Fuel Pressure questions

So. Im having a low / hunting idle issue.

fuel pump 2 years old.
filter 6mo.
fuel pressure regulator is junkyard.

I have a fuel pressure gauge on the line now permanently.

When the car runs good. and I check it. Im at 40 on the dot with the line disconnected.
and 34~ with it connected.

when I checked the other day when it was running crappy it was at 15-20.

two things I am thinking causing this.

I know there is crap in my gas tank. could be blocking filter.

could the fuel pressure regulator cause this ?


I bought A new fpr but it has yet to arrive.

I think the results sound right and the FPR is working.
My guess is it's going to be crap in the tank, but a filter change is easier.

Does it happen randomly or at certain tank levels?


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You can back flow the fuel filter and reuse it for testing. Also gives you an idea of just how much junk is in the system, save what you flush out into a clean container.

I've had that happen numerous times for various reasons. The most common is a faulty ballast resistor, jump it and re test.

Once I found a crispy connector in the fuel pump harness. 5-6 connectors between the firewall and the fuel pump. Funny but my voltage was almost normal (with the ballast resistor jumped), the issue was amperage. I finally figured it out after running a temporary wire directly from the battery to the pump. The bad connector I found was the one near the rear wheel well/C pillar and it was seriously crispy.

Ohm the fuel pump ground.

Low pressure can sure be a likely cause. Sounds like the first place to look.

Vacuum leaks have most times been the cause of my hunting at idle. Another cause can be TPS ground.

The issue seems to be worst coming to a stop from driving.
or switching between park and drive. the rpm's drop off. as does the pressure.

Does anyone know.
Can I remove the gas tank skid and will the gas tank stay in place with it removed ?
or is the skid what is mounting the tank to the chassis ?

On mine the tank is held up
By the straps and the tank skid plate bolts to the frame


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Green Mesa XJ said:

On mine the tank is held up
By the straps and the tank skid plate bolts to the frame

Click to expand...

That is what I have seen on the '96 and down design.

I don't know about '97 and up, but I suspect it is the same.

Okay.
I checked it again today and its fluctuating.
gotta be crap in the tank...
Im going to pull it. see how bad it is. then decide on cleaning.. or replacing.

So I wanted to confirm something.
When I turn the key to on position. I can see fuel pressure jump up. then fall back to 0
if i turn it 3x. and move to the front of the car its just falling down.
is that normal ? or is it supposed to keep pressure ?
when its running most of the time I am at 35 with hose on 40 with disconnected.
mostly when its cold she runs 15-20 psi sometimes.
just wanted to make sure before I pull the tank.
thats not electrical is it ?

I think for the leak down fuel pressure test you have to clamp the return line . Not sure. I'll look it up .

Did you try to bypass the ceramic ballast resistor ? (Driver side fender in the engine bay) you can use a wire to bridges the (wire) plugs, or use a fuse from the dash fuse box .


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In my crap Haynes manual (my fsm are electronics) it say to test the regulator disconnect the vacuum line and test running. 39 psi, attach a vacuum pump and draw a vacuum you should get 31 psi.

Fuel pressure below specs , momentarily pinch off return line.
Do not allow pressure to exceed 60 psi.
Inspect fuel line,
If pressure sides replace regulator
If pressure is above specs check return line for kinks

Leak down test :
Checking for leak past regulator and check valve in pump. Run engine, pressure at normal levels turn off, note pressure reading and let sit for 30 min, a Drop 0-20 is acceptable ,reading 19-39.
(Honestly most check valves leak)

Let engine idle and Pinch off return line , fuel pressure will rise to 95 psi. Shut off engine allow to sit 30 min comparing readings. If it drop 20 psi replace regulator, if it drop more than 20 it's the check valve in the pump.

Of course I'd still try to make sure the fuel pump has a good ground and is getting full power .



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