High Idle after warm

96 XJ 4.0

Starts just fine. Afer about 5-6 minutes of running, the idle slowly starts to rise and stays there once it hits a certain point. It rises slowly enough that you don't even really notice it. I do not have a tach, so I don't know what that point is. It has not thrown any codes.

While running(idling high), I unplugged the IAC. When I did that, the idle rose significantly. When I plugged it back in, it went back down to the high idle.

I have sprayed all vacuum lines with brake cleaner and found nothing. I have removed all of the vacuum lines and plugged the vacuum ports individually and that did nothing, still idles high.
After I did all of this, the CEL is now on. I would imagine that is due to me playing with AIC.

I can not find a method anywhere to test the AIC. Is there a way to test it?

I found a thread that was similar, and it was found that a grounding problem was the culprit. All grounds appear to be fine.

What next?



TIA

Hale

My guess is exhaust/intake manifold gasket is either loose or needs replacement.

Below is one of many threads containing some great information on this subject.
http://www.naxja.org/forum/showthread.php?t=81838&highlight=High+idle+issues


Hope this helps,

I replaced my exhaust manifold probably 6mo. ago. At that time, I installed a new gasket. It ran fine up until now.

Best I can tell, It appears to be just fine.


Hale

symptom..idle increase as it warms up... your vehicle is doing the opposite of normal....
so maybe whatever changes fuel/air ratio under different conditions is a problem..kind of like a choke kickdown problem that won't kickdown?
I use an Autoxray scanner i got off EBAY for $60...shows me when I am in open or closed loop operation, and what most of my sensors are saying along the way..certainly helps to eliminate the obvious at times. If I was gonna shotgun a part it would still be the IAC.

MAP checks out OK according to my FSM.
IAC checks out OK according to my FSM.
Intake air temp sensor checks OK according to my FSM.

TPS acts goofy according to my FSM. However, if I check another 96' 4.0 XJ(that runs fine), it checks the same.

I sprayed brake cleaner all over again, and still nothing.

I plugged off the intake with my hand and it ran just fine.(I did have mechanics glove on.)


C'mon guys, I need help here!

Hale

My vote is still exhaust/intake gasket . A refined guess would be one or more bolts have loosed up.

xj4fun said:

My vote is still exhaust/intake gasket . A refined guess would be one or more bolts have loosed up.

Click to expand...

Well, I sprayed brake cleaner all the way around it and there was no change in idle. No spitting or sputtering, no increase in idle...nothing. Gasket appears to be seated and tight, all the way around.:dunno:

Hale

Keep it simple and cheap, work to the most difficult and expensive. Both xj4fun and "M" have their points. Just because you put it on 6 months ago, doesn't mean it's not loose now. Not familiar with wiring, but with the cold setting in, is it possible wires are mixed up on open/closed? Just a guess there. It really is odd that this thing kicks up after it warmed. Don't have definitive fix, just echoing other's responses and adding my 2 cents to the process.

Haleyes said:

I sprayed brake cleaner all the way around it and there was no change in idle. No spitting or sputtering, no increase in idle...nothing. Gasket appears to be seated and tight, all the way around.:dunno:
Hale

Click to expand...

Just to verify, you checked the bolts under the intake manifold as well? I still suspect the gasket. The reason being the problem appears to be related to warm up. The block and manifold are made of differing metals that expand /contract at differing rates. I've dealt with several high idle issues on 4.0L. In all cases with your symptoms, the problem amost always related to the intake/exhaust manifold. Granted all of these were on Renix or OBD I XJ’s.

Grounds are certainly high on the list as well, which you have already indicated you’ve checked. These can also be affected by temperature (but rare) with corrosion being the common cause.

Can you further elaborate on this statement below? This implies the possibility of an intake leak. However I wouldn’t recommend testing this in this manner. Could be dangerous.

'I plugged off the intake with my hand and it ran just fine.(I did have mechanics glove on.)'

I plugged off the intake with my hand and it ran just fine.(I did have mechanics glove on.)
If, by this you mean you sealed off the throttle body completely, and it still ran fine? You have a big vaccuum leak somewhere, did you spray the brake booster for leaks?

I've got a 99+ style intake to put on. I'll grab a new gasket and see what happens. Hopefully, I can get to it sometime this week. Daymn, I wish I had a garage!!!



Hale

Well, you guys can call me a liar.

I went out and braved the cold nasty sleet, and put a wrench on all of the intake and exhaust bolts/studs. Let's just say that some were less than the disirable tightness.

It has been running for 20 minutes now and it's purring like a kitty......just like it's supposed to.


Thanks guys.



Hale

Hale - Super glad this worked out .

I hope I didn't come off as arrogant sticking with my beliefs. I was just hoping you would check this idea 110% before spending money, time etc on other possibilities.

The first high-idle I dealt with was 90 XJ. I did 'everything' over 3 months; Cleaned everything, replaced most of the sensors, added new grounds, manually changed the idle by altering the factory set screw, etc. I think I dropped about $280 in sensors. I saw checking the gasket on . Within 10 minutes I found two loose bolts and problem solved

Glad this worked out with having to spend any money..

xj4fun said:

Hale - Super glad this worked out .

I hope I didn't come off as arrogant sticking with my beliefs. I was just hoping you would check this idea 110% before spending money, time etc on other possibilities.

The first high-idle I dealt with was 90 XJ. I did 'everything' over 3 months; Cleaned everything, replaced most of the sensors, added new grounds, manually changed the idle by altering the factory set screw, etc. I think I dropped about $280 in sensors. I saw checking the gasket on . Within 10 minutes I found two loose bolts and problem solved

Glad this worked out with having to spend any money..

Click to expand...

Not at all.
I apperciate it.
:thumbup:

Thanks again.



Hale