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Old 08-10-2009, 01:42 PM   #1
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Extremely high idle problem

Hello everyone!

I am having a problem with my 97 TJ, 2.5L engine. I just replaced my water pump, thermostat, and serpentine belt. When it was time to start it, I had to jump start due to the battery getting drained. It now fires up right away, but is at around 4000 RPM. The throttle cable is not bound or anything, the butterfly valve is closed...tried replacing the TPS and the IAC - nothing changes. Tried a smoke machine looking for a vacuum leak, found nothing. Nothing on the OBD II Computer either. I have spoken with our mechanic @ work, as well as several fairly knowledgable people familiar with auto repair and we are all at a loss as to why this is happening? I'd be indebted to anyone with an idea as to what to check next, I would hate to have to take it to a shop!

Thanks!

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Old 08-10-2009, 01:50 PM   #2
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thats a weird one, usually its the throttle cable binding and sticking open, or a large vacuum leak. sounds like you covered those. your positive the plate is closed.

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Old 08-10-2009, 01:56 PM   #3
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Yes, it's fully closed. If you give it gas (almost like trying to 'kick' the idle down), it opens then and revs even faster.

Pretty frustrating...seem to have the experts around here baffled, figured I'd throw this to a broader audience.

Thanks for the reply!
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Old 08-10-2009, 02:09 PM   #4
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U scrambled the brain w low voltage .Disconect batt for a couple minutes and make sure u tighten all cables good and re start some times takes a couple minutes or tries.
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Old 08-10-2009, 03:40 PM   #5
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ok lets start out simple, is it actually at 4k rpm or is the gauge just saying it is? sounds like a stupid question but iv learned that it needs to be asked.
check voltage across battery terminals when off and when on. bad ground or bad alternator connection can cause some weird stuff. check the coolant temp sensor, i dont know if the 2.5 has a seperate sensor for the gauge and the computer or if there is only one but the coolant sensor can give those symptoms when it dies. if you still cant figure it out try resetting the computer like jerryj said. sometimes that will help if the computer has lost its settings but it does take some time for the thing to get its bearings again so you might want to just take it out for a drive and see if it will reset by itself.
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Old 08-10-2009, 06:22 PM   #6
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Thanks for the replies. Yes, it is running @ 4000 rpm, screaming loudly! I have disconnected the battery for 30 min or so, would it help if I disonnected the brain in addition to the battery?

