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Old 12-07-2004, 02:14 PM   #1
myaccord7
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engine revs up and down by itself?!?!

i just got the car. its an 89 prelude 5speed. when its in netural it revs up and down without my foot on the gas. only does it in netural. do these cars have a throttle sensors? or would it be the egr valve? thanks



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Old 12-07-2004, 02:21 PM   #2
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vacuum leak, fast idle valve, cold idle temp sensor.
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Old 12-07-2004, 05:30 PM   #3
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Quote:
Originally Posted by Slipslap
vacuum leak, fast idle valve, cold idle temp sensor.
It could be any of those, do you live in the north where it just got cold? I seriously doubt the egr valve has anything to do with it
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Old 12-07-2004, 06:39 PM   #4
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bleed the coolant, that could also cause it
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Old 12-07-2004, 07:05 PM   #5
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If the fluctuation is in the 700-1,000 rpm range it may be easy to fix. You will find a slotted set screw at the top of the throttle body facing towards where the air filter elbow is attached. The fluctuation is caused by the idle mixture being too lean. Turn the screw (with the engine running at idle) clockwise to enrichen the mixture. This screw is call an "Idle Speed Adjustment Screw" Actually, it adjusts the idle speed, but, by leaning out or enriching the idle mixture and not by moving the throttle plate.

If the fluctuation is at higher rpms like 1,500-1,700, this will not work !!
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Old 12-07-2004, 09:48 PM   #6
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Of course a Leaky EGR valve can cause this. Thats the first thing I would check. Ive seen this happen many times....
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Old 12-08-2004, 12:21 AM   #7
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Dano is right about most of that, but the screw doesn't control idle mixture, it's basicly a calibrated vacuum leak, allowing some extra air into the intake manifold, which the computer detects via map sensor, just like it would detect air getting in any other way, and adds fuel to match.
However, the computer doesn't like the idle being that high, and tries to turn it down via the idle control motor, and goes into a crazy loop.

If the fluctuation is 1500-2000(ish, very ish, but higher then 1000-1300) it's a largeish vacuum leak, either from a vacuum line or from the fast idle valve.
If you do a search for fast idle valve you should find recent instructions on how to deal with that.
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Old 12-08-2004, 01:59 PM   #8
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I had this problem before doing my engine swap... was my EGR.
Now I have a rev that's somewhat stable, it moves about a cm on the tach.
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Old 12-08-2004, 04:36 PM   #9
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Mine moves a little also. It seems to do it when it's warm but hasn't really been driven too much. It goes away when it's nice and warm. I've been wondering if it was cold idle temp sensor but it does it when it's warm but not hot so I'm still not sure. I am going to check some of the things mentioned here but it sounds like here and the other "bouncing idle" threads are more obvious than my slight rough idle.
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Old 12-08-2004, 05:34 PM   #10
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I have also looked into all the fixes listed in various posts and none seem to be my minor trouble...or maybe it is a vacuum leak that's really small.????
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