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Old 05-14-2009, 11:28 PM   #1
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Engine idle question.

Hey guys,

I am new to this forum and i like what i see so far. There is a lot of information in all of the sections. I have a question about my new to me project 97 TJ.

I picked up a Black 97 Sahara with a 4.0 and an automatic transmission. All stock. It has 199,998 miles on it. It was sitting for 3-4 months before i got and has some problems. I am working through the drive train issues first.

I'll start with the engine: The previous owner did very little up keep on this TJ and was not into cars. So this poor baby has been neglected. It ran rough when i got it, but started everytime you turned the key. The idle was all over the place. Acceleration is good almost normal but when you maintain a speed like 45 or 50 I would get some good sputtering. I gave her a standard tune-up (plugs, wires, dis cap, air filter, and oil change). She runs really good now with an occasional very light sputter. The problem is when sitting at an idle the tach is about 600rpm then it will rev to about 1400 and maintain it for a minute or two then go back to normal. After another little bit it will do it again. Its like when you first start a cold engine and it goes to the high idle.

What might cause this?

The rear seal has a leak but nothing major. We were thinking injectors are ok because there is no irratic running or flooding of the intake because if you hit the gas pedal it revs like normal then goes back to the high idle. A minute or two later its completely normal at 5-600rpm then repetes the high idle on it own.

Could there be a fuel pressure problem? A vacuum? The computer? O2 sensor? the check engine light has been on since i got it due to all the rough running before. It is still on and no i haven't had it read yet, but I plan on going to Discount Auto to get it read.

I am just looking for some opinions right now till i get the message reading.

Thanks for taking the time to read this!

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Old 05-15-2009, 07:05 AM   #2
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Sounds like your Idle Air Control valve may be giving you a problem. Remove it carefully and clean it out using carb./injector cleaner or buy a new one.

In my opinion, I would throw in a couple of cans of injector cleaner to clean them and treat the old gas that was laying in the tank.

You can check for any vacuum leaks using a spray bottle of water. When the engine is running, spray any vacuum hoses down with water and if you run across a hole it will be temporarily plugged with the water and the engine will change RPM's, you'll know. Once you find a suspected hole, repair the line with a replacement hose.

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Old 05-15-2009, 09:50 AM   #3
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All the original gas is gone. I put a can of STP injector cleaner in and filled up with fresh gas. Where is IAC valve located? Is there a spec sheet/diagram that shows the vacuum line layout? Basically how many, where they are located. Thanks for your thoughts!! I appreciate it.
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Old 05-15-2009, 10:08 AM   #4
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Here's the IAC location. It can normally be cleaned and restored to 100% functionality, it doesn't often need to be replaced since cleaning normally makes it work fine again. Be sure to also clean the oriface its solenoid actuated plunger fits into which is how it provides all of the engine's air at idle. Before cleaning the IAC, clean the inside of the throttle body with a good aerosol TB cleaner like 3M makes. Use the same TB cleaner to clean the IAC plunger and hole (oriface) in the throttle body.

The IAC is held to the throttle body with two T25 (T20?) torx head screws that are also Locktited in place. So use a good sharp (new) torx-head adapter to remove it, being sure to hold it firmly and square into the screw because it can strip out if you're not careful.
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Old 05-15-2009, 11:25 AM   #5
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Jerry,

Thanks for the pic!! I take it you have the same opinion for this problem description?

I read about your transmission swap on another thread. Great information!! I was reading about trannys because the one on this project jeep does not go into reverse or 3rd gear, the rear of the pan is leaking, and the ATF I put in is not red anymore. i don't know if something else is getting in there or if it is burning. I figured I would try and get the engine as good as possible first.

Here is a pic of the project I bought it for $500. Seriously!!

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Old 05-15-2009, 01:34 PM   #6
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Ok, I just got the check engine read at discount auto. There were a bunch of codes associated with the tune-up (dis-cap, #5 cyl misfire, mapping, etc). The one at the top of the list was "Idle air control". The discount employee said he would reset the codes, but when I left discount the check engine light was on again. Probably because I need to clean the IAC. So i plan on doing the cleaning and seeing what happens.

If I disconnect the battery for 4-5 minutes will the computer re-check the senors and possibly turn off the light if this works? If it has to be manually reset is there a way for me to do it at home? I figure if can do the cleaning and get the computer reset and the light comes on again i can go down and get a fresh code readout and see if the is something else.

Thanks guys for all your input!!

If i disconnect the battery
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Old 05-15-2009, 02:26 PM   #7
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Disconnect the ground from the battery for about a minute to reset the codes.
Get some new plug wires.
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Old 05-15-2009, 02:37 PM   #8
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I put new wires on with the tune-up I just did. I also put a new rotor with the Dis-Cap. So just disconnecting the neg terminal is enough? i don't have to drain the system at all it will self discharge after a couple minutes?
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Old 05-15-2009, 02:54 PM   #9
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I always give it about 2 minutes to give it a chance to fully reset. I haven't found value in going beyond that for that task though.
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Old 05-15-2009, 11:30 PM   #10
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Well, I cleaned the solenoid and the whole throttle body. The solenoid was so gummed up I used a whole can of cleaner. The check engine light is no off!! The engine is running normal and responsive. I still have the high idle problem after driving then sitting in park and letting it idle. If i put it in drive and sit with the brake on all is normal, but ifI go back to drive or neutral it goes back up to 1400rpm.

I have a major transmission problem. No reverse or 3rd gear (3-speed auto). Could this be part of the problem?

I am having the tranny looked at this weekend. I just wanted to get the right first.

Thanks for the help guys. Anymore thoughts on this problem would be greatly appreciated.

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