Utahfisher

New to site...very helpful so far.

Have a 90 Cherokee Laredo AT that sat for 5+ years, 97k mi. Before I got ahold of it some youngsters tried "everything" and couldn't get it running. I took it over and immed saw no fuel - replaced tank and fuel pump. Starts! but won't stay running, dies within 15 sec. and has no spark. Has to sit long time, colder is better, starts and dies. If in garage overnight sometimes wont start at all..has to be cold to restart. ?

Here is the list of replaced, some by me some by youngsters.

1.Crank position sensor, once me once youngsters
2.Coil - youngsters
3.Ig control module (used) - me
4.Cap/rotor, plugs, wires - youngsters
5.New computor 2 times they say, left new one in -youngsters
6.Checked fuel pressure w/o gauge but seems good.
7.Fuel pump kicks on fine
8.Near relay new silver looking relay ? w bunch of wires -youngsters

Had a fairly experience XJ owner/builder look at it...he's stumped.

Any next steps very much appreciated.

Slick761

Man that has to be the ckp sensor. Sometimes new ones have a real problem with getting good signal. Unplug it and set your multi meter to ac volts. Probe the wires coming from the sensor and crank the engine. Should have .4-.5 ac volts.

That Jeep Guy XJMJ

The white object to the valve's right, held in place w/ a single screw in it's center 'nub' with the two orange wires going into it is the ceramic fuel pump ballast resistor. The ballast resistor is a source of intermittant stalling / hard start issues as well. Cheap to replace... easy to break.

you may be thinking "the fuel is ok" but something this cheap is worth a try

cruiser54

Do this^^^. and also check to see if the CPS is mounted properly and that it's wire harness isn't pinched somewhere if you get low voltage readings.

Utahfisher

Man..at work but feelin "sick" because I can't wait to get home and try these suggestions!

One thing we did ID is that when it kills, and it just takes a couple of minutes or less, it's like the key has been switched off...no sputtering etc. So we tested and there is NO spark when it dies..and won't return until it sits for some time, usually overnight and again colder seems better meaning if it's really cold outside it takes longer to elec. kill ?? weird.

Also noticed when I replaced the Ig Cntrl Mod when my socket brushed past one of the in-line fuse links there was a small arc over to the base of the coil. ? Batt was connected at the time. Thougth perhaps just voltage from the coil or could this be something?

BTW I'm elec/voltage reader pretty lousy ... but I can usually figure it out with a bit of detailed explanation.

cruiser54

Try these for testing the CPS.
Renix CPS Testing and Adjusting
　
　
Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.
Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue.
The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.
Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.
You should get a reading of .5 AC volts.
If you are down in the 3.5 range on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.
Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.
A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Utahfisher

Well if I'm testing right what I'm getting is exactly .2 AC volts from the CPS during cranking. This is a new unit from NAPA. Next step is to get an OEM from the dealer and give it a try.

cruiser54

A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Utahfisher

Ok pulled the OReilly purchased CPS and replaced with an OEM version. Jeep started right up and ran for longer than usual...so decided to test drive. Turned on lights and immed. died as if the light light switch were the ignition "off". Could be coincidental but I'll test again when the Jeep will start again in the morning. Also, sometimes the tranmission P position will not allow turnover and N will or..move the shifter back to P and wiggle a bit until key will start cranking. Could this be the whole problem somehow?

Also last "fixers" of the Jeep left several wires down when they switched out the CPU - I can't see an obvious place to reconnect so perhaps this is part or all of the problem?

cumo000

Damn. Carpet looks nice and clean.

cruiser54

Make sure your battery connections are good at the starter and block. Also be sure the ground strap is attached between the head and firewall.

Over at the engine dipstick tube stud are some ground terminations. Remove the nut, scrape the stud to bare metal, polish the terminations and reattach.

Utahfisher

Was there more than one "type" of ECU for this year. I've heard something about a half year change on this XJ. We've tried two new ECUs on this but it appears the ECU may not be working properly so..possibly the new unit wasn't matched correctly with the original. If there are variations of the ECU available would we be alble to find the right one by matching to the VIN?

tjwalker

This problem is related to your "neutral safety switch", a very common failure on the XJ.

Here are a couple of links to "refurbishing" the NSS. It often can restore full function to that piece.

http://go.jeep-xj.info/HowtoNSSrebuild.htm

http://bc4x4.com/tech/2010/jeep-neut...ty-switch-nss/

This NSS issue however will not result in no spark. You certainly have addressed the usual suspects regarding that one. I'd follow cruiser54's lead on this as he is pretty well versed with 87-90 XJs.

cruiser54

Barking up the wrong tree here. 87 to 90 completely interchangeable with the later years (ending in 428) providing the most power and response. There is no 1/2 year change.

What is the CPS output when cranking right after it stalls with the new CPS?

Caish

Take your tester and measure the battery voltage, off then with it started.
Then again with the lights on and running.
I would also clean ALL the ground connections.
Big cable to the block, small strap from back of engine to fire wall, and the second small one to the fender.

Off voltage should be 12.6 approx, running voltage 13.6 to 14.0, it shouldn't drop when you turn the lights on too.
Your under dash wires looks almost as gnarly as my AMC Jeep. AMC was never known for good wiring.