AFARR

Put the Bosch fuel pump in (to replace the cheap one with the bad check valve...!!don't buy cheap stuff!!...).

Got out the cheap (Harbor Freight) fuel pressure gauge...hooked it up to the schrader valve (I assume that's where I should check it).

Get a reading of about 34psi. The new pump holds pressure well... Turn the engine on (not start) a couple of times and the pressure came right up...held fine.

Started it...idled nicely.

But the pressure stays pretty steady at idle at 34psi.

So, I push on the accelerator cable to gun the engine up a bit, and it stays steady at 34psi...release it and it momentarily drops to about 27 or 28psi, then goes back up.

Clamp off the vacuum hose to the fuel pressure regulator and it stays at a steady 40psi at all times (higher engine RPMs and when suddenly letting off on the accelerator).

Now I'm wondering if all the stuff I did to it (see my other post) was just a bad Fuel Pressure Regulator!. The hesitation comes when you let off the gas pedal to maintain speed...just about when the fuel pressure dropped off to 28psi.

Opinions?

A way to narrow it down further?

Thanks!

AFARR

AFARR

I saw several posts on here saying it should be 50psi as the normal fuel pressure....

Then I saw a post that said it needs 31psi.

This is for a '95 cherokee sport 4 liter. Both cheap (generic) pump and the Bosch give the same fuel pressure readings.

Thanks!

AFARR

cruiser54

You need the factory spec for your year. It is tested with vac hose on, then vac hose off.

Might try another gauge as I know you should be in the low 40s with vac hose on and high 40s with it off, just don't know the exact numbers.

tjwalker

Always post the year/engine of your XJ when you start a thread. There are significant differences through the years and this is one example.

The fuel pressure spec varies depending on what year 4.0 engine you have, as the fuel delivery system design changed starting in 96.

For 87-95, it should be 31 psi at idle

For 96-01, it should be 49 psi (plus or minus 5 psi)

cruiser54

Thanks for clarifying that Mr. Walker. If you'd gotten up earlier, you would have saved me a post!!

So if the OP has an 87 to 95 he should be good to go then.

AFARR

It is a 95. 4L engine. Sorry. I normally post that.

BigzXJ

When I tested my fuel I did the exactly the same thing as you. Same results. I did swap out the regulator on mine and it did not change my problem down the road. I don't know if the pressure drop is normal but mine does that to have you push the pedal. My Cherokee is 90 4.0, so you think they have something in common to cause this problem on different years. I am at a lose here.

AFARR

I'm starting to think it's something in the electrical (since I've replaced the pump and filter, fuel pressure regulator)...and the wires, rotor, cap, plugs, coil.

Running out of things to swap out.

It does seem heat related. Once the engine heats up sufficiently it starts...seems to run fine cold.

AFARR

Slick761

Hook up a vacuum guage. Could be a clogged muffler or cat.

BigzXJ

I put a brand new muffler in and cut the Cat. out. Did not change the problem at all.

Slick761

On the fuel pressure its normal that it dips. When you drop off the throttle you pull more vacuum causing the regulator to lower the psi a little.

AFARR

Wouldn't that cause a power loss across the board?

Mine doesn't have ANY loss on acceleration or deceleration, but only occasionally on maintaining speed (as a hesitation/stutter...bad enough that it really shakes the vehicle).

AFARR

Slick761

Ya, that's a tough one

DFlintstone

Maybe a tad off topic....I had a similar symptom forever. Turned out my wiring "Gang" to my injectors was worn/sometimes grounding where I couldn't see very well, on the bottom. Not likely, but easy enough to check.

BigzXJ

I have a vacuum gauge and do you mean the intake vacuum pressure?? If so where should I hook it up and what should the pressure be?