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Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

NeXJ

NAXJA Forum User
Location
Los Angeles
I have no idea what's happening - though I wonder if it's related to my cooling system having a blockage - been only using my vehicle for short local trips as a result. previously - throwing the hot air blower on has kept things to temp for trips over ten min... but suddenly now my revs have been climbing to 3000 plus at idle once i've been driving and engine running for five minutes or so... I'm going to take it in to have someone fix up my cooling system (not something i relish doing myself) but I'm wondering where this behavior might be coming from. Anyone want to take a stab in the dark?? I checked the TB cable etc - and no issues there. it's not moving - the rest position doesn't change...
 
Could be that you Idle Air Controller solenoid that is attached to the passenger side of the TB is dirty and hanging up. IIRC the IAC is held on with two T15 torx screws. You can clean the TB, IAC and ports with throttle body cleaner.
 
Could be that you Idle Air Controller solenoid that is attached to the passenger side of the TB is dirty and hanging up. IIRC the IAC is held on with two T15 torx screws. You can clean the TB, IAC and ports with throttle body cleaner.

hmmmm - okay thanks sounds like an easy enough thing to check... open to other ideas too... in case there are others
 
one thing confuses me however - how does the IAC increase the fuel flow to the TB? Surely it must be doing that if my revs are off the chart... seems the engine can't rev higher without more fuel flowing? or could this possibly be a computer/fuel injector problem?
 
The IAC is allowing more air in and the computer is on a rich fuel mixture due to cold engine this higher revs. Pull the throttle body, clean with carb cleaner, reinstall. Fixed my issue with the same problem. You could also try using amsoil powerfoam or whatever but I'd rather all that gunk not go through my engine.
 
OK thanks! were you going up so high as well? (3K ?) or just 1200 rpm or so?

though mine's not doing it from a cold start - only when it gets hot enough ... anyway I'll take your advice - just trying to make some sense of how this works...
 
Mine did it occasionally when warm. It would start then slowly rev to about 1800. Cleaned it good and never another issue. Your problem could be something else, never know. But a $5 gasket and some carb cleaner or TB cleaner is worth a shot.
 
The IAT/MAT Intake (manifold) Air Temperature sensor can do that when it goes bad. You can test the hot and cold resistance of the sensor with an ohm meter when the power/ignition is off versus the OEM specs in the manual. The data is in many old threads here.
 
OOPS! thanks guys for the additional responses ... for some reason I didn't get any notifications about them since... I haven't touched it yet but it's nice to have the additional advice... thanks again and sorry for the delay
 
High idle is commonly caused by vacuum leaks. Do a quick search on tracking them down.
 
High idle is commonly caused by vacuum leaks. Do a quick search on tracking them down.

well - i'm not sure if i'd call it 'high' so much as 'accelerating' or 'runaway' idle - it just keeps increasing and increasing - i had to cut the motor a few times when it got to be around the 4000 plus mark - i'm sure it would go as high as it needed to throw a rod though... if i just left it...
 
I doubt this is a vacuum hose leak since it is not there at start up, and only starts it terro campaign after a 5 minutes. Sounds exactly like an issue I had with a bad/dirty IAT air intake temp sensor on my 87 Renix. Read my earlier comments on testing it hot and cold and comparing it to the Factory specs.

But I guess it could be a growing vacuum leak as metal expands on the intake manifold like Old_man is suggesting, I think.

Also, a bad TPS could do this. It too can be tested hot and cold at idle for huge changes in the TPS resistance (engine off,cold and hot)

You need to change your preferences for notices of posts.

I have no idea what's happening - though I wonder if it's related to my cooling system having a blockage - been only using my vehicle for short local trips as a result. previously - throwing the hot air blower on has kept things to temp for trips over ten min... but suddenly now my revs have been climbing to 3000 plus at idle once i've been driving and engine running for five minutes or so... I'm going to take it in to have someone fix up my cooling system (not something i relish doing myself) but I'm wondering where this behavior might be coming from. Anyone want to take a stab in the dark?? I checked the TB cable etc - and no issues there. it's not moving - the rest position doesn't change...
 
well - i'm not sure if i'd call it 'high' so much as 'accelerating' or 'runaway' idle - it just keeps increasing and increasing - i had to cut the motor a few times when it got to be around the 4000 plus mark - i'm sure it would go as high as it needed to throw a rod though... if i just left it...

Throttle return spring broke, sticky throttle plate, sticky throttle cable, or defective IAT !!
 
Throttle return spring broke, sticky throttle plate, sticky throttle cable, or defective IAT !!

last time it was acting up I checked the throttle PRETTY thoroughly - and it didn't seem to have much to do with the return spring or cable - though I didn't check internally... yet
 
I doubt this is a vacuum hose leak since it is not there at start up, and only starts it terro campaign after a 5 minutes. Sounds exactly like an issue I had with a bad/dirty IAT air intake temp sensor on my 87 Renix. Read my earlier comments on testing it hot and cold and comparing it to the Factory specs.

But I guess it could be a growing vacuum leak as metal expands on the intake manifold like Old_man is suggesting, I think.

Also, a bad TPS could do this. It too can be tested hot and cold at idle for huge changes in the TPS resistance (engine off,cold and hot)

You need to change your preferences for notices of posts.


yeah I think I may just throw a new IAT sensor in it (if i'm taking it apart anyway) and see what it does ...
 
had to go to pep boys to find a T20 - as my torx male set seems to be on vacation somewhere in my storage area.

Anyway- so I mustered the minor dexterity it took to pull the IAT/IAS and got this out... i was a bit surprised to discover how clean it looked - compared to what I expected from seeing videos on the subject.

Can anyone tell me what the pinout on this thing is so i can check the resistance with my multimeter - there are four pins and an infinite number of unknowns ...

IAS.jpg
 
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