Low idle when car is in drive but stationary
 

Hi everyone I have a 1994 Honda Civic LX. I recently changed the head gasket and timing belt on the car. The engine has around 190k miles on it. When the car is in park it idles fine, when I am driving it drives fine, however when I come to a stop while in drive the idle drops really low. The idle however is ok if the lights are on. At first I thought that the issue might be related to the head gasket issues that I had had in the past but the fact that it is fine when the lights go on puzzles me. I am thinking that the problem is electrical. Does anyone have an idea as to what the issue might be? Any help on this would be greatly appreciated. Thank you

RE: Low idle when car is in drive but stationary
 

Basically your idle will go up when you turn on accessories like your a/c or your headlight b/c the car needs more power to run these things. you should be able to test it out by turning on your a/c and see if your idle is higher.

most of the time, the idle is struggling when there is some sort of vacuum leak or if the idle controls are dirty with carbon build up etc.

you should check to see if you can hear any hissing sounds coming from the car. folks have recommended hearing stuff with a long hose. be careful not to get near anything that is loud, only near the hoses.

RE: Low idle when car is in drive but stationary
 

Check your spark timing with a timing light. This has to be done after changing the timing belt. Be sure to jumper the test plug near the ECU to put the computer into timing adjustment mode before adjusting.

If you still have trouble check the idle adjustment. Warm up the engine and run it in neutral. Disconnect the wire plug to the idle air control valve on the back of the throttle body. If the engine stalls out, open up the air screw on the top of the throttle body until the engine will barely stay running (about 400 rpm) with the IACV disconnected. (Do not adjust the stop screw on the throttle lever!). Reconnect the IACV. The CEL will probably come on because of this procedure, reset it by disconnecting the battery (with the key off) if necessary.

Vacuum leaks on a MAP-controlled engine such as a Civic will cause the idle speed to increase.

RE: Low idle when car is in drive but stationary
 

Is the CEL ON?

Try cleaning the IACV with carburetor cleaner. Look for DIY on forum.

RE: Low idle when car is in drive but stationary
 

Thank you for this info. I have an '03 Windstar that had a very rough idle after a fix and it turned out to be a vacuum hose that was not plugged back in. Does this mean that Windstars are nto MAP-controlled? I thought they were. That's why I was saying what I said.

I have heard that vacuum leaks tend to make the idle higher but would they have the opposite effect also?

I always appreciate your and RonJ's comments here on this forum.

RE: Low idle when car is in drive but stationary
 

If you have an air flow controlled system (mass air flow or MAF sensor), all the air that the engine uses has to go thru the sensor or the mixture will be too lean. Vacuum leaks will cause the engine to idle rough or not at all. The air from a leak comes in after the sensor so the computer is not aware of it. It only injects enough fuel to meet the needs based on the air flow that has been sensed.

The MAP system, on the other hand, will sense the increased pressure in the intake manifold due to a leak and simply inject more fuel to compensate. The engine will take that extra air and run too fast.

Most newer cars have an air flow sensor because it is more precise, though also more expensive than a MAP system.

RE: Low idle when car is in drive but stationary
 

Cool thank you everyone for the advice I will check that out this weekend. Cheers

RE: Low idle when car is in drive but stationary
 

Thanks mk378. I appreciate your thorough and clear explanation of the differences. I now know that my Windstar is an MAF controlled car and that's why the leak made it stumble at idle. So I assume then that my 98 civic is an MAP controlled system. Are the newer civics then MAF controlled like past '06 or are they still MAP controlled?

This is very important for me since I recently had a rough idle and thought it might be a vac leak as some of the other posts responses said. This is good to know.


My question then is what does the above procedure do? It looks like you are disconnecting the IACV. Does this air screw adjustment recalibrate the idle? Or does it reset the computer somehow? This is very informative for me. Thank you for your contribution to this forum.

RE: Low idle when car is in drive but stationary
 

For 5th generation Civics, the EACV must be disconnected to properly adjust the idle speed because otherwise the EACV would prompt the ECU to maintain a higher specified idle speed, thereby interfering with the idle adjustment procedure.

For 6th generation Civics, disconnecting the IACV is unnecessary for adjustment of the idle speed.

RE: Low idle when car is in drive but stationary
 

What you're trying to do on a 5th generation anyway is set up a minimum air flow that is not quite enough. (With the plug disconnected, a properly functioning IACV will close fully). Then the ECU can open the IACV partway to establish a proper idle. The ECU always tries to maintain programmed idle speed, it cannot be adjusted. The air adjustment sets up the engine so it can idle properly with the IACV partly open. The ECU cannot control the idle speed beyond what is possible with the IACV fully closed or fully open. If the air flow gets outside of this range the idle speed will be wrong.