Misfire when warm.

[George B]

I am digging back into my memory here.
You said it only does it warm. That should mean it does it when in closed loop and reading the O2 to adjust the AF ratio. The MAP sensor plays a role then too. You can check the map sensor for proper range with a hand vacuum pump and multimeter but I don’t recall what the readings should be. You would need to use some software to watch the O2 readings.
Just spit balling here with ideas.

 

[exp500]

Recheck your distributor. And thinking after typing a little, you have backfires. Have you adjusted valves? What else did you find when you replaced timing gears? You were not the first in there correct? Problem sounds like a timing/ignition issue now, especially the tach bouncing at idle (on a vortec that is a crank sensor issue indicator).TBI motors were very picky about cams from my experience, Many of the replacement cams ran well but not perfect. Have you ever put a vacuum gage on it?
If it were mine, I would replace Fuel pressure regulator kit to rule it out, recheck distributor, firing order, Timing. Push on egr valve diaphragm, does it move? Down is closed up open. Hard to diagnose your problem over the computer.

 

[grouch]

When I replaced the timing chain, I was the first in there. The engine has 121K on it but I suspect this truck saw a lot of short runs when it was previously in service. I ordered a rebuild kit for the TBI. I put a little Sea Foam in with the fuel after I replaced the fuel pump and it seems to be running better but I'm thinking this goat pee we call gasoline just ate the 28 year old gaskets inside up. The sputters and back fires are like a carburetor starved for fuel. I'll clean and rebuild the TBI and see what happens then.

The engine is pulling good vacuum. About 15 inches. I'd prefer an extra inch or so but with t running like it is, I think that's as good as it'll be for a bit.

 

[OR VietVet]

That vacuum should be closer to 18 inches or higher unless you have compression issues. I forget, did you do a compression test? That engine has low enough mileage that the vacuum should be better, again unless there are compression issues.

 

[grouch]

That vacuum should be closer to 18 inches or higher unless you have compression issues. I forget, did you do a compression test? That engine has low enough mileage that the vacuum should be better, again unless there are compression issues.

I generally like to see 17+ inches but with it running like it is, I'm pretty sure that's why it's a little low. I'm trying to think back a few decades and treat this like a pre-fuel injected engine and it would be a carb. rebuild running like this. I ordered a TBI rebuild kit today and it should be in tomorrow. There's not much else I haven't checked.

 

[OR VietVet]

Did you do a compression test on all 8 cylinders with all plugs removed during the entire test?

 

[exp500]

Which way did you adjust Timing chain? 2 teeth retarded I'll bet. And Again, explain why. Were you using an adjustable kit?

 

[grouch]

Which way did you adjust Timing chain? 2 teeth retarded I'll bet. And Again, explain why. Were you using an adjustable kit?

The timing set had different notches for timing adjustment but I put it to stock. The original had jumped two teeth and when I first fired it up, the engine really woke up. I didn't do a compression test on the engine. What ever the problem is, it's gotten worse fast. I ordered a rebuild kit for the TBI unit and will rebuild it when I feel up to it. I've been symptom free from Covid for 48 hours but I run out of energy fast at times.

 

[grouch]

Well, the saga continues. I pulled the TBI unit and rebuilt it yesterday. It didn't look too bad but with it's age, it was probably due. I have to admit, I held off as I remember rebuilding carburetors. This is a LOT easier. I did have one issue though. My eyes aren't what they used to be and I missed a little bitty piece of old gasket on one injector. I sprayed a stream of gasoline over the grill of the truck. I must have dumped 1/4 to 1/2 a gallon of gasoline in the engine. I tore the unit down, found the debris and cleaned it and reassembled it. I had to let it sit overnight as the engine hydro-locked with gasoline. I took it to a friends shop for an oil change and warned the tech NOT to smoke as it had gasoline in the oil. At least I've probably cleaned sludge out of the engine.

It's running and it seems to not be falling on its face under throttle but I've got some fouled plugs so they are next to replace. I really like Champion plugs and I've used them for probably nearly 50 years, but they don't like gas fouling.

 

[exp500]

Glad you are progressing. My experience with this vintage was they liked NGK or delco, NGK lasted longer, nearly double. Keep posting your progress and will help you fine tune it. Get your MPG up with good throttle response.