Low fuel pressure recurring even after new pump and tank

[LiftedDisco]

I have had a recurring problem with the truck that two mechanics have been stumped by. After about 45 minutes of driving city or highway, I start to lose fuel pressure.

It?s 32-36psi normally, but then drops down to as low as 15 after driving for a while (measured by a remote in-cab fuel pressure gauge off the injector rail). Blew out the fuel lines, changed the gas tank, fuel pump, filter, fuel pressure regulator and combination fuel pump master relay.

Had the intake pulled and cleaned, changed vacuum hoses and did the MAF; no joy. Power drops to nothing and it won?t rev over 1200 rpm.

Any ideas?

 

[Rocky]

D1? D2? Year? Tank Full? Tank empty? Did you measure fuel line pressure before you went to all the expense of a new pump? Why do the tank?

So basically much more information,

 

[Robbie]

Is the pressure dropping while at cruising speed and stalling out the truck on the road? or does it just drop while idling? Have you checked the fuel pump harness? on a DI it runs on the frame near the driver's side rear tire and the connection is very exposed to the elements. possibly loosening up connections or bad connections making a weak pump.

 

[Mrmerlin]

Hi replace the gas cap , it sounds like the cap is plugged and not letting any air in to replace the fuel that you use, otherwise try just unscrewing it when the lo pressure is encountered if you hear a rush of air then that could be your culprit; If this fails I would carefully check the fuel lines and look under the truck for a crimped line , if the fuel tank was replaced then they may have crimped a flexible line and this will do the same thing as a bad fuel cap, Stan

 

[LiftedDisco]

Sorry about missing detail.

'96 D1 4.0 5spd 165K miles. Runs mid-grade or premium, Magnecor wires. No codes or Check Engine light.

The problem used to be made worse below 1/2 a tank; pump would whine badly. There was huge buildup of air in tank when cap pulled, so I drilled the cap to let it breathe. It helped for a few days, but the problem came right back.

I changed out the pump myself with a crossover part referenced from DWeb and it helped for a month or so. Problem came back. Truck was eligible for the tank recall and I got LR to replace the pump and filter while they were there. (grasping at straws at this point)

Helped for a few days, then came back. (Tow #2 for wife):banghead: Back to LR, they recommended the fuel pump relay was bad and replaced the large relay mounted next to pass. side fender. Fixed for about a week this time, then problem came back.

Went to ex-LR tech now working for an independant. Fuel pressure watched during test drive showed drop after 15-25 mins of driving. Pulled intake and MAF, cleaned out but didn't see anything out of the ordinary. Pulled pump and inspected wiring, found cooked ground wire (discolouration of wire, warped insulation due to heat) and replaced with new. Physically checked fuel lines and found nothing. Pump isn't making noise like the old ones did, but still drops in pressure.

Recommended changing the fuel pressure regulator, thinking it might be allowing too much flow and killing the pressure. Did this and fixed it for 6 days! Problem came back same as ever. No buildup of pressure in the tank anymore and the pump doesn't whine, but I still lose most of my fuel pressure when it's warmed up fully.

He's thinking ECU-related at this point and that there may be something haywire with the chip. I'm at wits end and just drove home at a maximum road speed of 25mph. Damn you Lucas!

 

[handz87]

check the voltage to the pump I bet you have a corroded wire right under the drivers side wheel well. This happened to me.

 

[juju6886]

This is no help but I feel your pain I had a f*$#in Vauxhall (GM) in the UK that was a two week car two weeks on the road two weeks off the road finally sold the damn thing and it now resides where it belongs in the Junk yard. Ahhh Thanks for lettin me vent.
Sounds like you have gone well above and beyond to fix it. Take a good look at the wiring might be heat related dunno. Odd problem.

 

[robisonservice]

The ECU does not regulate fuel pressure. All the ECU does is switch the pump on or off. Your problem should not be hard to solve. As others have suggested, it is caused by one of these things:

1 - low voltage at the pump or a bad pump
2 - Blockage in the lines or pump screen
3 - Excess return flow back to the tank
4 - Vacuum in the tank preventing fuel flow
5 - Excess demand from the ECU - gross overfueling

You have seemingly eliminated all the above, yet you still have a problem.

