92 Jeep Cherokee Laredo, 4.0 H.O. I6

So, my wife took the jeep to the store and a block from our house it died and wouldn't start. Hasn't started since. It's getting fuel at the rail (not sure how much psi) and it's getting spark, and it cranks like a champ, but will not turn over, even though it sounds like it kinda almost wants too.

The only time it started was when I had the wife crank it while I was jiggling the wires for all the sensors by the throttle body. It started for like, half a second. Then I replaced the crankshaft position sensor (with one from the junkyard) and it started up for half a second again but that was it.

I had to replace the computer (with one from the junkyard) a while ago because I apparently fried it while installing the alarm. Even though it fried before I ever hooked the alarm up to the battery.

Then, while replacing the valve cover gasket I apparently jiggled the wire harness too much while moving it out of my way because ever since then its had the engine light on with codes 12 and 24.

(12) Battery or computer recently disconnected
(24) Throttle position sensor over 4.96V

It drove for about a month before it died the other day. When I disconnect the TPS the code 24 is gone, but when I reconnect it it comes right back, so I'm gonna go buy a brand new one after work, but I have a bad feeling that even though the TPS seems to be bad, it probably isn't the cause of the jeep not starting.

 

UPDATE: New TPS did nothing, but the code 24 is still there. How can a brand new TPS have problems with its voltage?

 

The code is for the entire "circuit", not just the sensor. Have you performed a comprehensive inspection of the wiring for the TPS circuit?

I would also get a can of electrical contact cleaner, thoroughly spray the TPS connector, add a dab of dielectric grease to the reconnector and reconnect. Connectivity is also in play here.

The fact that it started when you were "jiggling" wires around the TPS might be the big clue here. You could easily have a wiring problem. Sometimes they can be tough to isolate.

Also, do you have a strong, BLUE, snapping spark? Yellow/orange/white indicates a weak spark which may not be strong enough to start the engine.

 

Spark seemed blue enough but I'll double check. Taking a look under the cap was one of the things I planned to try tomorrow.

As for the wires, I'm afraid you're probably right. I hate working with wires in the cold, but I'm gonna have to just grin and bear it tomorrow and follow those TPS wires around till I either get to where they start, or find an issue.

Side-note: I'm not expert at FI vehicles, but what should it look like when you push the leak valve on the fuel rail with the engine off? I did it while my wife was cranking and fuel shot a good distance out of the engine bay, but I did it again with the engine off and it only shot up about 2 inches for a sec then the pressure was empty.

 

Unfortunately, it's really not possible to judge fuel pressure without a gauge. You can rent one from many parts stores inexpensively. For your 92, you need approximately 31 psi for the engine to fire up. And yes, double check that spark. It is possible that you have a couple of things going on......

 

i recently went through this about a year ago. i bought a new tps and it still did what urs is doing.
xj's are nortorious for grounding issues. i had codes thrown too, but never read them. i did all the stuff below and the problem went away. i did have to buy any parts. i just did everything i'm about to tell you to do. a bad cps will cause this too. so you may want to clean/replace it. but only after this other stuff is checked. why spend the money.

i agree you should confirm fuel pressure first. electrical contact cleaner would be my next task once confirming fuel pressure. also check all grounds even the one to the cpu and chassis. clean the grounds with sand paper or emery cloth and apply diaelectric greese the them too.
once all the connectors are cleaned add dialectric grease on all electrical connectors. while doin this check for any loose wires in their connectors too.
that being said and done;....

if you are getting week voltage with a new tps you may have a cpu problem. the voltage being sent from the cpu is not enough to power the tps. obviously.... i never buy electricals from jy. you would be surprized how much moisture sets in when vehicles are not moved or run. electrical components are the first to go. this is why jys never warranty electrical components.

good way to tell is to remove your cpu and see if it can be pulled out of its case. then check for signs of moisture. any white filmy residue is a good indication of failure. while you have the cpu out flip it over and check the pathways for burn marks. if it were me these are the things i would do first.

judging from the codes you are getting if the cpu is not able to be inspected and you did all the above; as previously suggested; just buy a new cpu.