Recently I took a chance on a 1996 F-350 diesel that was for sale. 244K. It was at Union Gospel Mission Motors, a program that takes donated vehicles and reintegrates street people by employing them and teaching the basics of detailing, oil changes, etc. The head mechanic is a good guy, but not much on diesels. Was really honest about the truck. Cylinders 5 and 7 failed the contribution test, was kicking and smoking like an acrobatic plane on startup - but the truck looked well taken care of and the mechanic said it was a fleet vehicle that belonged to a local construction company. He felt it was a sound truck with a problem he didn't understand. They didn't want to buy two new injectors (thinking that was the problem), so they sold me the truck for $995. I changed out the glow plugs and relay, compression test was 400 psi in all cylinders. Starting the engine was a trauma, obviously still running on 6. Once warm and on the highway, it ran great, but seemed odd to me that #5 and #7 injectors would fail at the same time, so I figured it was more probable that part of the harness may have failed. I pulled the harness apart, removed all of the shielding, etc. but found nothing burnt, melted or out of the ordinary. I separated all of the wires and tried starting the engine. Normally when cold (about freezing), several 30 second cranking sessions would get it coughing and bucking, but this time it started beautifully. It smoked slightly for about a minute and cleared up. I think this harness is bagged, but has no outward signs of deterioration. Is the shielding (foil and wire wrapped injector wires) normal? Thinking of rebuilding a harness for it and would like to do it right.

 

i don't remember ever seeing any "foil" wrapped wires on my truck, haha. which wires did you seperate and it started right???? the wires going to the UVC harness on the passenger side i'm guessing???

 

Cody did a writeup recently about rebuilding the harness and going through the whole thing and looking for spots and he said some of was foil wrapped and he was replacing the foil with stuff he stole from the wifes kitchen.

 

Hers the link to his writeup. https://www.ford-trucks.com/forums/8...l-writeup.html

 

Yes, the external harness for #5 and #7 running along the top of the engine toward the passenger side is the problem. Assuming this is the UVC. The harness had the normal plastic armor, a rubber membrane, then inside the injector wires were wrapped (separately from the GP wires) with wire foil and wrapped with a steel grounding shield. Quite elaborate, but obviously not working.

Thanks for the link. I searched, but external harness didn't cut it. If the foil is necessary, I think I will use foil tape for furnace ducts.

 

Yes, the foil is important. to prevent cross firing.

 

Quick question, did you pull the valve covers and check the UVC (Under Valve Cover) wiring harness's/ They are known to melt. Also did you check the continuity of the valve cover gasket electrical connections? What shape are the pins and plugs at the valve cover gasket connection (harness side and valve cover side)? I would chcek these before rebuilding the harness, just in case it's something easy.

 

Yes, checked continuity on the pins and nothing melted. Gaskets and internal wires were good. I am pretty sure the external harness is the problem, as it seems to have self corrected when I stripped the harness apart and separated all the wires from each other. My best guess is that voltage was leaking or there are broken wires and moving / separating the wires has improved both starting and running. Doing that wouldn't improve a problem inside the valve cover. I don't think making a harness up will be much of a problem, it's really just replacing wires.