Engine/Transmission (1994 - 1998) Lift Pump Supply @ 20 psi = Low Power ?

[Greenleaf]

With the school bus setting in the shop, I see 20 psi at idle speed. At 1/4 throttle it moves up to 30psi then at 1/2 throttle to WOT it's back to 20 psi.



I replaced the overflow valve, fuel filters and lift pump. Still the same deal. Is this about all I can expect? I read that it's to be 35psi at WOT



I'm troubleshooting a low power issue on a 96 Cummins "B" Thomas Bus.



I pinched off the return line with both the old and new lift pump and the pressure went :--) I have a 30psi and a 60 psi liquid fill gauge w/snubber.



Perhaps I should shim the new overflow valve to 35 psi ?



Scott

 

[Joe G.]

That's plenty of fuel pressure. If you have low power it is probably something else. When my overflow valve failed I didn't have any problems at 6 PSI except that it would not start when I got it home. Have you checked to see if the timing slipped? What does the boost look like? You may have a leaking intercooler boot. Have you checked the linkage? Especially the two little screws that fasten the linkage lever to the injection pump lever. If the linkage looks ok, is it contacting the throttle stop at WOT?

 

[Greenleaf]

I have checked everything. Also advanced the timing a little. I cannot drive it with a bus load of kids so I have to take the word of the driver that it's low on power.



I have to remove the throttle linkage(s) to check pump time so it's all good and goes to WOT as it should.



If it did have a leaking boost hose the driver would have reported a loud shrill/whistle. Setting directly next to these engines, any boost leak is very easily noticed,even by students inside the bus.



Thanks for the help.

 

[Greenleaf]

After a couple of minor adjustments to said fuel system, the boost gauge sweeps past 35 and the black smoke emitting from the tail pipe will never clear up. I figgure the engine is recieving fuel now and any complaints concerning low power are mearly BS.



I also removed the little throttle limiting screw on the governor housing and now it goes to WOT