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  • Really fast idle

    I just finished putting on a new exhaust manifold on my 90 project YJ (2.5 4 cylinder). I took the whole intake manifold assembly off in one piece. When I start her up, the idle rises up to 3,000 r.p.m. and stays there. I checked and double checked all the electrical connections and vacuum lines. Everything seems to be in order. The one thing I am thinking right now is the automatic idles speed motor is not working. The plunger on the tip does not move back in once the Jeep is started. My Chilton's manual only has testing info for 91 and up, it doesn't mention the 90 having one (but I'm pretty sure that's what this is). I counted the threads on the end of the plunger bolt (where it contacts the plate that the throttle cable connects to) and then ran it in all the way and the idle settled down to 600 r.p.m. It runs fine and has power, but I'm thinking it is the ais motor? My thought is that in taking the assembly out, it got bumped or something which caused it to fail. Anyone else had this problem or can give me some additional ideas for things to check? Thanks in advance for your help.

  • #2
    The idling adjustments on FI vehicles are pre set from the factory and shouldn't be messed with. If you adjust it, it is just a band-aid fix for a problem that hasn't been resolved yet.

    Sounds like you have an intake leak. Get some carb cleaner and spray the engine down. When the RPM's drop, you found your leak.

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    • #3
      Yeah, I agree that screwing that screw down is a band aid. I forgot about the carb cleaner trick. I'll have to give that a shot. Originally, when it did it, I discovered that I had not tightened the EGR tube into the bottom of the intake. I was really hopeful that that was the source, but it wasn't. If the carb cleaner trick doesn't reveal anything, can you pull codes from the computer on a 90?

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      • #4
        O2 sensor?

        if your Idle was good before the manifold, and no changes have been made to the idle, you may want to check the usual suspects like the O2 sensor, that might be it.
        Last edited by rippin YJ; 10-05-2006, 12:08 PM.

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        • #5
          Originally posted by rippin YJ
          if your Idle was good before the manifold, and no changes have been made to the idle, you may want to check the usual suspects like the O2 sensor, that might be it.
          An o2 sensor would not cause the problems he is having.

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          • #6
            There is also a chance of a vac line disconnected.
            My O2 is cut on the 91 and not a hugh dif in idle speed after first start up.

            Check both your ported and manifold vac. readings.
            it's free and easy

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            • #7
              I'll check that, although I sprayed down the intake manifold, hoses, etc. with carb cleaner and didn't discover a leak (but man is it clean!). After looking at the Alldata at work today, I'm going to test the AIS motor and throttle position sensor. I'm still leaning towards the AIS because the plunger will not move at all. According to the testing instructions, you should be able to pull it all the way out. At least it is running and with the plunger screwed down, it idles at 800-900 r.p.m.

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              • #8
                Intake gasket!!!!!!

                Originally posted by GDJeep
                I'll check that, although I sprayed down the intake manifold, hoses, etc. with carb cleaner and didn't discover a leak (but man is it clean!). After looking at the Alldata at work today, I'm going to test the AIS motor and throttle position sensor. I'm still leaning towards the AIS because the plunger will not move at all. According to the testing instructions, you should be able to pull it all the way out. At least it is running and with the plunger screwed down, it idles at 800-900 r.p.m.

                Heh GD, Ole Jeep Tec here, you replaced the intake gasket as well as exhaust! If ya didnt have this problem before, then the gasket slipped. Not always does spraying the intake show up a vaccum leak at the intake. Also make sure that connections to all sensors are in straight and tight. The older connectors can get snapped back into place and not make good connection. 91' and newer were different, thus better connections. Also make sure that throttle cables not binding.... Good luck HMR-RSQ

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                • #9
                  Right, the gasket I used was a combination unit. I gues I'll have to go back and check that further. I did take care to make sure it was lined up on the pins on the head, but I guess anything is possible. Would a leak make it run differently (low on power, hesitation)? It runs just fine now except for the high idle.

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                  • #10
                    check the torque on your bolts. The carb cleaner trick only works for a rough or low idle.

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                    • #11
                      Well, I found out the the brushes on the idle control motor are bad. I am going to look for replacements, but I have a feeling that I will have to replace it. I went back and rechecked my bolts, hoses and lines again. I even had one of the mechanics from work stop by to check it out (has a 90 also) and it looks good. Does anyone have a 90 YJ with a AIS motor they don't need anymore?

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