Engine/Transmission (1994 - 1998) Overflow valve or lift pump?

[FordCummins]

Ever since I did my Cummins swap, the truck has had 40-42psi of fuel pressure at idle, and going down the road. Hammer it hard, and it would drop down to 30-32, then jump right back up. Lately, its been a bit under 40 at idle, and will drop to almost 0 on a WOT run, and down to 20 on a mid throttle run. Less smoke, and doesnt feel quite as quick anymore either.



I pinched the return line off, and at idle it the PSI jumped to 65 and then blew the rubber return line right off the fitting on the pump. I was gonna order a new OF valve from Piers and go from there. Could the lift pump be the problem?



The filter was just changed today, with no difference in fuel pressures or performance. Thanks for the help.

 

[bmoeller]

Since the PSI jumped up when you pinched the return, I think it would most likely be the OF valve like you were thinking.



The 42# is MORE than enough psi. Don't recall what you said the engine came out of, but at least in the Ram application, the psi should be 18-24 psi at idle, 28-36 at 2000 rpm with no load.

 

[Missouri Mule]

Seems to be your Over flow valve with that much pressure your lift pump has to be working.

 

[FordCummins]

Thanks guys, Ill try the OF valve first and see what happens. I will post my results.



On edit, the motor came out of a 96 Ram. I know that my fuel pressure is higher than what is spec'd, but thats what it has been since the day I fired the motor up after the conversion. I just left it alone because it worked fine.

 

[JDutton]

It's pretty normal for pressures to drop at higher flow rates. Pumps are rated for Head (pressure) vs flow, such that with higher pressure there is lower flow, and higher flow at lower pressure. The 0 psi would concern me though, especially if you're noticing less power. Could be the lift pump getting old and cruddy.



Good luck with it

 

[JGrover]

Before I bought my fuel gauge. Thought maybe overflow valve issue. Upper end power seemed a little low. Stretched the spring a tad over 1/2" No change in power.

Just bought fuel gauge. Idle psi was @30psi. Compressed spring to 1/2"+. 020 Now im at 22psi@idle and 30+ at 2000rpm

That overflow spring is very sensitive to various lenghts

 

[The patriot]

It's pretty normal for pressures to drop at higher flow rates. Pumps are rated for Head (pressure) vs flow, such that with higher pressure there is lower flow, and higher flow at lower pressure. The 0 psi would concern me though, especially if you're noticing less power. Could be the lift pump getting old and cruddy.



Good luck with it

This is very un scientific, and it's at least how mine reacted.

But when my LP was going bad, whenever I drained the filter while running, it would stall out.

Now with the new LP it keeps running even with the drain wide opened.

JM. 02 YMMV.

Eric