to each plug wire got a consistent pulse. I'm going to pull the plugs next time I can work on it and see what they look like. They are fairly new but I have gotten faulty plugs before. Especially if the parts guy gets cute and slaps them on the counter.

Warm it up at night, pop the hood, and check for spark plug wires arcing in the dark.

 

[OR VietVet]

A mist of water out of a spray bottle will help with the light show.

 

[grouch]

Warm it up at night, pop the hood, and check for spark plug wires arcing in the dark.

I've done that. I've also soaked my hand in salt water and grabbed each one. Not pleasant but it will let you know if the insulation isn't up to *****. No ghost flashes. If it was all the time I'd say it was a bad plug. The first time it did it, it was going down the road at a steady speed.

 

[grouch]

Got off on time today and spent some time trying to track this down. I'm beginning to think it isn't ignition related. I hunted for a pressure port and couple find one. Went on Youtube to see how these are worked on and it turns out I need an adapter to get a reading. I'm suspecting a weak fuel pump as it idles just fine. Going down the road, the higher the gear, the rougher it runs. It has a steady stutter in overdrive. Less in lower gears. I'm suspecting the pump isn't up to the demand. I tried to find an adapter to put in the line but it seems these older trucks aren't supported very well.

I called a friend, partially to see if he's still alive, as he trained me back in the 60's and we've been friends ever since. He retired when these were fairly new back around 1990. He's in his early 90's now but still active. Due to Covid, I called on the phone and not in person. He mentioned that since it works fine cold, there may be a problem with a ground under the hood. It seems the late 80's and early 90's TBI systems are really prone to ground problems on the fuel system. I'll check that if the pump checked out.