Green99XJCherokee

99 XJ misfires during acceleration and going down the freeway at 2000 or under and sometimes over. I cannot figure it out. I’ve change crank and cam sensor, all new distributor, new coil, it’s getting fuel and good spark. I’m not sure if it’s a sensor on the manifold or a low compression cylinder, I don’t have a clue

99 4.0
Auto
32x11.50

tjwalker

Have you pulled codes? If it's a misfire, the computer should have stored codes. Codes are very important in starting the diagnosis process. Post them here for comment.

As for fuel pressure, your 99 needs approximately 49.2 psi (plus or minus 5 psi) to run properly. This must be verified with a fuel pressure gauge, which can often be rented inexpensively or with just a deposit at your local parts stores, call around for availability.

Compression is a good "snapshot in time" of the internal condition of your engine. Easy to perform on this engine. The compression spec for the 4.0 liter engine is 120-150 psi, with no more than a 30 p[si variation between cylinders. This tool can also be rented from many parts store.

Good luck and be sure to keep us updated!

Green99XJCherokee

No misfire codes. I’ll check fuel pressure. i know for sure it does heat soak pretty bad but it usually goes away but it still misfires under 2000

tjwalker

Two sensors that come to mind that can/should be tested IF fuel pressure checks out would be the MAP sensor and the TPS (throttle position sensor)

Have you ever seen the check engine light come on or does it come on when it's misfiring? You may want to try a different code reader or scan tool to see if there are codes. OBDII is reasonably decent at picking up on misfires and setting a code(s).

Green99XJCherokee

I changed the TPS, MAP sensor, cleaned the idle air control valve, new spark plugs. Still haven’t checked fuel pressure, will be the next thing i do

Green99XJCherokee

I did a fuel pressure test and it read 28 when primed, 35 at idle and it is leaking down to about 12

Green99XJCherokee

under load it goes down to about 30 lbs

tjwalker

Assuming your fuel pressure gauge is accurate (I always use two gauges to make sure), your fuel pump is toast. You are way out of spec low with your fuel pressure numbers.

The spec is 49.2 psi plus or minus 5 psi.

exasemech

^^^^^^
Although another possible suspect would be really leaky injector (s). You could perhaps look at the long term fuel trim and see if it's running lean(pump) or rich (injector)