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

Shesh, that sir is not the droid you are looking for. That is the IAC Idle Air Control valve.

The IAT Intake Air Temp sensor is in the top rear of the intake manifold. The IAT has two pins, it is a variable resistor.

The IAC you pulled is a stepper motor valve.

Google "jeep Cherokee IAT sensor image" for the image and location

126472d1340599456t-iat-sensor-resistance-101_0166.jpg


126471d1340599456t-iat-sensor-resistance-101_0171.jpg
 
well i was wondering why it looked a bit different!! the only two sensors I could find with T20 bolts were the one I removed (which is?) and the TPS - right next to it... hmmmm...

but now I have a new problem - after cleaning both the TPS and whatever it was i removed (blush) - it won't even hold an idle now... mostly i just cleaned them up both with carb cleaner and did a deox-it job on the pins after wiping them clean...

gosh I thought i was following that video pretty closely... but alas it is DARK now - and I was barely able to get those two back in. So I will have to pick it up in the AM!

thanks for the pointer.


Shesh, that sir is not the droid you are looking for. That is the IAC Idle Air Control valve.

The IAT Intake Air Temp sensor is in the top rear of the intake manifold. The IAT has two pins, it is a variable resistor.

The IAC you pulled is a stepper motor valve.

Google "jeep Cherokee IAT sensor image" for the image and location

126472d1340599456t-iat-sensor-resistance-101_0166.jpg


126471d1340599456t-iat-sensor-resistance-101_0171.jpg
 
so - i was going by the vids posted earlier... so I see i've been mixing up IAC/IAT and thought they were both the torx mounted thingie on the throttle body. But the one i'm looking for is actually bolted down to the manifold and I didn't need to buy a replacement torx driver at ALL? KRIKEY

it seems for the first half of the thread everyone seemed to think it's the sensor mounted on the side of the TB that was the one to be cleaned...
 
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The REAL IAT has no bolts, it is a pipe threaded male brass body.

Make sure you reinstalled the face sealing o'ring on the IAT idle control valve properly!!!

Never use carb cleaner on wiring. The IAT has super fine motor wires. Must use an electric parts cleaner safe solvent. Carb clean can strip the varnish off the motor wires and short it out if it made contact too long, like soaked it.

What year was your rig? If Renix 87-90 you must calibrate the TPS output voltage by positioning the clamp down spot precisely ( rotation of the TPS body ). Proper Idle is at 0.82 Volts. 91-01 does not need calibration.

The IAC is not a sensor, it is controller valve to be precise, kinda like the fuel injector valve that controls fuel flow, it regulates the air flow rate but only at idle.

The IAT is a sensor. The TPS is a sensor.

They are all thingies, LOL. or watchamacalits and thingabobs, LOL. :)
 
The REAL IAT has no bolts, it is a pipe threaded male brass body.

Make sure you reinstalled the face sealing o'ring on the IAT idle control valve properly!!!

Never use carb cleaner on wiring. The IAT has super fine motor wires. Must use an electric parts cleaner safe solvent. Carb clean can strip the varnish off the motor wires and short it out if it made contact too long, like soaked it.

What year was your rig? If Renix 87-90 you must calibrate the TPS output voltage by positioning the clamp down spot precisely ( rotation of the TPS body ). Proper Idle is at 0.82 Volts. 91-01 does not need calibration.

The IAC is not a sensor, it is controller valve to be precise, kinda like the fuel injector valve that controls fuel flow, it regulates the air flow rate but only at idle.

The IAT is a sensor. The TPS is a sensor.

They are all thingies, LOL. or watchamacalits and thingabobs, LOL. :)

Thanks Ecomike - i was just a bit confused because everyone else at the start of the thread was telling me to clean the valve (IAC) to investigate the problem. And you're telling me something different - but I guess - in for a penny , in for a pound - so i'll do that one in the AM too...

not to worry - i didn't use carb cleaner on the 'wiring' just on the 'business' end of both sensors - and made sure I gave a good attempt at drying them too after - I used deox-it on the electrical connectors. nothing wrong with that stuff. That's what it's made for .

my engine is a 92 - well my computer thinks it is anyway - it's actually a november 97 but I'm using all my 92 accessories on it (swapped it in 00 because It needed it) so that I didn't need to put a whole new computer in it. So all the sensors etc are 92. The only mod is that I had to tap into the back of the head to put in a 92 style temp sender
 
Because the IAC controls air flow and thus idel at idle, most think it is the culprit of idle problems. But it can not control vacuum air leaks that by pass it. And if the computer gets bad sensor data the computer tells the IAC to over compensate thus causing a wandering, or low or high idle. Then you must look at the sensors that can cause the bad data, and they are the TPS, IAT, and maybe the CTS, and maybe the O2 sensor for a wandering idle.

