1988 F250 7.3 IDI misfire starting at 2,500 rpm

jdbaugh1

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I've been searching the internet for people that experienced similar symptoms to what I am seeing and have found a couple that appear to never have been solved. Essentially the truck starts fine and it runs really will up until around 2,500 rpms. The truck only has 130K miles on it and the oil has been changed ahead of schedule most of the time. If I rev the engine up in neutral it will start misfiring about right at 2,500 rpm on what seems like a single cylinder. It will continue to rev up with more throttle but the misfire is persistent from 2,500 on up to redline. The smoke at misfire is whitish blue.

This truck has been parked for extended periods of time. Before I acquired it they probably used it a could to maybe 4 times a year. If anyone has any guesses your input would be greatly appreciated.
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hesutton

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How many miles on the injectors and injection pump?
Used oil analysis?
Miss only when rev'ing in neutral?
Oil consumption between changes?
What trans?
Video of this happening?

Heath
 

DaveBen

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He just acquired the truck, so no history.
 

jdbaugh1

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How many miles on the injectors and injection pump?
Used oil analysis?
Miss only when rev'ing in neutral?
Oil consumption between changes?
What trans?
Video of this happening?

Heath

No oil analysis yet.

Miss is in neutral and I'm not sure if the mis is there under load. It seems to die off up top more than I'd expect it but I don't really have a baseline. This is the only IDI I've driven. I assume it could be missing under load as well.

Haven't had it long enough to monitor oil consumption. What is excessive?

5-speed transmission. The ZF-S4 I think is designation from memory.

I can get a video of it happening. Basically it can hold revs very smooth until 2,500 and at that point it starts smoking and shaking a bit. Like a single cylinder constant misfire. It cleans up some with higher rpms but it's still there to an extent. Right at 2,500 is where it is the worse.
 

jdbaugh1

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Forgot to say injectors and injector pump are original so 130k on those
 

hesutton

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Definitely time for an IP and injectors. They are way past their service life. Could be fuel related, but a video would help. Drive it and see how it acts in that RPM range.

Heath
 

aggiediesel01

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Lot's of little easy things to try before throwing in the towel on pump, especially if it's sat a lot. First try a new fuel filter filled with ATF, start it and let it idle until the sound noticeably changes and shut it off and leave it over night. Then drive it as normal or a little heavier than normal for the next day or two and see if it resolves. The chemicals in the ATF can help clean and lubricate sticking pumps and injectors. If that doesn't work then put a fuel pressure gauge on the filter header at the shrader valve and monitor fuel pressure as you slowly rev the engine up to 3k. I believe the pressure should be between 3 and 6 psi throughout the range. If it's low you likely have a weak fuel pump and that could be contributing to the problem. If the miss is very consistent you can hold the engine at that RPM and loosen each injector line to see if one of them doesn't change how the engine is running. This can pinpoint a specific injector that is bad. It's not uncommon for an injector to stick open at elevated RPMs. If you can isolate one that is giving trouble, then it can be rebuilt or replaced. With that many miles and not much history it would be advisable to pull the injectors and have them pressure tested/shimmed to bring them back up to spec. With a fuel system that's been cared for it's not uncommon for these pumps to last over 300k. So, if the previous owner changed the fuel filter like you say they did the oil then the fuel system might be fine if only a little lacquered up from sitting so much.
 

jdbaugh1

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Lot's of little easy things to try before throwing in the towel on pump, especially if it's sat a lot. First try a new fuel filter filled with ATF, start it and let it idle until the sound noticeably changes and shut it off and leave it over night. Then drive it as normal or a little heavier than normal for the next day or two and see if it resolves. The chemicals in the ATF can help clean and lubricate sticking pumps and injectors. If that doesn't work then put a fuel pressure gauge on the filter header at the shrader valve and monitor fuel pressure as you slowly rev the engine up to 3k. I believe the pressure should be between 3 and 6 psi throughout the range. If it's low you likely have a weak fuel pump and that could be contributing to the problem. If the miss is very consistent you can hold the engine at that RPM and loosen each injector line to see if one of them doesn't change how the engine is running. This can pinpoint a specific injector that is bad. It's not uncommon for an injector to stick open at elevated RPMs. If you can isolate one that is giving trouble, then it can be rebuilt or replaced. With that many miles and not much history it would be advisable to pull the injectors and have them pressure tested/shimmed to bring them back up to spec. With a fuel system that's been cared for it's not uncommon for these pumps to last over 300k. So, if the previous owner changed the fuel filter like you say they did the oil then the fuel system might be fine if only a little lacquered up from sitting so much.


I had someone on facebook recommend adding ATF to the fuel tank but never answered back about appropriate concentration. Would it be effective to dump some in the tank and go for extended drive or just douse it via filter fill and let it hot soak work better?
 

aggiediesel01

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I had someone on facebook recommend adding ATF to the fuel tank but never answered back about appropriate concentration. Would it be effective to dump some in the tank and go for extended drive or just douse it via filter fill and let it hot soak work better?
In the tank is more of a periodic maintainence thing. If you have an issue, high concentration directly applied to the injection system by putting it in the filter and running it into the pump and injectors and letting it sit is the most effective and fastest way to make a correction if it's going to work.
 

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