Vacuum on fuel filter
 

After driving the truck 40 miles. The fuel pressure starts out at 14 psi then slowly drops as the truck warms up. Eventually getting to 23 in hg of vacuum at the filter. This has a new injection pump (second one), new carter lift pump. New pick up unit in the tank. Rewired the lift pump. Any ideas?

Roll over valve or fuel cap are my guesses.

http://img204.imageshack.us/img204/6...lovervalve.jpg

we suspected the tank not venting. So we removed the fill cap. The Roll over valve came new with the sender. Is there anything I'm missing? thanks for the post

Has the fill cap for the tank been replaced? Could be a cheap fix.

you may not want to hear it, but that carter pump could go out tomorrow. your much better off with a fass, airdog, or raptor fuel system. it's cheaper than another injection pump.

we ran a vacuum guage on the inlet of the lift pump and the vacuum stayed the same at 2 inches. At the same time we had a pressure gauge on the filter assembly and the pressure would start at 10 to 14 then slowly drop off to 0 then pull a vacuum while cruising. Gonna try another transfer pump.

Ya, defiantly the lift pump. Especially when the suction side stays the same.

There was a fass on the truck and we were having the same problems and all we had available was a carter. The fass is a em990 can these be rebuilt?

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put the new carter lift pump on and pressure etc was good and thot the problem was fixed. Drove 120 miles and pickup started surging, bucking etc. brought it back, hooked gauges up etc and had no pressure at the filter again. Had scanner on it while driving and it said the lift pump was running etc. No codes as far as timing on injection pump etc. We have put a new sender in tank, new fuel lines from lift pump to tank. On our second injection pump and second lift pump and still having these problems. Any info would be greatly appreciated.

leave fuel cap loose, if problem goes away it is vent problem. i have had same problem twice on 2 difrent trucks.

We ran it with the fuel cap off.

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We have talked to the diesel guys we know and have tried pretty much every way we know to diagnose this problem. Were stumped.