7.3 Intermittent misfire

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HAPPY

Mike B.
Joined
Feb 21, 2010
Location
Granite Falls
I have another diesel issue, this time on Dads truck.

It's a 96 250 5 speed all stock with 155K. It has a intermittent misfire. It will start and idle fine. When you take off down the road, SOMETIMES it will miss for a mile or two and clear up, SOMETIMES it won't mis at all and SOMETIMES it will miss so bad it will just shut off.

Injectors will usually stop working all together right?

I tend to think it's the under valve cover harness. From fumbleing around on a few diesel forums that is my best guess/

Dad seems to think it's fuel related. I know there are pick up pumps in both tanks but isn't there a high pressure fuel pump also? I thought about the high pressure oil pump also.

If there is a high pressure fuel pump, is it electric or is it driven with the high pressure oil pump?

Any ideas? I'm completely stumped, how do you find the problem when it isn't consistant. I'm just looking for a good starting point. Dads on a fixed income and doesn't have a lot of money to throw into it.
 
The high pressure pump is your injection pump (mechanically driven), and yes there is a "lift" pump on diesel to get the fuel to the injection pump. As far as the problem I have no idea. It does kinda seem like a wiring issue though.
 
Definitely the under valve cover harness. Had it happen to my 99. Be careful driving it like that. If you short the injector driver module, they are big dollars to replace.

The part to fix the under valve cover harness is cheap.

I'm saying this assuming the fuel filter has been serviced properly.
 
One other note. Not sure on the 96, but the crank position sensor will cause the same issue.

Also, the HPOP is happy with good oil change intervals.
 
Has it ever had a new CPS? It's most likely not fuel related and there are no in tank pumps just one mechanical pump in the valley of the engine under the fuel bowl. High pressure fuel is provided from the injector by a High pressure oil system. Any Oil leaks? Though I think an IPR sensor is mostly low end problems but not %100

Id start By checking Valve cover Harness Connections. first at the valve cover then underneath from the inside connector too the injector its self you can even fire it up with the covers off and you should see little splurts of oil coming out of every injector. If one is not shooting out oil its bad. best way to actually test an injector is a cylinder Buzz test but need a scanner to do that. A 7.3l injector has a fuel side and a High pressure oil side with a solenoid on top. You can also test the continuity and resistance of the UVCH with a DVOM once you take it out to test for any internal breaks.

But If its never had a CPS with 155k on it I try that first they sell them now for only 30 bucks. Its located on the front of the engine and held in with 1 10mm bolt. As already said this assuming the truck as been well kept.

Take it somewhere and have the codes checked it probably stored one when it did it.
 
Dad is a fanatic when it comes to service, oil every 5k fuel filter every other oil change. It has never had the CPS changed that I know of. He bought it at around 80K, he's the 2nd owner. I think we will start with the CPS and then move on to wireing harness.

I will for sure post up the fix if we ever get there.
 
CPS doesn't always throw a code but if you have had continued problems there might be on there. Know anyone with a 7.3l power programmer that can check codes or advance or somewhere should be able to.
 
I have a programmers but its for my '00. I will probably run it by advance

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Put a CPS on it and fixed it. Thanks for the help guys!
 
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