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Mp8 loosin prime


Kyle 537
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I'm workin on an 08mp8. I have tried runin a line from the tank to the inlet of the filter housing and ran still looses prime, check valve on front of the head is good pump is good filter housing new. Bypassed line from housing to pump inlet, pump to secondary filter, and housing to head still no fix hooked it all back up the way it goes. Had a code for #2 injector fmi 7 replaced and inspected cup, it looked good no sign of leaks or wash out, but it dose still have the copper cups. I'm guessing maybe the one bad. So I'm hoping the is a way to find out without tearing down. If I plug the fuel galley in the head and pressure it up with a regulator would I be able to call it cups or injector o-rings? If so what psi should I use? Any other ways or ideas much appreciated!!

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I run a clear fuel line from fuel return overflow valve at head to the return at fuel filter housing . Look for excessive air in fuel; return this would indicate a cup is leaking combustion into fuel rail/return. Also look for debris at ecu cooling plate inlet and outlet fittings. Inspect overflow checkvalve for damage also , Good luck.

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You can also run a clear line from the fuel filter housing up to the rear of the head. If you have air going through this line it would mean the air is before the head.

You can plug off the fuel galley in the head and apply regulated air pressure and see if it holds. We use to do that when the cup problem first came out. Fuel pressure is somewhere between 50-80 psi, so apply 65-70 psi. If the pressure drops you probably have a cup issue again. The clear line method is the "go to method" now.

The fuel pressure FMI 7 code is probably because you have air in the system.

Make sure someone didnt mix the lines up on the fuel pump. Look at another truck and verify your lines are routed correctly.

If you dont find anything, start over at the beginning. Pretend it a new truck and start again. Clear lines first.

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Thanks for all the help, you guys are awesome. Ran the lines def comin from the head. Tore down #1 leakin coolant and 4 seems to be the problem hole. Company is to cheap to upgrade to stainless and conical. $2700 vs. $650 o well I guess. So ig I'm gettin one injector and 6 new copper cups. And difference in r&r or the cooper than the steel. I've never done the copper. The only real difference iv seem Is the conical are tapered were the copper set had the tip washers. One more question if ya don't mind. Bein as it was pushin compression in the fuel system, with the engine at operating temp could I have done a compression test on PTT and found the problem hole?

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