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100 over injector rough idle 10mm pump

Practical Diesel Tuning

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Recently I have installed 100 over S&S injectors and a 10mm S&S CP3 into my 2010 LMM Duramax. I have a very rough idle and fuel knock only at idle. Also when driving the truck if I hold at 3/4 throttle for more than 10 seconds I throw a P0087 code. I have a 3rd party 5 stage tune in the truck now and current tuner (starlight diesel) can’t seem to figure out what is going on hence why I am taking action into my own hands. This is the second bran new CP3 and second set of injectors so I’m sure the fuel system is sound. Is there any advise as somewhere to start so my truck does not throw low fuel rail pressure codes? This is a fully built motor I have built from the crank up. I have a snap on Zeus scan tool and when I try to command full fuel pressure at idle my actual and desired fuel pressure will not go above 75 mpa. Could this be do to the tune not allowing any more fuel pressure at idle?

Sorry forgot to add I have a FASS 165 lift pump oem fuel filter delete. And fleece 68 mm vnt turbo, high flow intake, mishimito intercooler and boost tubes, 5 inch turbo back exhaust. PPE up pipes no egr no dpf race use only! Also at 1/4 throttle EGT are running 800degrees.


I would suspect a mechanical issue there. If the truck will not achieve more than 75mpa I would suspect a loose loose block-off valve in the rail. You can bet the fuel is going somewhere instead of the engine (building pressure). Finding which part is returning it is your best bet. It's likely either the FPRV, injectors, or pump. If you take your return lines off the previously mentioned parts one at a time and run them to a bucket you should be able to find the culprit.

Obviously, I'm assuming you have good pressure on the low side of the pumps 5-8psi is plenty ( measured at the CP3 Inlet).

Hope this helps,


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