I will check the coolant temp sensor, it was the only sensor I messed with during the repair.
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Old 08-10-2009, 07:02 PM   #7
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They say it takes about 10 min to relearn I drove mine for about that and disconected the battery 1 time. once ur in gear its ok u just look stupid at stop signs drive it around and see what happens.It happened to me trying to start w loose cables after welding on the cage I hooked the battery up loosely to move it and it started doing it.Try to give her 10 min if your driving its not as bad as screaming in the drive way.I guess that would only work if its a stick.
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Old 08-10-2009, 09:11 PM   #8
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i have a 98, and i've had problems with the choke sticking. (i guess it's the choke) it would start up fine, drive fine, but when i got to a red light, the idle was reving up around 2 grand. i took off the throttle body and cleaned it well. the by-pass for the throttle intake, what i'm calling the choke was also dirty. once i cleaned these, it purrs like a little kitten.
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Old 08-11-2009, 04:08 PM   #9
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Well, I have tried everything suggested, and have had no luck. I even attempted to drive it around our parking lot and it ended up getting so hot so fast that I cracked my radiator...
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Old 08-11-2009, 06:16 PM   #10
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Thats crazy I was sure that was it I just drove mine home from the shop like that w no issues a little barking leaving the stop sign once in gear the weight of the jeep in gear smoothed it out and ran pretty normal.It still shouldnt over heat that quick though.I also ran on the highway though good air flow.It was fixed in about 10 min.Lucky I guess.Mine started at 2500 and a steady climb to 3600 Gotta be it??!!??
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Old 08-11-2009, 07:36 PM   #11
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If the butterfly valve is closed and the IAC is seated properly and you're getting extremely high rpms, you have a vacuum leak... 99.9999999% chance of that, at least that's my thought. I'd be checking all the vacuum hoses, especially the big one going to the power brake booster. No intake manifold work has been done lately has it?
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Old 04-25-2010, 03:33 PM   #12
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If the butterfly valve is closed and the IAC is seated properly and you're getting extremely high rpms, you have a vacuum leak... 99.9999999% chance of that, at least that's my thought. I'd be checking all the vacuum hoses, especially the big one going to the power brake booster. No intake manifold work has been done lately has it?
What would the problem be with the engine reving high if the intak manifold had been remove for and exhuast manifild replacement? That's a problem I'm haveign with my 1997.
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Old 04-25-2010, 04:04 PM   #13
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Not getting the intake manifold re-seated in the correct position properly is the universal reason for a high idle after having it off. It must be aligned properly over the little alignment studs which few notice when replacing the intake manifold. Be sure to get it properly positioned before tightening the bolts down, don't just try to tighten the bolts more tightly.
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Old 04-25-2010, 04:38 PM   #14
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Seams I got everything right Intake is set tight. Will have to recheck. Anything else you recomend to check? All hoses and elctrical connections have been checked over and over. Thanks for a quick reply.
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Old 04-25-2010, 04:52 PM   #15
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Seams I got everything right Intake is set tight. Will have to recheck. Anything else you recomend to check? All hoses and elctrical connections have been checked over and over. Thanks for a quick reply.
Nearly everyone that thinks they got the air intake seated properly when they experience high engine rpms quickly disover after loosening it & repositioning that the problem is cured. If you got all the vacuum hoses reconnected like the big one that goes to the brake booster and the other that goes to the HVAC system, the intake is just not seated properly. Even the smallest leak between the head and intake manifold will cause high rpms as a leak there is just like opening the throttle body's butterfly valve to let air into the engine.
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Old 12-29-2010, 01:10 PM   #16
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U scrambled the brain w low voltage .Disconect batt for a couple minutes and make sure u tighten all cables good and re start some times takes a couple minutes or tries.
good advice fixed my problem thanks
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Old 12-29-2010, 03:09 PM   #17
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When you jump started it there was a surge or spark generated. It loaded that "spike" into the computer's memory. The PCM was trying to read it as information. It's much the same thing as we use surge protectors for on PC's.

Sometimes disconnecting for a few minutes does the trick, sometimes not. It depends on where the spike is in memory. Sometimes you need to "deep clean" it - leave it disconnected for at least 8 hours. That puts everything back to default - just like it was when brand new.

If you have other strange problems or get codes, try that.
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Old 12-29-2010, 05:21 PM   #18
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Ive heard having a low battery itself also do funny things to the TJ computer. Make sure your using a new battery, let the memory deep clean 8hours or more with the battery disconeccted, and make sure there really is no vaccum leak. The symtoms sound that of the computer compensating for extra air and adding fuel to prevent the engine from buring too lean.
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Old 12-29-2010, 06:00 PM   #19
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He He - there are no "blue screens" on TJ's.
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Old 12-29-2010, 06:18 PM   #20
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in regaurd to the power serge from bump starting it.

i had to bump start my truck once and it did something similar but only 2100 rpm. did everything this guy did. even left the computer unplugged for a day, and still did it.

my problem was the ecm. the power serge made my computer retarted lol i had to send my ecm out to get reprogramed (and tuned while i was at it)
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Old 12-29-2010, 06:20 PM   #21
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Confusing...especially when you said it overheated and broke your radiator. I agree with those that think you may have a vacuum leak, but getting hot after you replaced the water pump and thermostat is a huge problem? Is it possible the belt is not routed correctly?
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Old 12-29-2010, 06:32 PM   #22
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It definitely sounds like an air leak. Mine did the same thing and it turned out it was the seal around the intake manifold was cracked. I knew because as it was revving I could push down and the revs would lower, then go back up after I let off. I also thought it was connected tight but apparently not...
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Old 12-29-2010, 09:12 PM   #23
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Post #16 - IT'S FIXED!

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