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XJ stumble & stall

W

wgregt

NAXJA Forum User
Location
SoCal
My XJ has had a little stumble when started up on rainy, cold days (or what passes for them here in SoCal). It starts fine, but runs noticeably non-smooth. Once I get it moving it runs better usually. Only happens after a rain or when it's cold in the morning.

Yesterday it lasted all day. It cut off on me 10-12 times (always restarted fine afterwards) in rush hour traffic, stop and go. When stopped, the idle seems fine (around 750 RPMs or so), but when you push the pedal to move again it stumbles, tach drops for a split-second, and then recovers. Many times it would not recover and stalled...and it's an automatic trans.

Also stalled out many times when backing out of a parking space later last night. It really feels like a teenage kid learning to drive a stick-shift and stalling it out.

I don't think it's a gas-quality issue, since it's OK @ speed when the road opens up.

I also heard a little pop/burble from the exhaust a few times when it stumbled and recovered.

** forgot protocol: It'a an 88 XJ Limited, 4WD, 4.0, 300K miles or so.**

Any ideas where to start looking next week? There goes my xmas break....
 
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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 as the CPS will test good when in fact it is bad.
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 .35 AC volts range or lower 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 the upper mounting hole, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

　
　
Revised 07-30-2012
 
Anytime you hear the pop, go check the vacuum line at the side of the throttle body that goes to the MAP sensor. It tends to pop out when there is a backfire.

Your problem sounds like a bad TPS sensor signal at idle, which is normally caused by age and wear on the TPS, it is a wear part, or by a poor sensor ground that screws with the low voltage readings from the TPS near idle, or BOTH!!!!!

The TPS and MAP sensors (data to the ECU) are very critical to controlling the Air/Fuel feeds up front during acceleration, as the O2 sensor only senses the back end of the process, the TPS and MAP see what is going into the engine first, the O2 sensor last.

Also a failing CPS or weak spark (bad rotor, cap, wires and plugs), can sometimes cause the same symptoms.
 
This is weird: Driving home tonight there was no stumble or hesitation at all. Also, the weather was dry. Coincidence? It always seems to idle rough/hesitate only when it's wet/cold. Would the TPS or MAP sensor be worse in these conditions? (I'm sorta ruling out he Crank Position Sensor as it was recently replaced.)

Would plugs/rotor/wires/cap be worse in wet/cold weather?
 
The factory recommends plugs, cap, and rotor every 30K miles, wires at 60K. At the dealership, we just began replacing the wires also at 30K because swapping them over to the new plugs and caps many times led to premature failure of the wires.

That said, yes, bad wires act up when the humidity is high. Get the basics done first before chasing sensor issues.
 
Didn't know you were a tech @ a dealership. Can I go non-factory on all of these items, or is the OEM stuff any better and worth the extra cost in this instance, in your opinion?
 
cruiser54 said:
That said, yes, bad wires act up when the humidity is high. Get the basics done first before chasing sensor issues.
Click to expand...

Also dirty wires, especially if they have had antifreeze, oil.... leak on them.....

IIRC condensation was always an issue under the used caps on the old rigs (pre 80s), before the caps got bigger and voltages higher? I had dry brass dust on my 87 XJ after 30,000 miles that even with out humidity was causing arcing and leakage of spark current.
 
When it's acting the worst, pull the distributor cap, see if there's any moisture at all inside. Maybe a spray of WD40. If it helps, fine. If not, maybe wires, as has been suggested. This problem was common back when there were breaker points under dist. cap.
 
Ecomike said:
Also dirty wires, especially if they have had antifreeze, oil.... leak on them.....
Click to expand...

Really? Just having oil, etc dripping on the wires? I can see if they had a break/crack in the insulation, but the wires themselves need to be clean in order to work well?

If so....news to me.
 
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It def. feels like a moisture thing. Drove it today in the dry, and it ran 100%. No stumble, no hesitation. This must be the often heard of, and yet rarely documented case of an XJ fixing itself, without even opening the hood.
 
wgregt said:
Really? Just having oil, etc dripping on the wires? I can see if they had a break/crack in the insulation, but the wires themselves need to be clean in order to work well?

If so....news to me.
Click to expand...

Yes, especially near the spark plugs, and the spark plug towers and wire cover on the plug need to be clean. Antifreeze is loaded with salts, so a little condensation can make the cover very conductive, and ground some of the HV current. If you get a weak spot in the wires where some of the spark can leak out, and there is conductive crud on the cover, it has a short circuit path to ground across the length of the wire, a fast and easier path to ground that avoids jumping the higher potential gap across the spark plug gap.

I was flushing my heater core last week and got antifreeze all over the new plug wires (easy to do, LOL), and went to the trouble of washing them down with tap water, then DI water, then fast dry brake cleaner.
 
wgregt said:
It def. feels like a moisture thing. Drove it today in the dry, and it ran 100%. No stumble, no hesitation. This must be the often heard of, and yet rarely documented case of an XJ fixing itself, without even opening the hood.
Click to expand...

Gremlins like to hide during good weather, and run amok during real cold or real hot, rainy weather when it is harder and more painful to look for them. It is called "Max Pain", and is one of the Gremlin bylaws of Human torture, LOL!!!
 
Any tips to swapping out the cap and rotor? Wires (one @ a time) and plugs I can handle and have done before.
 
The rotor is a no brainer, pull it off, push the new one on, it is under the cap. make sure it is seated all the way down and locked in place.

If the newer jeep distributors are like mine, there are two philips head screws, and the cap comes off. I take the cap off, then swap on wire at a time on the cap from the old cap to the new cap, then get the cap on (make sure the screws line up!!!!!!) and then tighten the screws, also real easy.
And while you are in there check for oil (shaft seal leak down under), and check for excessive shaft play side to side, both signs of needing a rebuilt distributor.
 
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wgregt said:
Didn't know you were a tech @ a dealership. Can I go non-factory on all of these items, or is the OEM stuff any better and worth the extra cost in this instance, in your opinion?
Click to expand...

Go to Napa and get Belden wires along with their premium cap and rotor which should have the brass terminals. You'll be fine.
 
Do yourself a favor. Take the existing rotor to the store with you, to match up to a new one. Somehow there's two different items, and it may be a crap shoot. I've had a problem with my '92, '96, and the long gone '98 ZJ. Buy a good cap and rotor, too. There's junk out there, with contacts that burn up faster. While the cap is off, see if there's any play in the shaft. I had the dizzy shaft go bad in my '96, wiggling around and tearing up the contacts inside the cap. It layered the inside of the distributor with a layer of fine metal dust. Maybe I'm lucky, but in forty years of wrenchin', I'd never come across that before.
 
The junk parts use aluminum (silver color) the best ones use brass for the inside collectors in the Caps.
 
#1 is usually cast #1 on the top of the cap, so line it up at 5 o'clock and tighten the 2 screws, and the rest can not be installed in the wring place as far as the cap and rotor go IIRC.

The trick is make sure the 6 wires are in the right spots.
 
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