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Got a 94 F-250 PSD in the shop. Got the call from the customer about a week ago that they needed me to come read the truck at their shop and see why it wouldn't crank. So got there and after a buzz test found hi-lo side shorts on cyl 1-7. Told the customer to check the UVC harnesses since they wanted to try to fix themselves. They found one harness melted on the driver side and 2 glow plug wires melted one on each of the passenger valve cover connectors on the pass. side. They replaced the harness and spliced in 2 new pins for the pass side glow plugs. It fired up but was smoking white/blue really bad and smelled like raw fuel. Got the oil heated up to run the cylinder contribution test and it showed #5 and #7 cylinder contribution fault. Pulled the valve covers and ran a compression test and all cylinders were within 20 psi of each other (around 420 psi). Replaced the 2 injectors and the wishbone line that goes from the rear of the fuel pump to the rear of each head. Cranked it up... still smoking like crazy and still showing #5 and #7 on contribution test. ICP press was below spec so replaced IPR valve per Ford manual. When you unplug the harness for those 2 cylinders you can tell they are firing just not fully. I'm thinking PCM or harness. Any ideas?? Oh yeah and installed test IDM still does same thing. Fuel pressure 70 psi.
 

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Also got some blowby coming from rear pushrod tube in rear of pass side head. Wouldn't a pushrod, valve, or lifter show up in the compression test.
 

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Did they also replace the valve cover gaskets, and have you done a buzz test after their repairs? I would test the harness from injector connector, to the idm connector, should be no more than .8 ohms for entire run including connections, test out their repair job.

If those injectors are clacking louder than number one or three, I would suspect a lack of fuel to this bank which would be really weird considering they get da fuel first, I dunno.
 

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Yeah man I ohmed out the valve cover gasket itself. Both UVC harnesses on that side. Then ohmed out from the valve cover connector to the IDM and everything ohmed out good. They don't seem to be knocking (clacking) any louder than others and when you unplug the 5-7 connector you can hear the engine miss a little bit but when you unplug the 1-3 connector it skips like hell. No codes on the buzz test that's what is weird. No codes anywhere.
 

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Yeah man I ohmed out the valve cover gasket itself. Both UVC harnesses on that side. Then ohmed out from the valve cover connector to the IDM and everything ohmed out good. They don't seem to be knocking (clacking) any louder than others and when you unplug the 5-7 connector you can hear the engine miss a little bit but when you unplug the 1-3 connector it skips like hell. No codes on the buzz test that's what is weird. No codes anywhere.

I have seen a burned or tuliped exhaust valve not show up on a static compression test believe it or not, yet result in a misfire while running. might do the ol' paper on the exhaust pipe while running test, I guess :dunno:
 

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Yeah I hear ya. I've been wrackin my brain over this truck all day hopefully a light bulb will go off in my head tonight. Thanks man.
 

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Backtracking a little, number five did not enter the misfire equasion until after the repair, correct? Hmmm, I'm gonna pull up the IDM harness diagram, cylinder five and seven are on the same connector I believe, don't spose someone got a wire crossed?

If you need a diagram for this circuit, I can email it to you.
 

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I pulled the diagram and ohmed out from the injector connector all the way to the pin at the IDM and the wires tested ok for the right pin in the IDM. According to the customer the truck was not skipping at all and was running great then the engine cut off an would not crank back and that's when they found the burnt valve cover connectors. Then I scanned it and found 6 buzz codes and they then found the burnt UVC harness. Then when it finally started it had the miss and smoking.
 

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Any PS wiring geniuses out there?
 

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Hey man just to update yall it was the engine harness. I never found the spot in the wiring even after tearing it apart but I knew it could only be the harness or the PCM and I went for it. Cranked up and no smoke no skip. Thanks
 
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