Random misfire - Ford Powerstroke Diesel Forum
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post #1 of 8 Old 10-13-2019, 06:28 AM Thread Starter
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Random misfire

Ok so I need some help. All started about 2 months ago when I was driving down the road it started to act up and have no power and ran rough so I pulled over and after turning the truck on and off a few times it fixed itself and ran fine up until about 2 weeks ago. Randomly when I would start it up it would act like it wanted to bog down and cut off and come back up to idle....if I cycled the truck on and off a few times it would be fine and I could go. Everytime it would be the same and no smoke. Then I finally got my dad over to listen to it hoping it would run rough when he was here because like i said it wont do it all the time and I may go a few days without it doing it and sometimes it will be everyday. Well when he was here I started it up and it acted different....it had a bad misfire and sounded like I was missing a cylinder and had white smoke coming out of the exhaust. Turning the truck on and off didnt fix it. So the next day I tore it all apart and took the valve covers off. All the injectors ohmed out fine and I started the truck up and it was running perfect. All injectors pumping oil like they should. Unplugged each injector one at a time and it changed the idle so my injectors I believe are good. I put everything back together and started the truck up and it was back to the bad cylinder again. I changed the valve cover gaskets/harnesses back in the spring. Its sounding like a wiring issue. Could I have got a bad set of harnesses? Or could there be another issue. This truck has me stumped. Any help would be great. Oh it's a 03 f250 7.3l with 252,000 miles.
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post #2 of 8 Old 10-13-2019, 08:55 AM
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No codes?

Did you do the 25 cent mod the the UVCH?

Can you run a buzz test?

03 7.3l 550, FRx, 6637 mod, 4 inch turbo back, ccv delete, aih delete, HPx, Riffraffs OCR, Turbo-pyro-boost-coolant EV2 gauges, Bullydog GT tuner.

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post #3 of 8 Old 10-13-2019, 10:29 AM Thread Starter
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No codes. And I dont believe the 50 cent mod will help nor doing a buzz test because I dont think it's the injectors. Like I said with the covers off the truck runs fine but when I put everything back together it misfires.
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post #4 of 8 Old 10-13-2019, 11:13 AM
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Originally Posted by Mlong7.3 View Post
No codes. And I dont believe the 50 cent mod will help nor doing a buzz test because I dont think it's the injectors. Like I said with the covers off the truck runs fine but when I put everything back together it misfires.
If the truck runs fine without the valve covers on, then you put them on, and it misfires, the problem has something to do with the UVCH in my opinion. If you did not stick a quarter under the clip on the UVCH plug it is very likely the problem.

A buzz test will tell you which side is giving you grief.

If it was something like a bent valve/bad fuel/bad compression/ etc it would NOT come and go.
It is electrical IMO and most likely injector related.

We own 6 different 7.3s. And we have fixed the UVCH on 5 of them.

03 7.3l 550, FRx, 6637 mod, 4 inch turbo back, ccv delete, aih delete, HPx, Riffraffs OCR, Turbo-pyro-boost-coolant EV2 gauges, Bullydog GT tuner.

03 X 6.0 175/30 warren sticks, Livewire TS w/ warren tunes, Blue spring mod, atlas 40 ficm, 270amp dc power alt

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post #5 of 8 Old 10-13-2019, 11:20 AM Thread Starter
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But when I have the valve covers off and the harnesses are plugged in, they still dont have the quarter and it runs fine. Could I have just got a bad uvhc when I replaced them? Could the pressure on the gasket after tightening the bolts down be messing with the wires that run through the gasket?
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post #6 of 8 Old 10-15-2019, 12:51 AM Thread Starter
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Well last night I started the truck and it had the misfire again so I unplugged one valve cover harness at a time and the drivers side killed the engine and the passenger side kept it running so I figured I narrowed it down to the passenger side being the bad cylinder. I pulled the cover off again and removed the gasket harness and found on the bottom side there was one red wire going to one of the injectors that had a bare spot and also 2 of the glow plug wires had a bare spot right at the plug. I'm figuring that this is my problem so I'll be getting a new harness and see if that fixes the problem.
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post #7 of 8 Old 10-15-2019, 04:14 PM
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Quote:
Originally Posted by Mlong7.3 View Post
Well last night I started the truck and it had the misfire again so I unplugged one valve cover harness at a time and the drivers side killed the engine and the passenger side kept it running so I figured I narrowed it down to the passenger side being the bad cylinder. I pulled the cover off again and removed the gasket harness and found on the bottom side there was one red wire going to one of the injectors that had a bare spot and also 2 of the glow plug wires had a bare spot right at the plug. I'm figuring that this is my problem so I'll be getting a new harness and see if that fixes the problem.
Sounds like you found it!

03 7.3l 550, FRx, 6637 mod, 4 inch turbo back, ccv delete, aih delete, HPx, Riffraffs OCR, Turbo-pyro-boost-coolant EV2 gauges, Bullydog GT tuner.

03 X 6.0 175/30 warren sticks, Livewire TS w/ warren tunes, Blue spring mod, atlas 40 ficm, 270amp dc power alt

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post #8 of 8 Old 10-15-2019, 04:45 PM Thread Starter
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Here are pics of the wires
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