I'd make sure the diagnosis of low pressure is correct . . . make sure you have an accurate gauge teed into the fuel rail in the right place. If indeed pressure is dropping to 15 then proceed on . .

I suggest you connect a meter right to the pump power terminals on top of the tank and drive the truck. I think you'd see a voltage drop and that will explain your problem - some wiring fault. That's by far the most common situation. Test at the pump.

Next I'd use a current probe on the pump power line to look at the current waveform (on a scope) That would tell if the pump was bad. If it were, I'd change it. I have had situations where people fit those aftermarket pumps into the old housing and the wires burn out, causing similar situations.

If not, your next step should be to fit a new filter and make sure the lines are all clear by blowing through them.

If that did not work I'd swap the regulator again and make sure there is no vacuum problem to it.

You have eliminated tank vacuum as a possibility by drilling a hole in the cap. As an aside, I have never seen tank vacuum issue stop the fuel pump without first collapsing the tank in on itself from vacuum

The final check would be to look at injector pulse width - if the ecu calls for more fuel the pressure can drop, but if there's no heavy load what would indicate some kind of engine managemetn problem. Fuel flow that dropped pressure like you describe would probably get the cats red hot.

 

[LiftedDisco]

The ECU does not regulate fuel pressure. All the ECU does is switch the pump on or off.

Thanks for saving me from needlessly spending another fist full of cash!

I'm going out right now to follow your diagnostic path. Thanks to everyone who chimed in, this has been painful for us.

:bangbang:



Any ingenious ways to tap in to the wires at the sealed plug at the pump? I'm loathe to cut the insulation as this is a corrosion-loving location.

 

[LiftedDisco]

#1 fully checked. Voltmeter at the pump road test. Read 13V at startup and stayed there throughout the test. Full power for the first 5 or so minutes, runs fantastic. Once warmed up a bit, the truck will begin dying off and lose pressure (although this not tested directly tonight).

Even when it was limping home and floored coming away from lights, the voltage never wavered at all. After 8 minutes or so of driving; in neutral WOT equals 1600rpm.
Interestingly with the carpet pulled up and the access hatch off, I could clearly hear that the pump never changed its sound throughout the test drive.

Ran out of light tonight, so on with the testing tomorrow after work.

 

[AfiRover]

try geting the injectors tested your relacement part #is err 722
good luck check the fuel rail for "rust" not to uncommon this will break free and hold an injecter open found one just the other day same problems but hade a tone of smoke when it lost power
just a thought

 

[Bouvier]

Now that the electrics appear to be OK how about the vacuum to the regulator? Could all of this have anything to do with high manifold depression when it warms up? I can't think of what could cause a sustained high vacuum after warming up but is this possible? WOT should crater manifold depression! From what I can figure the high vacuum causes the regulator to open sending fuel back to the tank causing a drop it rail pressure needed when backing off the pedal. Since this has been a long term thing eluding detection could it be off in this direction?

 

[LiftedDisco]

how about the vacuum to the regulator?

Now that is one that I haven't tried yet. I'll take a look there next.

I didn't get access to the fuel pressure gauge tonight so no news there.

Noticed something else that confuses me though. When driving and it has fully lost power and won't rev past about 1700rpm if I stop and shut if off for just a moment, it starts fine (as usual) and will run strong for about 3 minutes before resuming the problem. :ack:

By run strong I mean it pulls hard to redline with no hesitation at all, just like there is no issue. Then Prince Lucas waves his Wand of Mechanical Damnation and it quickly loses all power and barely limps along. :banghead:

 

[Reed]

Are you sure the LR changed the pump when you had the tank replaced?

The reason I ask is that I too had the same problem after I rebuilt my pump with an aftermarket pump. After going through all kinds of diagnostics, I finally pulled the pump again and found that the replacement hoses I had used to rebuild the pump assembly had an intermittent kink in them

When the engine was idling it ran pretty well and I could even rev the engine up and it would run fine. But when I put a load on the engine, such as driving the truck somewhere, there wasn't enough gas volume to actually run the engine under load.

