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Old 01-14-2017, 06:01 PM
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High idle -- ICV? Vacuum leak?

Hey folks -- I just rebuilt and reinstalled the 2.5 out of my '97 TJ, and it's running great. That said, it's idling at 1k rpm and won't settle in below that. I have checked and rechecked every vacuum line I can find (unplug each and manually plug the orifice to see if it changes the idle speed). No dice there.

I also removed and cleaned the ICV to no effect.

With the intake removed, when I put my hand over the throttle body, the rpm's drop significantly, but it will not stall out. There is a LOT of suction holding my hand to the TB, but I can't make it stall. Shouldn't it? Does that not indicate a vacuum leak? I sprayed carb cleaner here and there around the intake and noticed very little (if any) change in rpm, so I can't find an intake gasket leak either.

I suppose the next step is to see whether the accelerator cable stop can be adjusted down a bit to see if it's just hit a mechanical point where it can't idle any lower, but that seems unlikely, doesn't it?

Any other thoughts? I have no codes, and the Jeep runs great. I just hate the high idle. It also seems to "hang up" (take a while to settle back to idle) when letting off the gas to shift and that's annoying. I don't expect Ferrari-like throttle response from my little 2.5, but I would think that it would rev up and back down a little more quickly.

I did have a stuck thermostat (open) and it wouldn't warm up to temp, but rather only to 140 or 150 degrees, I'd guess. Might it be stuck in "cold idle" and need the battery unplugged to relearn idle speed when warm?

Any thoughts would be VERY much appreciated at this point. This is the last real stumbling block I'm dealing with and I'd love to get it sorted out. Thank you in advance!

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Old 01-14-2017, 06:22 PM   #2
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It is possible that the intake manifold is not seating properly up against the head. Seems to be a common problem after the manifolds have been removed and replaced.

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Old 01-14-2017, 07:31 PM
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It is possible that the intake manifold is not seating properly up against the head. Seems to be a common problem after the manifolds have been removed and replaced.
That crossed my mind, but I would have though that I'd hear more of an rpm change when I sprayed carb cleaner around the intake mating surface. That would explain it not dying when I put my hand over the TB though. It's wearing a new gasket, and I reseated it once (tightening in the correct pattern). Maybe I need to try another gasket as I did tighten out of sequence the first time.

Thanks!
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Old 01-25-2017, 04:14 PM
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Bumping this thread back to the top!

New ICV did not help. I am going to try to re-seat the intake/exhaust gasket next, I think. Shall I buy a new one to drop in, or just try re-seating the one that's in there (which was new after my engine rebuild)? I have only put about 15 miles on this gasket, but it's seen lots of idle time. Could it me deformed already? It's only $10 or so for a new one. Worth the expenditure?
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Old 01-25-2017, 04:48 PM   #5
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Bumping this thread back to the top!

New ICV did not help. I am going to try to re-seat the intake/exhaust gasket next, I think. Shall I buy a new one to drop in, or just try re-seating the one that's in there (which was new after my engine rebuild)? I have only put about 15 miles on this gasket, but it's seen lots of idle time. Could it me deformed already? It's only $10 or so for a new one. Worth the expenditure?
Thats really a judgement call when you get the old one out. I would probably pick up a new Fel Pro gasket either way.
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Old 01-25-2017, 05:08 PM
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I'm thinking the same. Cheap peace of mind with a new one. I didn't know if I might be throwing money away, but for the cost I think I'll pick one up. Thanks!
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Old 01-26-2017, 08:54 AM   #7
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Since you can't effectively reach the bottom of the intake runners with carb spray, it's possible that you have an intake leak (very small) in an area which you simply are not able to find.

From my experience, yes, if you put your hand over the whole throttle body, you should be able to choke out the engine. With your hand over the TB, you should definitely notice a substantial change in whistle - somewhere - from the engine pulling that air from the atmosphere. Can't imagine that it wouldn't make its presence known.

Saw a post on WF a while back about someone doubling up on manifold gaskets due to poor sealing of the intake/exhaust manifold. Can't say that is recommended, but if it works, so be it.
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Old 01-26-2017, 09:15 PM   #8
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If it's a gasket, spray some carb cleaner around the manifold where it joins to the head... Ensure you're fastening the manifold in correct sequence?
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Old 01-26-2017, 09:31 PM
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If it's a gasket, spray some carb cleaner around the manifold where it joins to the head... Ensure you're fastening the manifold in correct sequence?
Yep, already tried the carb cleaner trick -- very little different detected (if any), but then I can't get at the bottom. I did tighten in sequence.

Thanks though!
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Old 01-26-2017, 09:31 PM
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Since you can't effectively reach the bottom of the intake runners with carb spray, it's possible that you have an intake leak (very small) in an area which you simply are not able to find.

From my experience, yes, if you put your hand over the whole throttle body, you should be able to choke out the engine. With your hand over the TB, you should definitely notice a substantial change in whistle - somewhere - from the engine pulling that air from the atmosphere. Can't imagine that it wouldn't make its presence known.

Saw a post on WF a while back about someone doubling up on manifold gaskets due to poor sealing of the intake/exhaust manifold. Can't say that is recommended, but if it works, so be it.
Thanks. I have a new gasket to try this weekend (weather permitting)!
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Old 01-27-2017, 11:18 PM
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Hat tip, gents -- I installed a new gasket earlier this evening, and it's MUCH better now. I suppose the computer may still be relearning idle, but at least it eventually settles down to 800 rpm or so, and drops to ~1200-ish almost instantly even when cold. It was almost certainly the intake/exhaust gasket.

Thanks again!
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Old 01-28-2017, 11:58 PM   #12
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Awesome! Glad it worked for you! Wrenching on your own stuff is quite satisfying!
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Old 01-29-2017, 01:50 PM
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Indeed it is a good feeling. I had done lots of small repairs before I tackled the full-on rebuild of my 2.5 engine, but pulling that motor and rebuilding it from scratch was incredible satisfying. I'd done head gaskets, timing belts/chains, etc... but this was a first. My stomach was in knots when I first turned the key after reassembly, but once I got everything sorted out it was a real feeling of triumph.

I must have checked and rechecked torque on internal parts four or five times, and was super careful to check bearing clearances with plastigauge. All's well so far, though -- knock on wood!

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