Engine/Transmission (1994 - 1998) Max Fuel Pressure

[JoeBioDiesel]

I had a problem with my truck this week. It would missfire at high revs, boost would fluctuate rapidly, and EGTs would bounce around. I suspected a fuel problem and changed the fuel filter on the road, no change.

I put a pressure gauge on the pump when I got home and it was only 15 psi at idle and dropped to near 0 when under boost. I also had a boost leak and a kinked fuel line. Each thing I found wrong and fixed made it a little better, but didn't fix it completely. To address the low fuel pressure problem I removed the overflow valve and stretched the spring to . 550" and reinstalled. Fuel prerssure is now 30-40psi at idle but peaks as high as 70psi when at high revs and no fuel (decel). Should it ever get that high?

I tried a search but didn't come across anything addressing high fuel pressure.



Joe

 

[MLee]

Joe;

I believe that Illflem said one time that 30# was max at 2000 rrrrrrrrrrs. sounds like maybe that the return line is not returning, but you have checked all that, RIGHT. 22 to 24 @ idle.

Marv.

 

[SEmerson]

I run about 40 at any thing off idle. 40 is concidered high.

 

[Andy Redmond]

You stretched the spring to much. This is hard on the lift pump. I would squeeze it (compress the spring) a bit to see about 20-25 at idle.



What happens with the fuel psi gauge attached at idle, when the return line is gently squeezed with pliers? PSI up? then likely an overflow valve, no change then lift pump isn't supplying proper output.



Have you done a volume test to check the volume of fuel the lift pump is delivering? Do you have more then 4 hg of vac. at idle on the fuel supply or return? If so you have restriction.



Is your shutdown solenoid fully retracting upward (full stroke). You might zip tie it up and go for a test run. . be cautious you now have disabled your kill switch?



Sounds like its running out of fuel on the top end? Could the timing have slipped (recent re-adjust)?



Have you tested the innercooler charge air system for leaks after your repairs (other leaks)?



Good luck

Andy

 

[MLee]

Joe;

you streatched the spring 550" thousands, it is just a hair over 1/2 inch, this is what has worked for every on else. not to much. Keep checking

Marv.

 

[JoeBioDiesel]

thanks everone for the feedback.

Andy, here are the answers to your questions:



What happens with the fuel psi gauge attached at idle, when the return line is gently squeezed with pliers? PSI up? then likely an overflow valve, no change then lift pump isn't supplying proper output.

PSI shoots up rapidly.



Have you done a volume test to check the volume of fuel the lift pump is delivering? Do you have more then 4 hg of vac. at idle on the fuel supply or return? If so you have restriction.



I tried the volume test. Only 250ml in 30 sec cranking. The service manual says I should have over twice that amount.



Is your shutdown solenoid fully retracting upward (full stroke). You might zip tie it up and go for a test run. . be cautious you now have disabled your kill switch?



The solenoid lifts all the way.



Sounds like its running out of fuel on the top end? Could the timing have slipped (recent re-adjust)?



No. I've got the timing tools. Rechecked at 17 degrees.



Have you tested the innercooler charge air system for leaks after your repairs (other leaks)?



No, but I probably should. After failing the volume test I thought I should recheck for air getting into the supply line. I pressurized the fuel tank and checked for external leaks. None found. I can easily blow through the supply line. I'm now thinking the lift pump might be bad. Can deliver ressure, but not volume. I'll substitute and electric one for a road test, and lower the tension on the overflow spring.



Thanks again,

Joe

 

[Joe G.]

I see 40 PSI quite a bit. Idle is in the low 20s. High pressure depends a lot on air and fuel temp as far as I can tell. Normal pressure for me is about 35 or so pulling. Decelleration always shows 40 on my gauge. It's been doing that for over 150K miles. I stretched my spring to just a hair over . 5".



Andy,



I'm wondering how high pressure can be hard on the lift pump. Maybe I don't understand it. The push rod pushes a piston that compresses a spring. At the same time a check valve allows fuel to flow thru the piston. When the push rod retracts the spring pushes the fuel. So unless the spring breaks I don't think high pressure could cause a problem in the pump. I kind of worry about seals in the P7100 because of high pressure, but I have seen no sign of any leaks. Might high pressure be hard on the check valve? It seems to me that the power of the spring is the upper limit of possible pressure. If I don't understand this thing please enlighten me.

 

[JoeBioDiesel]

Well, after running poorly on an intermittent basis my truck finally died completely, 60 miles from home. After sitting for a short while it woud run at idle, and under slight acceleration. I limped it home on the back roads. It was really acting as ifit was sucking air, not just low on power but missing too. There were bubbles in the pressure line and fuel pressure was near 0 psi. After verifying that there was no holes in the suction line from the tank, and fuel heater, I decided to hook up a different fuel tank. The lift pump will not suck fuel from the new can, even if I prime the hose. The NAPA guy can have one today. It's made by Carter. Anybody heard good/bad about NAPA lift pumps?



Joe

 

[Joe G.]

Did the test have the fuel heater and pre-filter between the pump and the supply? Or did you remove the little curved hose between the pre-filter and the lift pump so that the pump was drawing fuel directly from the can?

 

[Cooker]

It's made by Carter. Anybody heard good/bad about NAPA lift pumps?

Joe

The Carter LP's are fine. I think even the Cummins ones are made by Carter. Just curious what you had to pay for the LP?



Does the primer button still work on your LP?

 

[JoeBioDiesel]

Hi Joe,

I plumbed it directly to the can.



Cooker, I think the pump was $130. 00 ish. I have an account there. I just signed the ticket and left. I'd remember better if I had to pull the cash out of my pocket though.....



The button seemed to work, but the pump would work a little bit too. Once you got fuel to the inlet it would keep the truck running, even though the gauge read no pressure. (the short answer... I don't know).



Joe

 

[RBratby]

i just bought a lp from cummins. it was $159. the lift pump is made by carter.

 

[Joe G.]

Joe,



I would be curious about the condition of the parts. Is it possible for you to take the bad one apart and take a close up picture of the pieces for me? Especially interested in the O-rings. There are three big O-rings and a small one in the push button if I remember correctly. One or more of the three check valves may have failed.



Thanks,

 

[JoeBioDiesel]

Hi Joe,

I'll dissect it for you, no problem. I bet it's a checkvalve failure because of the way it acts. It was giging smapp pressure pulses out the pickup hose when I had it plumbed to the external tank. The pulses were big enough for it to spray fuel several inches when I had my finger over the end to check for vacuum there... of which there was little.

Joe

 

[Joe G.]

That sounds about right. They are really simple gadgets so it almost has to be bad O-rings or check valves. If we could find a source for the check valves it would be easy to fix one. I asked Larry B about that and he said he would see what he could find. I suppose that he couldn't find them.

 

[JoeBioDiesel]

I finally got around to taking the lift pump apart. One of the checkvalves was damaged. There is a metal housing the supports the rubber seal inside. The housing center was cracked out if it, and missing. It must have come out of there in smaller pieces, because if it all came out at once it would have never fit through the fuel lines.

It was the outlet checkvalve that was damaged. The inlet one was fine.

Joe