High RPMs.....Aaaaaaaagggghhhhh!

Hey Y'all.....

.....I'm a newbie to the site but have owned and been slowly fixing up/ restoring my '87 XJ Laredo with the 4.0 for two years.......I've had intermittent problems with stalling, startup and occasional high rpms at idle, but lately the problem is consistent.....I had a mechanic go through my fuel and electrical systems about two months ago and told me it appears that the signal from my ECU may be faulty, but that his diagnostic computer is unable to pinpoint why the signal for the idle circuit stays open so long and only closes occasionally.......I've replaced sensors and had the TPS adjusted more than once during different repair phases.......I was wondering if anyone else had or has an older XJ and gone through this same problem? Lately, after the engine is warm, when I start it, the idle revs up to about 3500 rpms and stays there until I turn the key off.....after a couple of tries it starts normally, does anyone think I should replace my ECU? If anyone has some feedback, I would sure appreciate it. Thanks.

i had it for years and couldnt figure it out until someone on here told me to replace the TPS and voila....problem solved, now it idles perfect at start, cold or hot.

if the tps is replaced make sure it is set to 12%

Yeah, the TPS has been replaced......and the MAP sensor, and the idle speed stepper, and I'm tryin' to figure out if it's a computer issue.....PCM? ECU is suspect......need a reference.......

I had the same problem--- it was the TPS.

When you replaced the IAC, did you clean the passages in the throtte body which run to it as well as the seat where the point contacts the idle air circuit? That may be worth a shot.
Tony

Actually the most common problem causing high idle is a vacuum leak. They can be quite intermittent and cause a multitude of symptoms. On models with the vacuum bottle mounted behind the bumper are notorious about having the line running under the battery break. The ones with the vacuum bottle just in front of the heater blower tend to have a problem with the hard lines running behind the motor on the firewall.

A can of carb cleaner can be used to locate the source of the leak.

My vote is for dirty contacts on the engine harness connector mounted on a firewall bracket near the brake booster. It's a real pain to remove and clean each of the contacts but it solved my problem.

PaulJ said:

My vote is for dirty contacts on the engine harness connector mounted on a firewall bracket near the brake booster. It's a real pain to remove and clean each of the contacts but it solved my problem.

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I agree fully with this one... solved my problem also.

You dont really need to pull each terminal from the block... simply clean it with brake clean,,, it will wash all the old contact grease away along with the dirt.

after it has dried, fill each terminal hole with silicone die electric grease, doing so from the wire side of the connector, smear the face of the connector also before you reassemble the two halves.

Thanks, guys........I haven't cured the problem yet, but it does seem to be related to the wiring harness connectors, and the fact that some of the wires are pretty soft from getting hot over the years. I'm in the process of cleaning the contacts and using the grease in the connectors, and so far the idle problem hasn't come back!

Appleby said:

Thanks, guys........I haven't cured the problem yet, but it does seem to be related to the wiring harness connectors, and the fact that some of the wires are pretty soft from getting hot over the years. I'm in the process of cleaning the contacts and using the grease in the connectors, and so far the idle problem hasn't come back!

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Also watch for drips and leaks from the master cylinder onto that wiring harness, brake fluid will eat right thru it.