1998 xj 4.0 Driving home from work Monday afternoon and it quits running, no sputtering or anything. Just like I turned off the key. Messed with it on the side of the road, fuel pump turns on with the key. Will turn over but will not fire. Get it towed home. Sat until this morning. Go out and hit the key and it fires right up, let it run for an hour, no problems. It is my DD and i travel 40 minutes each way to work so she needs to be running. What can I check to see what the problem might be? Any tips are greatly appreciated.

 

Check the CPS first, most likely the culprit.

 

this the one under the distributor cap or on the bell housing?

 

bell housing

 

Tell him how to check the CPS.

 

Would love to know how. I will do every last thing iI can to avoid sending to the shop. I have the tools and know how to do about anything, if somebody can tell me how to check something. Kind of new to the jeep world.

 

It can be tested as follows:
https://www.cherokeeforum.com/f51/testing-cps-5563/

These tests are good for detecting the basic failure modes of the cps, where the internal wiring is no longer functioning properly.

The slightly harder failure mode, is when the CPS passes the tests above, but for whatever reason is sending improper signals to the computer. For example, missing teeth on the flexplate/flywheel, or the magnet on the sensor getting too weak to produce a reliable signal. That kind of stuff. When these things are suspected (after doing the tests above), a scope can be used to view the cps signals. Without a scope, the next best step is to swap the sensor with a known good part.

 

OK. Thanks guys/gals going out to check it now. But to find my ohm meter first. LOL One of those tools that i don't get much use out of so it's placed where i won't forget it.

 

Are you 100% sure the fuel pump turned on when the engine didn't fire up?
I'd think it seems like a typical crank sensor failure except for the fuel pump turning on... if true.

 

The fuel pump on the XJ will prime (turn-on) even if the CPS has failed. Unless there happens to be some unusual electrical problem that is effecting them both.

 

Are you sure about that? Maybe year specific ASD variant implementation?

All i know when my crank sensor went out on my 2000, no fuel pump prime.
After cranking for a while with no start, i immediately poked the fuel line schraeder valve with absolutely no pressure...nothing, only a few drops of fuel dribbled out.

 

CCKen might know if there were differences in the years. I suppose there could be variations.

I helped a friend troubleshoot and replace the CPS on his 2000 XJ, and the fuel pump never failed to operate. And on my 96 the fuel pump is not part of the ASD.

But I don't have enough experience to know if that holds true for all XJs in all model years. Or perhaps there is more to the equation that I don't understand.

 

You're right. The fuel pump operates independently from the ASD relay and will turn on for a few seconds at initial key to RUN. However, if the PCM doesn't see engine rotation signals from the CPS within 2 seconds of engine rotation it will shut down the fuel pump.

 

I can add only one thing for the CPS use an OEM one from Mopar.

 

Did it throw a code?