Engine/Transmission (1998.5 - 2002) Unusual Fuel Pressure...

[PKleine]

This has started happening again and with a transfer pump that is only 3 months old.



Condition / fuel pressure

Idle / 8 psi

up to ~1700 rpm / 6 psi

above ~1800 rpm / 10-13 psi

WOT / almost 10 psi



The above happens occasionally. Usually pressures are good; 14-15 psi at idle, 12 cruising and ~10 WOT.



Last summer when this was happening I spent almost enough $$$ in troubleshooting with DC & Cummins to buy a new lift pump. Recently, I fooled myself into installing a new transfer pump, when the real problem was bad fuel.



Any one have any idea what is happening? It's almost like low voltage to the pump... I haven't had a chance to check that yet.



Thanks in advance.



Peter

 

[SnoKing]

What is your volt meter doing when this is happening. Also remove then lift pump relay and reinstall it with some dielectric grease. SNOKING

 

[PKleine]

The IP voltmeter indicates normal & I used Dow DC-4 on the electrical connection to the pump.



PK

 

[Gary - K7GLD]

Sounds like you LP is cavitating at lower fuel flow and accompanying higher pressure - then once higher engine speed increases fuel demand, the pump resumes near normal operation.



I just went thru a similar situation caused by adding too much used engine oil into my fuel - but normal PSI returned once the oily fuel was eventually diluted with new fuel. Other similar suspects are any thing or condition that restricts normal fuel flow, plugged fuel filter, kinked or crushed fuel line, etc...

 

[PKleine]

Sounds like you LP is cavitating at lower fuel flow and accompanying higher pressure - then once higher engine speed increases fuel demand, the pump resumes near normal operation.



I just went thru a similar situation caused by adding too much used engine oil into my fuel - but normal PSI returned once the oily fuel was eventually diluted with new fuel. Other similar suspects are any thing or condition that restricts normal fuel flow, plugged fuel filter, kinked or crushed fuel line, etc...

Gary, that cavitation theory sounds possible. This began shortly after the last fuel stop; maybe something bad in the fuel. I quit trying to burn used engine oil as it was causing too many clogged filters. I've seen plenty of filters get clogged, but never seen one that clogged, cleaned its self and clogged again...

 

[Ron Bissett]

Gary, that cavitation theory sounds possible. This began shortly after the last fuel stop; maybe something bad in the fuel. I quit trying to burn used engine oil as it was causing too many clogged filters. I've seen plenty of filters get clogged, but never seen one that clogged, cleaned its self and clogged again...

Suggest you try a T or Y fitting to line to sender and add a mechanical guage to test the mounted guage to see if its the guage or the pump. Make sure you use a known to be accurate guage you can plug the test guage line or add a cut off switch plug and store the test guage . Ron Bissett in Metro Louisville KY.

 

[Gary - K7GLD]

The real tip-off to this particular issue, is the fact that PSI *increases* once higher fuel fow occurs - an excellent indicator of cavitation caused by reduced/restricted flow in a constant speed pump like our lift pumps.



In my case, as stated further above, the most immediate way I could regain normal fuel pressure, was to briskly accellerate momentarily, which increased fuel flow, and caused the LP to again begin normal operation - at least until PSI increased to high enough level to start cavitation all over again...



I can only assume some combination of increased viscosity due to the added used engine oil, or it's tendency to increase fuel foaming, was the basic culprit in my case - but all is well now that I have added fresh fuel to dilute the used oil content percentage.

 

[PKleine]

I'm bringing this back up because I have some new info but no fix yet.



On my last trip back from Oregon the fuel pressure indications became very unusual. At first indications were normal. Then as the truck warmed up they began to drop. Early on they would be around 6 psi and if I stabbed the throttle a little, they would come back up to 10 to 13. Then they would drop back down shortly after I returned to steady cruise. As the drive continued the steady cruise FP deteriorated to about 2 - 3 psi, but if I floored it, they would come up to 10 - 13 psi (much more than a little stab of the go-pedal). FP would drop right back down to 2 -3 psi the instant I reduced power. If it matters it was hot outside, 100F to 108F and I've been through 4 tanks of fuel since my last post, purchased from different sources.



Today I received a fuel pressure test gauge (Thanks Rip!) and attached it to the schrader valve on the inlet to the fuel filter. The pre-filter fuel pressure matches the post filter pressure. That is, they go up and down together. At idle there is less than 2 psig difference between the two indications with the pre-filter being the higher one. At high flow, (go pedal down hard) there seems to be a slightly larger difference almost 4 psig. Anyhow, the good news is my fuel pressure gauge mounted on the pillar is ok. I'm starting to think that having a fuel pressure gauge is ok, but what we really need is a fuel flow meter (is fuel really getting to the VP?).



Tomorrow I am going to recheck the electrical connections. Then try to hook up my voltmeter to the LP and go for a drive and see what is happening electrically.



Does anyone have any idea as to what is happening here? Am I missing anything (besides a reliable fuel transfer system )?



TIA



PK

 

[R-N-R]

Hate to say it, but you might want to pick up a LP and swap it out. If there is no change (not the problem) you can save it or turn it via the classifieds. Somebody or yourself will most likely be needing one sooner than later

 

[master1578]

I had a somewhat similar problem a little while ago. Look Here



It might be worth a shot to change out the pump. Although I'm not sure if my fluctuating pressures were a result of just the lift pump or a combination of a good pusher trying to send fuel through a bad lift pump.