Warm XJ wont start

i have a 93 4.0, it starts fine, and runs pretty good (feels a little weak), but after you shut it off, it wont start back up. i took it to a shop, and they said no codes came up, and they didnt know what was wrong with it. they said it might be the computer. i figured id ask here before i start throwin parts at it.

thanks

Hold off on that computer. Replacing the computer should be a "diagnosis of exclusion", meaning that everything else has been ruled out before you condemn the computer. Yes, the computer can of course fail, but generally speaking they are pretty hardy in comparison to other electrical components on the 4.0 engine.

If it starts good when cold and you only have symptoms when warm, you are experiencing a "thermal" (heat related) failure.

It is very important that you test for spark when the engine won't start. Testing has to be done when symptomatic, as these components may test as good when the engine is cold and starts. Has that been done? Pull a plug, keep it attached to the plug wire, place the plug electrode near a good engine ground, have a buddy crank the engine while you watch. Knowing whether you have NO spark or WEAK spark when the engine won't start is absolutely job #1.

If you have no spark, the #1 suspect is the crankshaft position sensor. Without the input from that sensor, you will have NO spark and you will have NO fuel going to the fuel injectors. I've included a link a thread on the crank sensor below. The crank sensor is probably the #1 sensor failure on the Jeep 4.0 engine.

The #2 suspect for thermal failure / loss of spark is your ignition coil. It can be tested with a meter and a manual. You test for both primary and secondary resistances.

Although more rare, it is possible for the camshaft position sensor (located inside the distributor) to fail thermally as well. That too can be tested.

Don't throw parts at this on a hunch. Terribly ineffective and gets expensive in a hurry. Test your way to a solution! Good luck and let us know what you find!

Link to crank sensor information:

http://www.naxja.org/forum/showthread.php?t=1044073

The computer is about the least likely of many suspects here, especially if there is no code, so I definitely concur with birchlakeXJ on this. Test methodically before throwing in a new computer.

Consider especially the ignition system, because this is one component which is not monitored by the computer, so secondary ignition failures (coil, cap,rotor,plugs) will not throw any code on a 93.

I'm about to go away for a few days, but if, after getting better advice, you find that it really is the computer, or can't determine any other cause, and if yours is a 93 4.0 with automatic, then PM me next week. I have a computer from a 93 which I don't expect to need.

I should add that one of the reasons I have that extra computer is that it was a suspect in a problem that turned out to be a defective splice in the wiring harness, and the injector wiring harness of the 93 seems to be susceptible to this. If conventional diagnosis and testing doesn't come up with anything, you might do well to get a multimeter and start testing the wires for continuity.

I had a five volt power supply short that never did throw a code, the CPS was toast. In the beginning it was a periodic hard to start, then it progressed to what seemed like a breakdown in the ignition system, occasional stalls, bad miss, loss of power etc, eventually no start at all.
The CPS in the later model XJ's (after 90) can be hard to test. The book says it is a simple continuity test, the reality is the other two legs of the CPS can ground or partially short to ground.
I'm fairly certain the sensor (supply) voltage for many of the sensors share a common driver or stages of the same driver. A partial CPS short caused my sync sensor to drop to 3 volts input.
I'm fairly certain the TPS and the CPS share a 5 volt driver and from my experience a partial short in one sensor can affect another. As well as cause a chain reaction, were one failed senor causes another to give faulty information.

Like mentioned, the coil has to be in the back of your mind, I've seen many that malfunctioned when they got hot on many different kinds of cars and trucks.

Kind of primitive but effective, splash a little water on the CPS when you get a hot no start, see what happens. Use a rag and squirt a little fuel out of the fuel rail test port and see if you have pressure and/or vapor in there.

My check engine light went out, I got no light when I first turning the key to run. Thought it was the bulb, turned out to be the CPS. I surly don't know exactly how the computer works and what is interrelated in the circuits and wiring. I do know that CEL codes and even readers can lie to you.

thanks eveyone for the great info! ill start checking things out soon.

thanks again