ISX 475 Air in Return Fuel Line

Engine:

2005 Cummins CM870 ISX (EGR)

Symptoms:

Runs rough
Unstable Engine Speed (loping)
Poor Fuel Economy (4.5 mpg)
Poor Performance
Knocking while under load at high RPM

Diagnosis:

After Cummins replaced the Exhaust Gas Pressure sensor, one timing actuator, and increased the idle speed, they sent the truck back to me and it wasn't fixed at all. Not one iota. So back for a second time this week, today they hooked up a sight glass to the fuel inlet and return lines on the IFSM. Going in there were a few bubbles, but not much. Going out there was A LOT OF AIR!!. Probably about 10 times as much air going out than going in. Cummins said 3 of the 6 injectors are failing and leaking air from the combustion chamber. They are recommending replacing all 6 at a cost of $5185.

My question is this: Are there other places for air to be introduced into the return? I thought a little air was normal in the return line, and that the ISX fuel system provided a mechanism to bleed off air before the fuel is sent to the IFSM and injector. The fact that the return line has 10x more air out than coming in on the inlet suggests it's sucking air from somewhere in the head. Where besides the injector & cup could this air be coming from? Thanks! I go back on Friday to have the injectors done, but It'll be a dark day for me if they do the job and it doesn't fix the problem. Rocky Mountain Cummins in Denver is doing it.

 

What really sucks is that I just had it rebuilt 2 weeks ago and they said the old injectors were fine and no problems. Also put a new cam and one rocker in at the same time, as well as some other stuff. Would've been a lot cheaper to have them put new injectors in at that time.

What about the drain-back valve in the IFSM? That could be a source of air in the return, too, yeah? I just don't want to condemn the injectors prematurely and and miss something else, especially after $5k. I think the tech did isolate each injector and determined 3 of them were leaking air past the o-rings, but I'm skeptical and am worried they are throwing parts at it again to hopefully solve the issue.

 

It ran rough before the rebuild, yes, but wasn't knocking and got decent fuel economy. Cummins implied that the fleet pride freightliner shop that did the rebuild may have cut the o-rings accidentally during re-installation. We'll know more on Friday when Cummins digs into it and pulls the injectors.

 

So did you replace all 6 injectors and it solved the problem?

 

Good point about the lift pump leaking there. Thanks.

 

What about that little brass screw-in bypass valve in the IFSM body on the back? I remember watching a cummins video on the ISX fuel system and they mentioned to check that if air in the return was present.