Hi all. I'm going to share what I learned in my quest to find and fix a vexing mis-fire. What happened on my 2000 F350 7.3l, was that on occasion I would notice a minor miss. Might be there for a day or two and then gone. A few days or weeks later, it would return. After a few months of this, my miss became a real problem. I lost so much power that I could barely get over 25mph. Shut down for about 15 minutes and she fired right up and ran fine.

I had checked for codes and found none which seemed to elliminate the UVC as a cause as I expected to see codes logged for injector failures. I then looked carefully at my fuel system and found the fuel pump relay was faulty. I noted a variable resistance which while not causing the pump to quit did result in a loss of fuel flow - hence my miss-fire.

I have swapped out the relay and after more than 3 months I am sure I have fixed the problem. So, if you're thinking you have UVC harness troubles, you may want to investigate your entire fuel system for not so obvious faults like the one I found on my truck.

Hope this helps others with difficulties they are experiencing to think outside the box.

 

Thanks for sharing the info, might want to share this with the 7.3 forum as well.

 

That an interesting failure mode...

I'd definitely post this in the 7.3L forum too...

 

How did you pin it down to a faulty relay?

 

I noted a low voltage on the fuel pump. Looking at the schematic diagram I worked my way along and found high resistance across the relay contacts.

BTW: Most of my electrical issues have been related tp faulty (wornout) relays.

 

Oh, and rep points for you

 

I am having the same problem at 1.5 to 2.0 rpm i have a minor miss no codes read but i know there is some misfire there Change fuel filter I beleive this might be my problem also cudos to you for the diagnosis. At 1.5 to 2.0 rpms get minor misfiring but doesnt show any codes iam on to partds house. Also loss of power is a symptom. You will not notice this problem unless you really feel your engine performance. Thanks again brother diesel riders

 

Man, Thanks for the heads up!!

I've had a similar problem that I thought I had solved, but alas it is back again. I noticed mine was real bad in the cold weather. This past winter it was not so bad.

Where did you get the relay from? What part number? I've never had to replace a relay on a vehicle.

Thanks again for the heads up. I'll be checking this tomorrow...

PS, I've changed both VC gaskets and UVC harnesses in my quest to fix this. Most of the miss has gone away, but it still reappears from time to time.