At least that was what I think I remember happened. The problem was however the hoses I had used to rebuild the fuel pump with.

The only other thing I can think of is that when the fuel pump fails, it putts a lot of debris into the fuel line. Maybe enough to not only clog one filter, but maybe two in a row.

Anyway, my first suggestion would to test the fuel flow at the pump. Disconnect the fuel line at the pump and put a hose over the output nipple of the pump. put the other end of the hose in a suitable container and have someone turn on the key. Just a rough check on the volume its putting out.

Then, pull the fuel pump and give it a good inspection. Making sure there are no kinks, leaks, or disconnects within the fuel pump assembly. And that the "new" fuel pump is actually in good condition with all its brushes and bearings and filter intact.

If you do this, please be very careful as there is a lot of potentially explosive liquid involved in these tests.

One last thing, don't try to run the pump on the bench or when it is not immersed in gasoline. Doing so can burn it out rather quickly.

Hope you can sort this out.

 

[texmotoII]

Well, did you get the mofo figured out yet?

 

[jhmover]

Here's one from Rangerovers.net

'Blocked Vacuum Relief/Rollover Valve: Another common reason for failure is a blocked vacuum relief/rollover valve in the evaporation tank in the rear quarter panel. The vacuum buildup causes the tank to collapse and crush the fuel pump. Initial symptoms are often a temporary cutout of the engine at high speed, progressing to a permanent failure. Loosening the gas cap to relieve the vacuum can often get you home in this case.'

Plus I didn't notice if anyone checked the fuel filter?

Hope this may help.

JH

 

[LiftedDisco]

Blocked Vacuum Relief/Rollover Valve: Another common reason for failure is a blocked vacuum relief/rollover valve in the evaporation tank in the rear quarter panel. The vacuum buildup causes the tank to collapse and crush the fuel pump. Initial symptoms are often a temporary cutout of the engine at high speed, progressing to a permanent failure. Loosening the gas cap to relieve the vacuum can often get you home in this case.

Thanks JH, I drilled a small hole through my filler cap to relieve any built up pressure but this didn't do it.

I bought my wife a used 626 on the weekend so I am in somewhat better shape in that department. No more "so are you gonna sell the Disco?" type comments.

I'll be hitting the ideas on this thread in the next few days and will update as to what I find. Something above about corrosion inside the fuel rail including rust caught my eye. Could a decent performance below 2000rpm dropping off to nil when trying more than that be caused by flakes of rust blocking the injectors inside?

I've never had the rail off and don't know what they look like inside.

Thanks again to all for your help!

 

[LiftedDisco]

Are you sure the LR changed the pump when you had the tank replaced? ... pull the fuel pump and give it a good inspection. Making sure there are no kinks, leaks, or disconnects within the fuel pump assembly. And that the "new" fuel pump is actually in good condition with all its brushes and bearings and filter intact.

Checked the attachment and vacuum hoses on the new fuel pressure regulator. No fault found. Pulled fuel pump and inspected, it was shiny new and has a 2005 LR part sticker on it.

Inside of tank is perfectly clean, no debris or scum. (it's only about 7 months old, but I'm checking everything now)

Test drive, fault still there:banghead:

Pump appears to have been eliminated, along with the tank. Next up I'm going to blow out the two lines between tank to filter and then filter to rail.

After that perhaps fuel rail? This is REALLY sucking big time.

 

[warner0188]

Hey, did you get it figured out yet?

 

[frickjp]

It ain't rocket science. The pump has one function. It pumps. Period. It scavanges fuel off of the bottom of the tank and pumps it. Through the line to the filter. Through the filter. Through the line to the fuel rail. Regulator limits the pressure to 32-ish and returns the remainder to the tank.
While it may have voltage, does it have ground? You have to watch both, right at the pump. When the pressure drops, take a pair of needle nosed Vise-Grips and pinch the return hose shut. That eliminates the regulator. Still low? Got voltage? The fuel pump connects in the top of the L/R wheelhouse with a short harness to the tank unit. They tend to corrode there.