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  #31  
Old June 18th, 2017, 18:27
NeXJ NeXJ is offline
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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?
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  #32  
Old June 18th, 2017, 19:02
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

Quote:
Originally Posted by Ecomike View Post
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!
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  #33  
Old June 18th, 2017, 20:43
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Ecomike Ecomike is offline
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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.


Quote:
Originally Posted by NeXJ View Post
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!
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  #34  
Old June 19th, 2017, 10:37
NeXJ NeXJ is offline
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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Originally Posted by Ecomike View Post
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
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  #35  
Old June 19th, 2017, 13:13
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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!!!!

Don't leave home with out it, LOL
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  #36  
Old June 19th, 2017, 22:03
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

Quote:
Originally Posted by Ecomike View Post
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!!!!

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?
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  #37  
Old June 20th, 2017, 08:06
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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.

Quote:
Originally Posted by NeXJ View Post
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?
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #38  
Old June 20th, 2017, 08:08
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

You might check with your local chapter and see if someone close to you has a scanner for OBD-I Jeeps.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #39  
Old June 20th, 2017, 08:12
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #40  
Old September 5th, 2018, 09:25
NeXJ NeXJ is offline
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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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  #41  
Old September 5th, 2018, 10:00
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

More likely the leak is the intake/exhaust manifold gasket.
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  #42  
Old September 5th, 2018, 10:02
NeXJ NeXJ is offline
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

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Originally Posted by old_man View Post
More likely the leak is the intake/exhaust manifold gasket.
oh ... can i ask why you say that? just because it's more likely to deform when expanded due to heat (not as massive as a head)? or because it happens more commonly?
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  #43  
Old September 5th, 2018, 14:56
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

Did anyone check the manifolds to see if they were flat, parallel, true and not warped?

If you hear a sucking sound the same time the idle ramps up, its obviously a vacuum leak. I hear water trigger spray bottle at possible vac leak locations can be informative?

Do not get the TPS wet!!!

If you suspect fuel injection controls, disconnect the IAC, and or TPS and O2 sensor right after cold start up and then see what happens.
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  #44  
Old September 6th, 2018, 05:39
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

Quote:
Originally Posted by NeXJ View Post
oh ... can i ask why you say that? just because it's more likely to deform when expanded due to heat (not as massive as a head)? or because it happens more commonly?
All of the above along with 20 years diagnosing 4.0L's and 50 years of wrenching. When the rpms start to go up, spray some carb cleaner around the intake area and if you see a sudden rpm fluctuation, bingo. Careful of flash back.

Are you using Bellville washers on the intake? If not, do it.
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  #45  
Old September 6th, 2018, 08:47
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Re: Idle speed going through the ROOF on warming up : 97 XJ 4.0 HO

Quote:
Originally Posted by NeXJ View Post
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!
In that case it is not a vacuum leak, the ECU is opening the IAC valve to add air. Bad IAT (even if it is new), wrong IAT, bad ground on the IAT sensor???
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