Hello, I have a 98 XJ with about 127,000 miles on it. Randomly this last fall my XJ failed to start. It would turn over, but wouldn't fire. I left it for a few days until I had enough time to try to fix it (about 6 days) and it fired up just fine, with me doing NOTHING. I had no more problems with it all winter and all spring until now. It is doing the same thing. Do you guys think this is just a crank position sensor on the fritz? It sounds like the fuel pump turns on and I have moved/rocked the Jeep so I don't think I am in a bad starter position. Any thoughts? Thanks!

 

When the crankshaft position sensor fails, it fails. No resurrection.

 

id say the cps, clean the throttle body and iac and get a new tps and do plugs and wires too and that should put you in pretty good standing on your ignition system

 

When it cranks but won't fire, is there sufficient gasoline and pressure at the fuel rail?

 

How do you go about checking this? Sorry I am new to the site and new to the XJ world. Spent too much of my time in the lab and am pretty embarrassed/pissed off at my lack of knowledge when it comes to vehicles. If anyone has the patience to help me diagnose and possibly fix my Jeep I would greatly appreciate it. I really want to know how my XJ works. Thanks!

 

My 90 cherokee did the same thing
It was the starter relay. Don't know if this will help because of difference in years. Also another thought on my son's XJ The cps plug got dirt in it. Cleaned it and works great

 

With your '90 did it not start and then run fine for awhile and then act up again. This is the most perplexing part.

 

So I switched out the starter relay with my cooling fan relay and still no dice. Also switched out my fuel pump relay and ASD relay and still no go. My fuel pump turns on and I unplugged the cps and the connection looked clean, reconnected and still no start. Didn't have my voltmeter on me so couldn't test. While I was under the jeep checking out the cps I did notice that the O2 sensor was missing about a foot of insulation..don't know if a bad O2 sensor can cause a no start. Another thing I remember is on the ride home the night before this last no start I experienced the "death wobble" which occurred quite frequently last fall, maybe this exposes a bad connection somewhere. Wobble went away last fall after I put on new tires...the new tires must have been masking what was really causing the wobble I guess.

 

are you geting any spark?? tested fuel pressure??

3 basic must haves for a running engine.
spark
fuel
compression

 

CPS had a resistance of only 0.6 ohms and at cranking an AC voltage of .07...guessing it is now completely shot.

 

Its also got to have proper timing and air.

 

There is a releif valve on the fuel rail that supplies the injectors with gasoline. The fuel rail is on the driver's side of the engine, up on top.

There should be a small plastic cap on the rail that you screw off by hand. Underneath there will be a fitting that looks like a valve stem Schrader valve core.

Press in this core like you would a tire valve to let air out. If fuel squirts out, you have fuel present in the rail and it is under pressure.

Obviously, only do this on a cold engine and wear eye protection.

 

Sorry, I don't agree.

They can have intermittent failure from time to time as they get old.

The failures will get more frequent until one day, it fails completely.

I have personally had this happen on two different Northstar Cadillac engines. They have two CPS'. I had them replaced before complete failure.

The symptoms the original poster had sounds very much like what I experienced.

 

Ok checked the relief valve, was nice of them to point it sideways straight up might have been ugly, a lot more pressure than I was expecting, definitely fuel is present. I am pretty sure the plugs and wires are original so I really need to replace these anyways. Is there a plug that is easier than the others to get to and pull out? Is there a best way to test the spark. I think I have read orange spark/no spark bad, blue spark good, yes?

 

#2 or #3 should be relatively easy to get to.

You should have a nice blue spark when grounding plug to block and cranking.

Don't get shocked. The electronic ignition packs quite a whallop.

Check your plug gap; it should be .035.