Fuel pressure

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
Trying to chase down the air/fuel problem on the XJ.

Shouldn't there be fuel pressure (measured at the injector rail) when you turn the key to ON? I can hear the pump motor priming in the tank, but pressure stays at 0.
As it cranks, slowly builds up to ~15 (takes many cranks). at which point it will catch. Once running, stays there steady, regardless of RPM. However once it's off, drops right back to 0, no residual pressure.

The way it behaves is that it dosn't like a sudden change in throttle. If you easy it up, wil lgo nice and smooth... but a big gas, it chokes. Have it rev, then let off altogether, it drops too far and stalls (unless you give it a little more and ease it down.

Bad fuel pump?
 
After some reading, yes it sounds that way.
It may be as high as 17-18... hard to tell since the gauge goes to 100... but it's definitely low.
That plus being 0 immediately when it isn't running before/after running tell sme there is definitely a pressure problem.
But (grrr) couldn't that also come from a leak?
 
I think you've got a bad pump on your hands! Or possibly a bad FPR. Come to think, it's probably the FPR. Somebody back me up here, but the regulator should be a bypass style with a built-in check valve to hold pressure in the system with the pump off to aid in starting. If that valve and the reg are bad, it'll probably read low and dump the residual back into the tank. I'm not a Jeep guy, though. The valve may be built into the pump. Either way, it should hold pressure, and probably put out more than that.
 
The regulator is right on the front of the rail; it has a vacuum line and is suposed to trade off vacuum for pressure (from what I've gathered by reading).
I disconnected the vacuum line from it, and thsi changed nothing.... there's definiteky good vacuum on the line, decreases w/ RPM (as it should?).
I'm thinking the problem is prior to the regulator.

If the fuel filter were full of crap - that would make the pressure higher, not lower, right? From restriction?
 
FPR, or fuel pump. I run a Mustang in-tank pump and the pump has a check valve. When this goes bad, all of the pressure leaks down.

I would guess the fuel pump is weak and not holding pressure, and not strong enough to maintain pressure. You can try clamping the retuen line while it is running to see if the pressure raises, but I think it is time for a new pump.

Exact think happened to me with the last bad pump I had.
 
catfishblues said:
Or possibly a bad FPR. Come to think, it's probably the FPR.
Click to expand...

Could be.... AFTER you change the inline filter (hard to build pressure if it can't push the fuel thru the canister :flipoff2:):

IIRC, when we were testing Brent's... we clamped the fuel return line off to see if that effected the pressure or flow. In his case it did nothing... we tested the flow by depressing the schrader while running & ended w/ plenty of pressure-VERY little flow... and a pump swap.

If'n you clamped the return line and the pressure rose, pretty good indication the regulator is pooched. If it does nothing, time for a pump. You may be able to skip the mega$$ intank replacement for a comparable inline (if one exists)... OTOH, you have nothing to lose by dropping the (empty) tank...could be a plugged sock, bad connector hose/fitting, or something else repairable.
 
Yeah, I was thinking I might take that pressure gauge, and take a measurement on either side of the inline filter. IIRC it's just in front of the gas tank? That'll be fun I'm sure.
If this thing sat for awhile (which clearly it has), there's no telling what kind of crap has settled and ocndense in the tank or line.
 
I would just pick up a new filter and change it. It will be the same amount of work to check the pressure fore/aft and should only be $10. If that doesn't do it, pump:bounce2:
 
kcarroll said:
I would just pick up a new filter and change it. It will be the same amount of work to check the pressure fore/aft and should only be $10. If that doesn't do it, pump:bounce2:
Click to expand...

I think it will be LESS work to just swap the filter! By the time he dicks with adapting the gauge (so it isn't leaking... unless it happened to come with tapered fittings :lol: ), he could change the filter 3 times!
 
Caver Dave said:
I think it will be LESS work to just swap the filter! By the time he dicks with adapting the gauge (so it isn't leaking... unless it happened to come with tapered fittings :lol: ), he could change the filter 3 times!
Click to expand...

DUDE....that's what he was saying in the post you quoted! :lol:
 
Well, not the filter. Pulled the old one, plenty of fluid spilled out. It was pretty dirty, but flowed nonetheless. W/ new one... no change in behavior. Grrr. Well at least now I have a good filter.
Put the gauge right on the line before the filter.... 16-17 psi, only when cranking.
Still no answer to this - it should have pressure just with the key to ON, right? Primed?
 
Does it have a check valve at the fuel pump? Some fuel injection systems use them. My old z car has one, it keeps the fuel pressure from leaking back into the tank when it is turned off. The pressure regulator sends all the extra fuel back to the tank, cause the fuel pump runs at one speed (wide open), regardless the engine speed, so at idle, alot of fuel is going back. An easy check for the regulator is pull the vacuum line off and it should rev higher.
 
You should have pressure for at least a few moments when the key is turned to the on position, and then it will slowly go down. You should have constant pressure when cranking and after the Jeep cranks. Sounds like a bad fuel pump to me.
 
td4runner said:
Does it have a check valve at the fuel pump? Some fuel injection systems use them. My old z car has one, it keeps the fuel pressure from leaking back into the tank when it is turned off. The pressure regulator sends all the extra fuel back to the tank, cause the fuel pump runs at one speed (wide open), regardless the engine speed, so at idle, alot of fuel is going back. An easy check for the regulator is pull the vacuum line off and it should rev higher.
Click to expand...

That's interesting... when I pulled the vacuum line off of the pressure regulator, nothing changed.
However, the pressure (17 psi that it is) stays constant regardless of RPMs...?
 
Survey says....

Fuel pump.
Got one from a junkyard, just swapped it in, soooo much better now, running about 35 psi at idle.
Still has a miss and a bit eratic... but that's minor.
Unf I didn't have a new o-ring, the old one is so swollen Ialthough still pretty intact) I couldn't fit the snap ring back on with it there. So for the time being, no o-ring... Stays pretty snug anyway...
 
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