Truck sputters and backfires when hot

Schurkey said:

Stop throwing parts at it. See post #7.

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All of the parts I listed were part of the engine overhaul, I wasn't throwing those parts at this issue, most of the parts were 30 year old original, and needed to be replaced anyway.

89_C3500_Larry said:

All of the parts I listed were part of the engine overhaul, I wasn't throwing those parts at this issue, most of the parts were 30 year old original, and needed to be replaced anyway.

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I have had it hooked up to a scanner, directly to the ecm, and the only out of spec reading was the throttle position sensor, and we adjusted that. It fixed the high idle. Also the O2 sensor reading rich/lean. Also checked the fuel pressure, and maintained 10psi, I thought this was low, but was assured that it was acceptable. I also failed to mention that the block was bored .030 over.

89_C3500_Larry said:

All of the parts I listed were part of the engine overhaul, I wasn't throwing those parts at this issue, most of the parts were 30 year old original, and needed to be replaced anyway.

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I disagree. Short of ICM's, I've found the OEM sensors to be more reliable than aftermarket crap. Matter of fact, I pick them when I go to yards. I have at least two spares of everything.

89_C3500_Larry said:

I have had it hooked up to a scanner, directly to the ecm, and the only out of spec reading was the throttle position sensor, and we adjusted that. It fixed the high idle. Also the O2 sensor reading rich/lean. Also checked the fuel pressure, and maintained 10psi, I thought this was low, but was assured that it was acceptable. I also failed to mention that the block was bored .030 over.

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10 is too low from what I've read around here. I'd like to see what it was under load.

X2 - 10PSI is marginally too low, you want 11 - 13 on a stock engine. I'd almost be willing to bet you lose pressure under power and you're getting "lean pops". The fuel filter can be partially plugged too.

A brand new, or almost brand new, after-market distributor can have a failed ICM already. My Spectra Premium distributor's ICM failed in under 3000 miles as did my Pertronix Flame Thrower ignition coil. It would run but had a bad stumble at idle. Use AC Delco or Delphi parts.

89_C3500_Larry said:

Have you figured out what the cause of your problem? I am currently experiencing a similar issue with my 1990 7.4 tbi. seems to only misfire, bog, choke, sputter, and lack power after running for more than 10 minutes, and seems to be worse when its hot outside, and also only seems to do it in 2nd or 3rd gear. Runs fine in park. The block and heads were completely rebuilt recently, but most all of the other parts were put back on the engine, with the exception of: new distributor, coil, plugs, wires, new coolant temp sensor, new water pump and radiator. All of this was done at a local trusted shop. My next attemp at fixing this issue will be replacing the EGR valve and solenoid and then a new map sensor. It has no cats, and occasionally throws a lean or rich code. Any advice or ideas welcome.

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After much research on this forum as well as other places, I traced the issue to an over heating Ignition control module. I removed the old one, thouroughly cleaned it, and reinstalled it with a new coating of thermal paste (aka heat sink grease). It immediately cured the problem. I purchased the thermal paste from the computer repair department at BestBuy for about $12.

Thanks for posting back. This is most certainly help others in the future.

I'm a day late and a dollar short. I'd had the thermal paste problem on my old RX-7, and I was scrolling down to recommend checking that. Glad you found it.

Great to hear it was

DerekTheGreat said:

I disagree. Short of ICM's, I've found the OEM sensors to be more reliable than aftermarket crap. Matter of fact, I pick them when I go to yards. I have at least two spares of everything.



10 is too low from what I've read around here. I'd like to see what it was under load.

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The only sensor I have replaced was the temp sensor and it needed to be replaced because its wire terminal was broken.

I did replace the fuel filter, forgot to mention that.

Thanks for the info. I'm at work now, but this weekend I'll look into that icm.

I checked the icm, it had a small amount of "thermal paste" on it, and it was a "transpo" brand (cheapest Chinese crap on Earth) I found them for less than 10 bucks on alibaba, anyway I decided to put in an AC Delco with a generous amount of thermal compound from best buy, and I can say it made a vast improvement, but did not completely solve the issue. Now it seems to do it randomly instead of only when hot. It will spit and stumble only when accelerating hard and not every time. I'm going to go ahead and replace the throttle position sensor since it was reading high on the voltage and causing high idle, so my fix was to dremel the hole out on the plastic mounting tabs on the tps to more of a slot, and adjusted it down to the right voltage at idle. I'm thinking it may be the cause of it. But I'm no mechanic, and I hate taking it to the shop. I'll be sure to update if that completely solves the issue.