89 XJ 4.0 intermittent stalling problem?

Intermittently my (5 speed) XJ will loose power to the point of barely idling (roughly) or flat out stalling. I found a broken vacuum line and fixed it but a week later it did "it" again. It will start and rev up to about 2K then drop to a very low, stumbling idle or stall. I have replaced the Idle Air Controller and Ox. sen., thoroughly cleaned all grounds/battery connections and done a major tune up with highest quality parts 6 months ago, prior to this (and between failures) it runs/ran fantastic. (Even good fuel mileage) Both times I farted around wiggling connections, tapping on the IAC and TPS with no changes for 10-15 minutes restarting it. It finally starts, smooths out and acts like nothing is wrong. Could this be a sign of a failing TPS? (I've gotten a replacement just in case to install.) The symptoms seem more like my new IAC is acting up, I replaced it due to an intermittent steady high idle problem. (It was in very poor shape.) These old Renix systems can drive me nuts, it won't let me find the problem, but it got me home! Thanks for any ideas! Neal

First read my old thread and links on it:

http://www.naxja.org/forum/showthread.php?t=905849

Have you used an ohm/volt meter and tested the grounds for the sensors? They are a common source of the high idle problem (an extra 8 ohms resistance on the TPS ground will add 3-400 RPM to the idle IIRC). It is very sensitive to a poor ground. You need to test the IAT and CTS temperature sensors hot and cold, at known temperatures, like in ice water and boiling water. I know the IAT sensor, when bad, can cause starting and random high idle problems. A vacuum leak can cause a high idle, and vacuum leaks on the intake manifold can grow or shrink as the metal heats or cools.

Thanks for the reply, that link is one of my permanent bookmarks for the last 10 months and much appreciated reading. I'm fairly well trained in electronics and know my way around meters and electrical theory/operations. (I was just getting OHM readings off the new TPS for baseline reference.) I've cleaned and checked grounds and connectors under the hood but have yet to "catch" any readings while the thing if failing. (Digital meter is now staying in console storage box permanently!) I replaced the gas cap with a new (Locking) Stant 10591 today as topics stated it could cause running problems. (?) The old one looked like condensation caused some rust stains in the center of it's vent holes. Finding the vacuum leak from the heater control line lead me to think that's all it was. (I'll be replacing even more vac. lines now, I replaced the entire emission/injection set with new from NAPA.) This last failure I did notice a loud kinda sucking sound but the engine was trying to idle @ 200 RPM's! Most of your topic deals with high idle and when mine "fails" it shoots to 2K upon starting but immediately drops to 200 RPM's or so. So far the 2 times it's failed the only thing common was the first time I had just put gas in it, it was below a 1/4 of a tank, and yesterday it was below an 1/8th but had been running for 30 minutes or so. I'm replacing the fuel filter and will cut it open to have a look inside.

Fuel level sensor may be faulty? May be running out of gas?

Well it ran fine without adding fuel for 20-30 minutes yesterday and for 15-20 before adding gas today, but I am wondering if it's at all related to low fuel. I'm going to keep more fuel in it from now on that's for sure!

I thought I had serious bearing problems developing earlier this year in my Daughter's 89, turned out the fuel pump is very loud if the fuel tank is at or below 1/3 full, LOL.:laugh3:

Good to know, my XJ only has 120K miles on it, my friend had it for about 12 years before I bought it from him. This forum has been a very valuable source of information.

Have you checked your CPS for AC output voltage? Since you have a 5 speed, the CPS could be coated in worn-off clutch material reducing it's signal strength.

Unplug the CPS. Probe the two wires in it's connector with your meter set on AC volts. Have someone crank over the engine while reading the meter. You should have .5 AC volts. Anything less than .4 and you can have intermittent starting problems.

Thanks, that's a good one to check! Both have been replaced not many miles ago but time wise it's been awhile as the previous owner drove it very little (For years.) when the cooling system started leaking. There could be crud building up from sitting then being used by me daily. It just runs so well MOST of the time. (I have a remote cranking switch I'll leave with the meter.)

b5blue said:

Thanks, that's a good one to check! Both have been replaced not many miles ago but time wise it's been awhile as the previous owner drove it very little (For years.) when the cooling system started leaking. There could be crud building up from sitting then being used by me daily. It just runs so well MOST of the time. (I have a remote cranking switch I'll leave with the meter.)

Click to expand...

I'm anxious to hear what you find.

No new failure yet but I have my meter and remote cranking switch ready!

b5blue said:

No new failure yet but I have my meter and remote cranking switch ready!

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Check the CPS now for the heck of it. A low output even when it's starting well now can indicate a weak signal which if all the stars are aligned just right becomes a no-start in the future.

Do you know the settings/reading I should get?

b5blue said:

Do you know the settings/reading I should get?

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Re-read Post #8.

My bad! Thanks! :cheers:

b5blue said:

My bad! Thanks! :cheers:

Click to expand...

Post up your test results.

Well 215 ohms across the pins. My "helper isn't up yet", he's working nites. I have a Renix page bookmarked that gives a 125 to 275 ohms reading as acceptable, will see what the A/C volts are soon.

Go with the AC volts for sure.

My son is off tomorrow we'll do it then! Pesky thing is purring like a kitten now, I drove it around for over an hour today. I've gotta go buy something to reach the bottom screw on the TPS to change it out, it's down to low for my drive set to fit. My buddy said to check for crap around the throttle plate, it could be hanging up every now and then. I cleaned the IAC port real well when I replaced it but didn't clean the whole throttle body.

Well we may be on to something now CPS has .2 volts cranking over, confirmed against both my analog meter on 15 ACV and Digital meter on 200 ACV settings. Who makes the best CPS for the money? (Jeep?)