AkiyoXJ

So I picked up a cheap little xj and soon after it quit starting but would crank. After some reading I found out about the cps and replaced that with a pos BWD one. A week later it quit working unless I would wiggle it before starting and read that I need a mopar sensor. I finally got a mopar one and it worked flawlessly for 3weeks until today. It will crank but not turn over.

In short what could be causing my jeep to kill my cps prematurely or is there something else that may be my problem? Thank you for any suggestions!

Its a 1991 automatic xj with about 131k miles.

Firestorm500

Check to make sure you're getting fuel to the injectors, and that you have good spark.

TwoXJS

You'll probably want to make sure your CPS is working correctly and sending the right voltage to the computer. There's a way to test it, but you'll have to google it unless someone posts it since I don't remember. No CPS signal = No spark. If something really is killing your CPS's, I'd start checking for a broken wire or something out of the ordinary.

AkiyoXJ

My injectors are all good and I recently put new plugs in and gapped them but il pull them out to double check. Thank you for the input.

AkiyoXJ

I remember reading about checking cps but il have to look it up again. My buddy and I also noticed it has a junkyard ecu in it when we where checking it out last time so that's why I might have a bad ecu.

DFlintstone

Supper simple to test a CPS in 87-90. Pretty tough with a meter I can't afford for 91 on. See if you have spark. See if it spurts fuel into the rag you are holding there when you depress the dealie there in the schrader valve on your fuel rail.

Check the connections on the fuel pump ballast resistor. That's the ceramic thing there by the air cleaner. Power from the rear of that goes straight to the pump, you can bypass it. It's mostly only there to quiet the pump a bit.

If you have spark it's not the CPS!

You might look at Cruisers stuff found in my Signature. ICU/Coil contacts may well be the same. Also the ground's at the dipstick are important for about everything.

DFlintstone

Electronic Control Unit till 90, then Power-train Control Module, (PCM), 91 on.

scottmphoto

Refresh your grounds. It can't hurt and is free.

RTorrez1

Is it throwing any codes?

AkiyoXJ

Sorry for the late reply I've been really busy. Anyway I replace the auto shutdown relay because I was told that could do it and it started perfectly the first 3 or 4 times and then went back to not starting. I took the cps out and cleaned a tiny bit of dirt off of it and it fired a in what sounded to be a couple cylinders and then went back to not firing at all. Then I unpluged the cps cranked it for a sec and then plugged it back in and cranked it then gave it gas because it fired a little again and it started. Im going to refresh my grounds like scottmphoto said. What ever it is its keeping it from sparking.

Rtorrez1 It isnt throwing codes at all.

DFlintstone

Well darned if I can't find "testing HO CPS" . Anyway it takes a scope or a very spendy meter.

Edit, OK, there is stuff in here somewhere on checking an HO CPS> http://www.cherokeetech.com/VBull/showthread.php?2129-Engine-Computer-Diagnosis-Videos

AkiyoXJ

Well I tested the resistance on the cps and it reads all good I just can't seem to find a way to test the plug that goes into the loom to see if there is a short in the wire.

Radi

In general you'd be removing the connector at the ECU and measuring from one end to the other to check continuity, then from one pin to the other to check for shorts, both while giving the harness a good shake. I don't have a drawing for your year so can't tell you which plug and pins @ the ECU end belong to the CPS.

cruiser54

Fuel pressure is being ignored?

Firestorm500

Yeah, check my #2 post.

Do the simple things first. No need to get complicated yet until basic things are verified.