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Old 09 Nov 2011, 03:01 pm
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Default IAC valve causing hard shifts?

My 98 accord v6's IAC valve has gone bad and revs and revs. When driving at low rpm the car shakes and sometimes shifts hard. The lowest the idle will be at is 1200 rpm. The trans was rebuilt in 08 and has only 6700 miles on it so there's no way that can be the reason. Anyone got any(positive) ideas?
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Old 09 Nov 2011, 03:16 pm
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Join Date: Aug 2007
Posts: 1,032

Idle hunting is a favorite Honda hobby, and the IAC valve gets blamed most of the time. I have never seen a bad (or even particularly sticky) IAC valve in a Honda. Most idle hunting is the result of low coolant or a bubble in the coolant, which drives the Fast Idle Thermo Valve nuts. The coolant can be bled by opening the bleed nipple on the radiator hose housing until a steady stream of coolant comes out, and the coolant can be topped up at the radiator... but that is not what is wrong with your Accord.

With the minimum idle at 1200 rpm you have a big vacuum leak. Honda uses MAP sensors instead of MAF sensors, so any leak on the unsplit side of the intake manifold is the same as opening the throttle a bit. A leak at the intake manifold gasket also raises the idle but the mixture imbalance typically confuses the ECU, which will throw codes for mixture and often for catalyst efficiency. When manifold gasket leaks get bad the ECU will go into hard revving interspersed with moments of near stall, like clockwork every 2-3 seconds.

The way to hunt for a vacuum leak in the Honda (which rarely responds to hunting with carburetor cleaner spray) is to remove the intake duct from the air cleaner, plug the breather hole, put the duct to your face, hold the throttle open and blow. There should be very little leakage. If there is leakage a helper can find it pretty easily by listening and feeling. Remember the intake manifold gasket can leak on the top or on the bottom.

When you get the idle straightened out the transmission should be good again.
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