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Old 08-09-2009, 06:41 PM   #1
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high idle after header install

I just put a new borla header on my 99 TJ and now it has a high idle. It ranges anywhere from 2000-3500 RPM and does not go down. I did the code test and it came back with code 25. "An open or shorted condition detected in one or more of the idle air control motor circuits" I checked the IAC and it is plugged in. When I started it with the IAC unplugged it did the same thing.

I sprayed carb cleaner around all the vacuum lines and nothing changed with the idle speed. I think it may have changed a little when I sprayed near one injector... but it definitely wasn't much of a change, if any.

Is it possible that I didn't seat the intake manifold properly and that is the leak? It looks like its seated right and I think the problem would be much worse if it wasn't.

I checked to see if the little L rubber piece from the MAP sensor is there, and it is.

Also, there are 3 vacuum ports on the side of the intake manifold, two are used and one is capped, right?

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Old 08-09-2009, 07:07 PM   #2
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spray around the header and intake. I replaced my exhaust manifold and didnt tighten it back on enough and I got this code problem. Also once to do it you might need to disconnect the negative on the battery for like 15 minutes. this will reset the idle memory on the ECM. Then once you put it back on let it run at idle for about 20-30 minutes. this will retrain the ECM idle memory.

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Old 08-09-2009, 07:14 PM   #3
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Let it run for 20 minutes at the high idle?

I'm thinking that the problem is related to me not using one of those 3 ports on the side of the intake. I capped it since I couldn't find a vacuum line that connects to it. I'm referring to the vacuum port that is closest to the front of the engine.
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Old 08-09-2009, 07:25 PM   #4
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Your high idle is caused by a vacuum leak between the head and intake manifold, you didn't get it properly positioned and seated. It's easy to miss the two small positioning studs on the head that the intake manifold must fit over to fit properly. Loosen your intake manifold, take care to get it seated properly and after retightening it you'll be fine.
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Old 08-09-2009, 07:38 PM   #5
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Here's a picture of the port in question. What connects to this? I'd like to figure that out before messing with unbolting the intake manifold... it's a pain to get to some of them.
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Old 08-09-2009, 07:46 PM   #6
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Put your thumb over that port and if the idle comes back down to normal, that's the problem. That's a different vintage manifold from mine so I can't tell you what it connects to.
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Old 08-09-2009, 08:01 PM   #7
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Doing that brought the idle down a few hundred RPM but it's still way too high. Could whatever plugs into that cause a high idle RPM if it's sitting open looking for vacuum?
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Old 08-09-2009, 10:04 PM   #8
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a few hundred is too high? sounds good to me. Where were you idleing before you did it?

Also I meant once you reset the ECM idle memory let the car run idleing and it would reset the idle back to where it should be (not high idle).
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Old 09-12-2009, 01:28 PM   #9
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It looks like you may be missing the line to the brake booster - it should be one of the bigger lines.
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Old 05-05-2011, 01:30 PM   #10
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I'm having this same issue. What ended up being the problem and solution?
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Old 05-05-2011, 02:06 PM   #11
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I'm having this same issue. What ended up being the problem and solution?
Did you replace the exhaust manifold? If so, i'm sure you missed an alignment stud and the manifold is not seated properly.
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Old 05-05-2011, 04:09 PM   #12
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Did you replace the exhaust manifold? If so, i'm sure you missed an alignment stud and the manifold is not seated properly.
I replaced the exhaust manifold with a pacesetter exhaust header, well my local muffler shop did anyway. Would missing an alignment stud still apply to a header?
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Old 05-05-2011, 04:17 PM   #13
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Would missing an alignment stud still apply to a header?
Yes. The factory uses an exhaust header on the 4.0L anyway, not a manifold.
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Old 05-05-2011, 04:26 PM   #14
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Jerry, I have 2.5 if that makes any difference. I just now removed the IAC and cleaned it... I had a K&N and noticed how dirty the throttle body was, so went back to paper filter. I'm really hoping it was the IAC. I don't like having to take my Jeep in for work, I prefer to do it myself, but the one area I haven't personally worked on are the exhaust components
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Old 05-05-2011, 04:41 PM   #15
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Get it running - spray a BURNABLE carb cleaner or starting fluid on the intake/exhaust/head joint.
If the R's change, you found the leak.
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Old 05-05-2011, 04:42 PM   #16
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The 2.5L comes with an exhaust manfold. The main difference being that a manifold is cast in one piece, headers are made with separate piping bent so their lengths are equal.

Good idea cleaning the IAC first since it can cause idle problems like the TPS can. Cleaning the IAC first is a good idea because doing so is free. The TPS can also cause idle problems but since it cannot be cleaned, only replaced which costs $$$, cleaning the IAC first is always the best first step when working on engine idle or rpm problems. At least after verifying the intake manifold is not leaking first.
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Old 05-05-2011, 06:51 PM   #17
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Good idea cleaning the IAC first since it can cause idle problems like the TPS can.
So after cleaning the IAC the Jeep ran fine for a few seconds then the idle started fluctuating with extreme randomness, anywhere from 500 to 2000 rpm! It didn't do this before I cleaned it. I turned off the engine and disconnected the IAC. I restarted the engine and it idled high at around 1800 rpm but was stable. I reconnected the IAC and it went down to 1500(same idle before the cleaning) and no fluctuations. I'm guessing the IAC is probably toast and needs replacing?

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