Crazymonkey

Hello,


I'm new to the forum, and I've been scouring the internet for some help and cannot find anything similar to remedy my situation.

92 W250. 280k owned it for 5 years, had injection pump rebuilt, new stock lift pump & installed low pressure mechanical fuel gauge after fuel filter with a needle valve, last summer. Was running consistently around 7 psi.


I don't see any leaks, the engine runs great, starts at the touch of the starter, if I wasn't looking at the fuel pressure gauge I wouldn't know anything was even wrong.

The issue:

This winter I noticed that when I would start my truck cold it would run at 7 psi, which is good from what I gather. As it warmed up the psi would gradually and slowly decrease all the way down to 0 psi after about 20-40 miles depending how cold it was outside. 15-35 degrees Fahrenheit. If I shut it down & started while engine was still warm it stayed at the same consistent low psi, whatever it was when I shut it off. After hours of sitting or overnight and the engine was cold once again it would start back up at 7 psi. I drove it all winter like this and just made short trips, not letting it get all the way to 0 psi.

Now that spring is here the ambient temp is in the 50's 60's & 70's

50 degree weather first start of the day I'm running @ 4psi and can only go about 15 miles before dropping to 0 psi

60 degree weather first start of the day 2-3 psi , not sure on distance, it was too low to risk driving anymore, won't get vey far before it drops to 0 psi.

70 degree weather 1st start .75-1psi

Haven't driven it for the past 2 weeks due to the warm weather, just starting it in my driveway troubleshooting.

What I've tried:

-Replaced fuel filter, old fuel filter looked fine with no contamination in the fuel

- new lift pump

- thought maybe I received a defective lift pump so ordered another. 2nd new lift pump still starting @ approx 1 psi in 70 degree weather, 4.9 psi in 50 degree weather.

-assumed the grommet seals in the union between the fuel filter and ip were maybe worn out so ordered those from cummins and also replaced the seals on the same line at the outlet of the fuel filter

-swapped out my mechanical fuel pressure gauge for another. Getting same readings

-removed my fuel cap to release pressure in the tank

-rigged up a 1 gallon fuel can under the hood connected directly to the lift pump

- I read somewhere on the internet to block the return line, doing this didn't really do anything until I shut the engine off, then the gauge pegged @ 15 psi until I removed the plugged line that sprayed diesel all over.

- opened up my needle valve all the way, no change

More observations:

70 degree weather running it is approx <1 psi, I notice when I turn the engine off the gauge jumps up to about 2-3 psi for a second or two then drops to 0

Not really sure what else to do, now with the weather being warm I don't want to risk driving anywhere & possibly damaging my ip.

My last resort is to bring it to a shop, because I'm out of things to try.

Any suggestions or ideas would be greatly appreciated

Thanks in advance!

nonrev

Good morning and welcome to the forum, what I would try is disconnect the fuel supply from the tank to the pump then rig up a can for fuel supply to the fuel pump, there is a possibility that the screen in the bottom of your tank is plugged ... if you get good pressure you will know that is the problem.
Hope this helps

oliver foster

Vernon,
He lists trying a 1 gallon jug under the hood as a direct fuel supply to the lift pump.
I am a little stumped as to the cause.
I have certainly seen fuel pressure be affected in cold weather as it gets thicker and flows more difficultly, but this seems to be happening the opposite way than I have seen...

nonrev

Oliver,
my bad...must not have had enough coffee yet

NJTman

Try installing a fuel pump snubber. See what happens to the pressures.


https://www.genosgarage.com/product/...ge-accessories

nonrev

T-man, he says he has a needle valve and has tried opening it all the way.. no change

Crazymonkey

no worries, it was kind of a long post.

Thanks for the suggestions!

Crazymonkey

Yes it is very strange! My thoughts were as the fuel is warmer it is more viscous and could be leaking somewhere, or a rubber seal warms up allowing pressure to escape? I just don't see any leaks.

Thanks!

j_martin

Wow, this is a wierd one.
Analysis, fuel is not being delivered as fast as it is being pulled.
Yes, I said pulled. The VE pump has a first stage lift pump built in to it. On some apps, such as a 4 cyl VW, there is no external lift pump.
Lets apply binary analysis to it. (divide and conquer)
A. fuel is being delivered by the lift pump at a reduced rate.

a. Lift pump bad.......you've covered that
b. Restriction in fuel line or fuel tank.
1. pickup screen intermittently blocked by debris
2. suction line (or it's liner) collapsing
B. Fuel filter plugged or restricted

B. Other first possibility Distributor injection pump is flowing more fuel than can be delivered.
1. Leaking somewere
a . Extermal
b. Into the engine, ie front seal
1. bypassing to return line

Now, given all that, it appears that you've isolated it to IP bypassing fuel to the return line. Maybe I'm wrong, and a blocked return line over pressured it only when cold. If restricting the return line causes increased pressure under all conditions, then the problem is internal in the IP.

On the delivery side, you've replaced the lift pump, so restriction is the only possibility. The auxiliary gas can and pickup hose is a good substitution test for this. Just be sure you bleed through the fuel filter before you start the engine, or you might air lock the injection system.

Crazymonkey

One thing I noticed when I had the fuel jug hooked up under the hood going directly to the lift pump, was that the disconnected fuel line coming from the tank had a steady stream of diesel coming out of it once I started the engine. After I turned the engine off it continued to come out until I released the pressure by removing the fuel tank cap.

That could be normal, I'm not sure.

Do you know any tests that I can perform to be sure that it is the IP pulling too much fuel?

Like possibly hooking up a pressure gauge to the return line and if that reading is higher than spec, concluding, it is bypassing too much fuel to the return line.

Any idea what that pressure is supposed to be?

Thank you!

j_martin

The fuel tank will siphon off when disconnected at the lift pump.

I can't believe the IP would be flowing way too much fuel back the return line and still running well.

Did this problem come on slowly, or suddenly?

The lift pump is drive by a push rod and the camshaft. Is it possible that the lobe has spalled and worn down?

Somewhere there must be a spec on the fuel pump push rod throw. It could be measured pretty easily.

dodgenstein

The arm on fuel pump rides directly on camshaft

Crazymonkey

I just noticed it this past winter, so I'm not sure if it was suddenly or gradual.

today it is 60 degrees out, I picked up a fuel pressure tester gauge and hooked that up directly to a T connected to the banjo bolt. Both my in-dash gauge and the tester gauge are both reading 2 psi. Tester gauge bypassed the needle valve.

If the lobe was worn out on the camshaft, would the ambient temperature have an effect on that?

Thanks again!

nothingbutdarts

Is there a spec. for lift pump pressure and flow rate in the manuals?? Sorry, I don't have any manuals available.

oliver foster

I have had a high mileage 2nd gen with a worn cam lobe that had very low / NO fuel pressure. After installing 3 stock LPs, I installed a cheap NAPA inline electric fuel pump to get it back up and running.
Maybe a worth while test to try...