Bad wiring too, but yours is obviously changing with temperature which points right at the IAT or a vacuum leak that grows with higher temps like an Intake leak at the Intake-exhaust manifold gasket on the side of the engine head.

Two other places a vac leak can be are the fuel injector o'ring seals and the throttle body bottom gasket to the intake manifold.

You can test for those by spraying carb cleaner or trigger spray water in places where the leak might be to see if the idle speed jumps.
 
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Just for the fun of it, once the engine is off for a minute or more and cold, disconnect the IAC and start it. If the idle problem remains my money is on vacuum leak at the throttle body, injector or intake manifold to head gasket.

With the IAC disconnected the idle should drop as it gets hotter I think, if it changes at all.
 
Pulled the IAT you indicated ... it was clean as a whistle but then again I only have maybe 100 miles TOPS since the new head - and get measurement of 0.1 ohms (dead short) while cold. Thinking this might be the (or a) culprit
 
Ya think? Maybe? LOL. Me thinks, found the droid you are looking for, you have, LOL.:cheers:

Be sure to also do the IAC disconnect test later I suggested as a way to look for vacuum leaks once the IAT is replaced.

"Just for the fun of it, once the engine is off for a minute or more and cold, disconnect the IAC and start it. If the idle problem remains my money is on vacuum leak at the throttle body, injector or intake manifold to head gasket.

With the IAC disconnected the idle should drop as it gets hotter I think, if it changes at all." and if it goes way up over time, I would check the intake-exhaust manifold bolts!! Disconnecting the IAC removes the computer's ability to change the idle air flow rate for test purposes.
 
HMMM - well that was a failed experiment I guess :(

after a THREE WEEK wait from JC whitney I FINALLY got the replacement IAT sensor - popped it in - and NO GO. Same result RPMS climbing through 8000 if I let it go that far (only let it go to 3000) while at idle... bummed... so you think check the vacuum hoses then?
 
Just for the fun of it, once the engine is off for a minute or more and cold, disconnect the IAC and start it. If the idle problem remains my money is on vacuum leak at the throttle body, injector or intake manifold to head gasket.

With the IAC disconnected the idle should drop as it gets hotter I think, if it changes at all.

incidentally i didn't do this until i had the new IAT - I wanted to test one thing at a time you know... anyway- with the IAC disconnected it behaves nicely and the idle doesn't climb ... hmmm... maybe i should drive it around with the IAC off for the time being ... at least it should be able to get me to the post office without having to turn off my vehicle in the middle of busy intersections!
 
I forget the story on this, did it ever throw any codes at all?

Is the TPS reading normal resistance at idle, with the power on and engine off? Or has it gone bad?

Is it running closed loop with the IAC disconnected?

What are the fuel trim numbers with the IAC disconnected? STFT and LTFT?

Have you tried disconnecting the battery and clearing any old data in the PCM now that it has a good(?) IAT sensor?

From what you just said, it is not a vacuum leak. This is major progress.


incidentally i didn't do this until i had the new IAT - I wanted to test one thing at a time you know... anyway- with the IAC disconnected it behaves nicely and the idle doesn't climb ... hmmm... maybe i should drive it around with the IAC off for the time being ... at least it should be able to get me to the post office without having to turn off my vehicle in the middle of busy intersections!
 
I forget the story on this, did it ever throw any codes at all?

Is the TPS reading normal resistance at idle, with the power on and engine off? Or has it gone bad?

Is it running closed loop with the IAC disconnected?

What are the fuel trim numbers with the IAC disconnected? STFT and LTFT?

Have you tried disconnecting the battery and clearing any old data in the PCM now that it has a good(?) IAT sensor?

From what you just said, it is not a vacuum leak. This is major progress.


Hi ecomike -

1. no - no codes 'thrown' (assuming you are referring to any 'check engine light' codes) - that only came on when i disconnected the IAC.

