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Old 02 Nov 2011, 01:31 pm
Join Date: Oct 2007
Posts: 69
Default High idle and constant up and down idle on my 94 civic ex?

Okay so im learning lots about Honda's, i have rebuilt motors and put them but i cant seem to figure out my own motor!! My car idles really rough, it sounds like a vacuum leak but i cant figure it out. It has a constant up and down idle between 1,200 and 2,000 rpms. Its beyond annoying and i have had enough with it! i changed the map sensor, Tps sensor, and the IACV sensor. Made sure my intake and intake mani were all on tight and all that good stuff but i just cant figure it out. And its weird cause after i changed the map sensor the idling was fine for a day then started again. But now it only rough idles like that every once in awhile. Im at a major loss! Any ideas would be much appreciated! Thanks!
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Old 02 Nov 2011, 02:01 pm
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Join Date: Nov 2011
Posts: 2

Have you checked your ignition coil pack.

If one of the coils has failed you will be running on 3 cylinders causing the ecu to adjust air/petrol ratio to compensate for the failed coil.

I know this will sound really stupid,but have you checked your air filter incase it isn't sitting flush in the

filter housing,My car did the same thing and it was a paper thin space around the filter causing it to suck in more air.

last and not least,have you checked incase any of your new sensors are not clipped correctly in and when you go over bumps or rough roads it makes a connection and the car runs smooth.
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Old 02 Nov 2011, 02:16 pm
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Join Date: Aug 2007
Posts: 1,338

very common problem on older hondas. The idle is regulated by an Idle Air Control Valve. It is essentially a plunger that moves in and out of an idle bypass passage in the throttle body to keep the idle even by regulating the amount of air that goes around the throttle plates when they are closed.

over time the passage gets clogged with carbon (think of soot in a chimney). that carbon does one of two things. it either blocks the air flow through the passage in which case the engine starts to 'gasp' for air (the surging up and down) or the carbon clogs the plunger so that it sticks in the 'passage closed' position. the same effect for both--an unregulated idle. If you've changed the IACV then it sounds like you have the clogged passage.

there is a 'hole' in the side of the throttle body wall on the air filter side before the throttle plates. take the air bellows off and you'll see it about half-way up the side. that is the bypass port. to clean it you'll need to take the throttle body off (or the IACV off) and spray carburetor cleaner (or throttle body cleaner--they are the same thing with different labels) into the port so that it runs out through the IACV port. if you take the throttle body off you can clean the port from the IACV to the intake as well by spraying in the second of the IACV ports.

this assumes that the connector on the IACV is good and that the valve is not electrically dead.

cleaning the port will in all likelyhood solve your problem. be sure to clean the throttle plates while you've got the opportunity.

hope that helps
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Old 02 Nov 2011, 02:31 pm
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Join Date: Aug 2007
Posts: 1,032

Idle hunting is a favorite hobby of older Hondas. The most common reason is a bubble in the coolant. The Fast Idle Thermo Valve is open when cold, letting more air in. (Honda uses a MAP sensor so a vacuum leak ahead of the manifold is identical to opening the throttle.) When the coolant is not purged after a fill hot coolant does not reach the FITV at normal idle speeds so it opens, raises the idle, which gets hot coolant to the valve and closes it, with the cycle repeating every few seconds. You purge the coolant by opening the nipple (looks like an oversize brake bleed nipple) on one of the radiator hose housings on the engine about half a turn, then closing it when a steady stream of coolant comes out.

A much more troublesome cause of idle hunting is mixture hunting. Rather than the lazy revving of a bubble, mixture hunting is abrupt. Every couple seconds it goes from very high rpm (3000 rpm is not unusual) to a near stall for a fraction of a second, then jumps back up, like clockwork. It is caused by uneven mixture from one cylinder to another, with a leaky intake manifold gasket being the usual culprit. Because of the MAP sensor you can't find the leak with carburetor cleaner or the like - if it responds at all it is only to slightly lower rpm.

No coil packs on your 1994, of course - just one coil and a distributor.
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Old 02 Nov 2011, 02:46 pm
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Join Date: Aug 2009
Posts: 166

Mine did this, it was the IACV
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