Help with 1989 Cherokee XJ LTD rough idle and acceleration

Hi:

I'm hoping some of you XJ fans can offer some suggestions on a problem that just happened to me today for the first time.

First of all, the specs: 1989 Cherokee LTD (XJ), inline 6 cylinder, 4 litre, with 150k Km's (ie: relatively low mileage, or more accurately 'kilometerage' for the year).

I drove to Costco this morning, about a 15 km drive. Everything was fine on the drive there but when I started the truck up to come home (1/2 hour later), it was idling roughly and would stumble quite a bit while driving (but it didn't stall) - it felt like it wasn't running on all cylinders. I did make it home where I did a thorough once over of under hood wires and hoses and couldn't see any obvious problems.

The last time I drove the jeep was about 3 weeks ago. It's rained a lot since the so it could be that some water has got into the gas tank, but I've lived here in Vancouver for over 20 years and the constant rain we get for 9 months of the year has never been a problem before.

My XJ has never failed to start since I purchased in new in 89, however after it was a year old, it began to take a bit of cranking (say 20 seconds) to get it going. It's been that way ever since. I took it to several dealers all of who said that a bit of cranking was normal. Since it wasn't normal when the jeep was new, I've often wondered if the CPS was faulty, but in all the reading that I've done, a faulty CPS seems to manifest itself in intermittent start failures and I've never had a start failure.

Anyone have any suggestions on likely candidate for the source of my trouble? Thanks for any advice.

Regards,

Dave

Reply to
NT
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How long has it been since you changed the fuel filter?

Reply to
DougW

Hi Doug:

Thanks for your response. It's been several years, but not that many Km's ago - probably about 45k Km's.

I wouldn't expect a fuel filter to be fine, then 30 minutes later be clogged enough to start causing problems. I don't know - can say I've ever had a fuel filter problem before (that I knew of).

Reply to
NT

They do that. The filter will pass fuel at the proper rate up till the point it gets too clogged. Usually it shows as a lack of power or pinging under load/full throttle.

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I change my filter every 10,000 miles and it's astonishing what junk can come out of it.

The other possibility is heat-soak related. That can be a failing sensor or even the coil. When my coil died the engine would run when cold but would sputter and buck when restarted warm. Finally it just stopped. Oddly, the coil ohmed out just fine. But I threw another coil on simply because I couldn't find another reason why it wouldn't start.

Worst off if you get a new coil, you have a spare. :)

The CPS sensor can also exhibit heat related failure. At least on my ZJ it's on the exhaust side of the engine just behind the downpipe. The only real way to know if that sensor is having problems is with a scope.

Reply to
DougW

Reply to
Will Honea

Hi:

Update on my troubles...

I finally got around to taking the truck into the dealer today. The Dealer ran all possible sensor tests and checked the fuel pressure and spark and couldn't find anything wrong. The problem is still there (as far as I know). The problem seems to happen when the engine is thoroughly warmed up; on the drive over this morning, I guess it didn't warm up enough as the problem wasn't there - but it was there on the weekend when I was out on the highway. The dealer assured me that they warmed the engine up thoroughly and still couldn't find any problems.

Not sure what to try next. I'm a little leery about starting to replace sensors and the like when the analyzer showed nothing wrong. The Dealer suggested that it could be the CPS (which has never been replaced) but when checked it seemed to be working fine. If the CPS is being flakey - why wouldn't it have been apparent when the dealer warmed the engine up for the test? - Maybe it's a vibration thing?

I've discovered a little more about the symptoms.......

The engine idles fine and runs well at all speeds and acceleration rates when cold.

Once warm (either that or it's a distance thing), there is significant stumbling (feels like all cylinders are cutting out not just one or two) on mild acceleration, although, if I hammer it down, there will only be slight stumbling and it will accelerate like normal. Even though slight accelerating will cause it to stumble heavily, I can very slowly bring it up to highway cruising speed and it will cruise just fine - but hit a slight hill and it starts to choke!

Once this problem starts happening, the idle becomes very rough as well, but it doesn't stall (at least not yet), but it does sound as if it's not running on all cylinders.

I wondered if it could be some moisture in the gas. I was low on gas so I added a half tank and poured in some injector cleaner that is also supposed to take moisture out of the gas and that didn't seem to make any difference.

Anyone else have any comments or suggestions?

Thanks.

Reply to
NT

I find cleaning all the sensor's plugs and sockets with a good electronic contact cleaner, then sealing their skirts with dielectric grease does them wonders. They are low power computer signal plugs that are very touchy for corrosion. They even look clean, but a spray fixes them.

If the fuel filter is of unknown age, I would change it just because....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: N>
Reply to
Mike Romain

There's a couple of sensors I would replace. The temperature sensor for the computer, not the one for the gauge, and the ambient air sensor that's screwed into the manifold. It's been my experience that as these sensors age they vary on the side of reporting too cold of a value and your engine runs rich. You could also have a vacuum leak somewhere. On a car almost 20 years old it may take you a while to isolate the problem. I had a 89 xj once and it ran ok but when I replaced the o2 sensor it really made a difference. The Crank Position Sensor normally causes hard or no starts as they go bad. As Mike has replied clean and seal connections under the hood. Don't forget the ground straps.

On the Renix systems the tester can only give real time readings and since yours was not failing when they worked on it they didn't see anything wrong.

JoBo

Reply to
Jo Bo

Hi

I had similar problems on my lifted 93 YJ, 4.0 liter. It took awhile to troubleshoot, but after replacing ignition, wires/starter, emissions, fuel filter, fuel pump, it turned out to be the OEM factory computer. This was a year after the engine had been replaced, fuel injectors cleaned, new plugs, wires...

the funny thing was I took it to the local jeep dealer, who couldnt find out what the problem was. This was after they ran diagnostics, and checked emissions, and exhaust. and billed me an extra $200 just to tell me nothing was wrong.

the problems were it didnt matter what the engine condition was, after running awhile the computer would just short out, and I would be stranded. It literally was like a lottery as to how long the engine would run, it could be 2 or 20 minutes before stopping. Symptoms would be sputtering and misfiring and loss of oil pressure, and then shutdown.

I think the reals reason that the dealer couldnt find anything wrong with it was because, if its computer related and the computer talks to their computer, it creates a false positive effect. also, I think that experience and training was also key. People with little experience in diagnostics tend to blame the equipment and not themselves.

hope this helps.

NT wrote:

Reply to
batty505

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