2. if by 'closed loop' you mean stable and with apparent negative feedback - then my best guess would be 'yes' - it seems stable and RPMs do not climb. It sounds exactly like it should. Or do i need to look into this deeper to find out?

3. sorry for my ignorance but I've never heard the term 'fuel trim number' nor STFT / LTFT and I'm not sure how i would determine this (?) - I'm pretty handy - i have a service manual and did my own engine swap (with the help of another naxja member) once but have never come across this before.

4. no - i haven't tried resetting the computer (disconnecting the battery) but maybe i should... I'll get back to you if this brings about any change in the situation - I'm GUESSING I should have the IAC reconnected (as normal) when I do this ... (?)

thanks mucho for your help... i am grateful
 
Since you have a 97 rig?, go to ebay and shop for and order a bluetooth OBD-II scanner (about $20 delivered), then buy the apx $5 TorquePro app for your(?) smart phone, and then with both you can monitor if the O2 sensor PCM is working, live data, and you can monitor the fuel trim, engine temp, air temp (it would have caught your bad air temp sensor right away) and so on and so on live engine data!!!!:cheers:

Don't leave home with out it, LOL
 
Since you have a 97 rig?, go to ebay and shop for and order a bluetooth OBD-II scanner (about $20 delivered), then buy the apx $5 TorquePro app for your(?) smart phone, and then with both you can monitor if the O2 sensor PCM is working, live data, and you can monitor the fuel trim, engine temp, air temp (it would have caught your bad air temp sensor right away) and so on and so on live engine data!!!!:cheers:

Don't leave home with out it, LOL

I think I'm ODB I though - i'm not sure if i can get that much data (?) - my computer is a '92. The engine is a late 97 but I left all the '92 accessories so that the computer would still work... the only real issue with it was having to tap a new hole in the head for a 92 temp sender...

should i get an ODB I reader? as far as I understood -the 'key switch' trick gives me about as much info? or is that wrong?
 
There is a lot more info you can get, yes. But for OBD-I you need to spend some bucks. I bought a used, Snap-On scanner, MT-2500, which all the shops are dumping on ebay as they are upgrading to newer scanners for newer rigs. I can now read Renix, OBD-1 and OBD-II live data with my Snap on. It takes some effort, a learning curve, and time to shop and get just what you need and limit dollar costs. I spend about $400 total on mine and a bout a month shopping and buying this and that cable, and cartridge to get all the stuff I have now. But I can scan 1984-2001 now, Jeep, ford, GM saturn.. and get all live data, not just error codes. Live data is much more telling.

I think I'm ODB I though - i'm not sure if i can get that much data (?) - my computer is a '92. The engine is a late 97 but I left all the '92 accessories so that the computer would still work... the only real issue with it was having to tap a new hole in the head for a 92 temp sender...

should i get an ODB I reader? as far as I understood -the 'key switch' trick gives me about as much info? or is that wrong?
 
You might check with your local chapter and see if someone close to you has a scanner for OBD-I Jeeps.
 
You can get live sensor data and test sensor grounds, with just a multimeter!!

Cruiser54.com has a lot of easy to follow right ups on doing those tests on 87-90 rigs, but most 92 stuff is very similar, except 92 has no EGR, the O2 sensor is different, it is 0-1 volts, and the TPS is different. The rest is the same I think.

Google "Ecomike O2 sensor test Naxja" for the thread on 91-01 O2 sensor testing.
 
OK - i'm opening this thread again because I'm just starting to look at this again - after a year or so (!) of letting my XJ sit (poor thing) - amazingly the battery still starts it!! But the problem persists ... SO - after having done some research into vacuum leaks (yes that's what it seems to be and the engine does make some whistling sounds) ... I am hoping someone might confirm or deny a hypothesis:

SINCE all this started shortly after having a local mechanic replace my head with an 0630 (IIRC) they found second hand - and SINCE it seems like the problem only starts when the engine actually reaches temperature after 10 mins or so of idling ... i'm guessing it's more than likely the problem is the head or head gasket (?) - since maybe any other kind of vacuum leak would still be there when the engine is running from cold (if you get what i'm throwing down here?). Or could it be that the problem only shows up when the engine goes into closed loop operation after stabilizing?

what say you? any suggestions? I tried listening around the engine compartment through a cardboard tube for the specific location of the whistle (not sure it's quite a whistle but it's something...) i couldn't accurately locate it ...
 
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