Low fuel pressure to high & pulls vacuum

Fordracer · October 20, 2020

I have a 2008 6.4 with 320,000 miles that the customer has to bleed the air out of the upper fuel filter housing to get it to run at times and now he can't get it to stay running after he bleeds it. It has a P2291 hard fault. After I got it I monitored the FRP and you have to crank it a long time before it starts to build pressure and then it starts and dies. I bled the high pressure fuel system and a lot of air came out and now it runs fine. Now for the weird part. I was monitoring the low pressure fuel system and koeo it has 3.5 psi but when you start it it jumps to 15 psi and if you rev it up it goes to 20 psi. Now for the second weird part, 15 to 20 seconds after you shut it off it pulls a 2 inch vacuum and holds it for hours. The longer it sits the longer you have to crank it to start it. I'm going to let it set over night and see what it does.

I have never seen this before. If the high pressure system is somehow bleeding pressure to the low system you'd think it would be more than 15 psi. If the low pressure regulator is causing it why is the pressure normal at koeo? Has anyone run into this.

lmorris · October 21, 2020

Run it with the bleed tool on with the hose returning back to the tank and monitor fuel pressure. My thought is it may cause some change in your readings if the transfer pump in the high pressure pump is in fact bleeding pressure back into the low side. Just a thought though because I have never seen this, but it does make sense that that is what's happening.