Cherokee Ppl

Hello xj aficionados,

1999 xj automatic completely stock, 120k mileage

I am having engine/ electrical issues. A month or so ago I started having problems with my engine cutting out, when I put it into gear. This just didn’t happen when it was cold but anytime. It also started to jerk and sputter when driving it. At stop signs or red light, the rpm gauge will hover over 1, and then drop below 1, when I give it gas. I had it towed to my repair shop. They hooked it to the computer and nothing showed signs of trouble. But when the graph setting was selected for viewing, when the throttle was adjusted it showed a sudden drop on the graph. As if there was a slight engine hesitation when given gas. So we Replaced the crankshaft pos sensor, map sensor, coolant sensor, distributor pick up cam sensor, new distributor cap, distributor rotor, new plugs, new wire set. And after all of this, still doing the same thing. So I put two cans of high mileage sea foam along with 93 octane fuel, no change. It’s only got 120k miles. The head is not cracked. My next steps could be: find an 1999 xj computer and switch it out. Maybe it’s an injector gone bad. I just do not know, and worse yet is neither does my mechanic.

Do any of you wonderful XJ Geniuses have any clue as to what the problem is?

tjwalker

Replacing the computer is a "diagnosis of exclusion" which means you have ruled everything else out. Don't think you are quite there, at least not yet. I wouldn't install any more parts on it though without diagnosis directly leading to it. Sometimes the new part you install may be of lesser quality than the original part that is now on your vehicle.

For starters, I would test the "TPS" (throttle position sensor). I also would test fuel pressure. 49 psi plus or minus 5 psi is the spec there. Take a look at fuel trims with a scan tool. Also never hurts to perform a compression test, a good snapshot in time of the internal condition of the engine. For your 99, you should have 120-150 psi with no more than a 30 psi variation between cylinders.

A tech that really knows their way around a scan tool should be able to help you figure this out....

Cherokee Ppl

Thank you TJWalker,

I do know that my tech said he hooked up a fuel pressure gauge and had 60 psi on the fuel. Is that too much? I will run this great info by my tech. I really appreciate your help and advice on this topic!
just one more thing to add, just prior to having these problems I discovered a large mouse nest under my back seat. Don’t know how it got inside the vehicle, I never saw the mice but saw lots of droppings. Could this be a gnawed wire somewhere on a harness?

thanks
Clint

lawsoncl

Damaged wiring is certainly possible. Two relatively easy things to check. Wiggle, and flex the wiring along the fuel rail and where it crosses to the firewall and see if you can cause it to stall. If the wiring has frayed internally, it can be intermittent as the engine moves. Bad motor mounts can contribute to this. Second, have a good look where the bell housing bolts up to the engine and make sure the bolts are tight. If those bolts are loose, the bellhousing can shift and cause the crank sensor to move out of alignment.

Saudade

What lawsoncl says.

Fuse 21 (20a) in the PDC powers all injectors. There are several splices and a connector between the fuse and each injector (S137, C107, S109, S113) . Each injector is then grounded through the PCM. There's a common splice (S113). The wire color is DG/OR (dark green/orange). S109 and S113 can be found in the locations in the pic below. Use a test light or voltmeter to check for voltage at the injectors. While checking for voltage, shake the harness around.



S137 also powers the ignition coil.




I had an intermittent FI #3 for a while. Mine turned out to be a bad splice in the harness to the #3 FI.