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I have a 1998 Civic LX. About 2 months ago I noticed the idle speed was pretty low after the engine was warm. It wasn't too bad at first. A few weeks ago I got code P0420 (CAT). I noticed the exhaust manifold was split nearly in two when I took the heat shield off so I replaced the manifold/CAT. That took care of the P0420 code, but not the idling problem. That got worse - to the point that anytime I clutched the car at low RPM's, it would stall. I then took the IAC valve off and cleaned it out. I noticed the inlet and outlet for the coolant were completely gunked up, so I cleaned them out. The valve itself seemed to move freely, but just in case I cleaned it all out with electronic parts cleaner. I drove it a bit and it seemed better, but still idled around 500 rpm's. I put some fuel treatment and dry gas in (I live in northern NY with temps very often in the negatives) just to see if there might be a problem with water freezing or something else in my fuel. I've driven about 60 miles since then and it still idles around 500 rpm's once the car is warmed up.

Anyway, any ideas/suggestions on what to try next. Do you think the IAC valve is toast? Fuel filter maybe?

 
Rep Power: 518

If there is a screen covering one of the holes in the IAC valve, scrape all the carbon buildup out of the screen mesh. This usually fixes the low idle problem on most for me.

 
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Thanks. I couldn't get the adapter off before because the screws were really stuck. I'll have to try harder.

 
Rep Power: 518

Hang on.... You have a downdraft throttle body with an IAC screwed to the front of if? It might just be stuck, the rotary valve inside can get caked with goo.


In my previous reply I thought you had the type of IAC that bolts to the rear of the intake manifold, it has a screen in one of the ports. My mistake, sorry.

 
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No worries. I just finished tearing it apart again. I cleaned it, and the throttle bottle out to the point that my g/f would eat off it. The valve seemed to move freely.

When I first start the car cold it idles very evenly at 1500 rpm's. Once the car is at operating temps while I'm driving it, it runs perfectly fine. When I stop at a stop sign it idles at or a bit lower than the 500 rpm mark for about 5 seconds and then evens out around 700. For those 5 seconds it acts like it wants to stall.

There is no CEL to indicate any sensors are bad.

Any other thoughts?

 
Rep Power: 518

After you cleaned it all, did you do an idle relearn?

 
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I didn't. What is the procedure for my car? Thanks!

 
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Basically, this:

 
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Great, thanks! Its about 15 below zero/f here right now, so I'll give it a try tomorrow/this weekend.

 
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I followed the process that you gave me. I also unplugged the 7.5 amp backup fuse next to the FI e/m fuse because I read that it is a backup ECU fuse. I drove the car 200 miles yesterday. The car is not quite idling where I feel it did before (about 1K), but it is now idling at a steady 7-800. This seems to have fixed it, but I'll post if anything changes.

Thanks alot for your help, ezone.

 
Rep Power: 518

1k is too high for when the engine is at operating temperature.
750ish sounds good.

The spec may be listed on the emissions label in the engine compartment or on the hood.