High Idle Question

I'm going to look at a XJ for sale that the owner says has a very high idle. He isn't able to tell me how high exactly he just said very high and he is scared to let it run for more than a second. Reading through the threads in a serach this appears to be a common problem with Renix XJ's and if it's a 97 and later the IAC is the main culprit. In all the threads I read the IAC was at fault when the idle was just a little high (no more than 1200 RPM) where as this appears to be idling much higher. I'll be taking some spare parts to try and swap out so what should I bring besides an IAC? Any ideas?

99 Classic 4X4 231 AW4

The most common problem isn't the IAC, it is normally a vacuum leak. If the rpms are really high, check the hose to the brake booster. Crimp it and see if it improves.

Check throttle body gasket
Check intake manifold gasket.

a while back, after an engine swap, we ran into a similar situation. the line that makes me think its the same thing is 'scared to let it run for more than a second'. it doesnt sound like a SLIGHTLY high idle, those arent scary. im thinking an uncontrolled roaring 4.0 singing at 3500+ rpm on startup. which can be scary.

MY PROBLEM, was the intake manifold was sitting on the alignment dowel, holding it off the head. (the pin got pressed in, it wasnt obviously sitting off the head 1/4", but you could stick a putty knife between the two at the gasket..

maybe ask them if they recently had engine work done by noobs? if they dont mind, try sticking a thin putty knife between the manifolds and the head. if thats the case, buy it on their mistake.

Hell, start it and see how it runs, even if you have to shut it off real quick. If it sounds good, and is priced right--considering the serious high idle, wink-wink--then buy it. Things that could cause the high idle are NOT going to be that expensive to fix, maybe a little sweat, but not that expensive. IMHO.

joe_peters said:

Hell, start it and see how it runs, even if you have to shut it off real quick. If it sounds good, and is priced right--considering the serious high idle, wink-wink--then buy it. Things that could cause the high idle are NOT going to be that expensive to fix, maybe a little sweat, but not that expensive. IMHO.

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I think the price is right regardless. It's a 99 classic and the guy wants 3500 which I'm sure I can get down to 3200 at least. Here is the list of goodies. RE 3.5" lift. 31" tires. Rock sliders. 231HD after market TC. Rock Sliders. ARB bumper with 8500lb winch and a roof rack. For 3500 I think it's worth it just for the goodies on it and I can worry about the high idle once it's in my driveway.

kdailey4315 said:

I think the price is right regardless. It's a 99 classic and the guy wants 3500 which I'm sure I can get down to 3200 at least. Here is the list of goodies. RE 3.5" lift. 31" tires. Rock sliders. 231HD after market TC. Rock Sliders. ARB bumper with 8500lb winch and a roof rack. For 3500 I think it's worth it just for the goodies on it and I can worry about the high idle once it's in my driveway.

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Get R Done!

Vacuum leak is first suspect for a "scary" high idle. Good advice above as for things to check on that.

A slightly high idle might be the idle air control, but that doesn't sound like the case here.

Thanks for info. I'll definitely check for a vacuum leak.