When started cold, the engine's idle speed is around 1200 RPM even when the ambient temperature is around 60/70F, and within five minutes, it drops to around 1000. After driving a few miles, the idle finally drops to 700ish.

One odd thing is that after a cold start and drive the car when its idle drops to 1000, if I park the car after driving just one or two miles, the idle normally go up and down between 1000 and 1500, occasionally up to 2500.

There's no CEL, no code. Just recently I cleaned the idle air control valve (though it wasn't too dirty) and replaced all spark plugs, valve cover gaskets, thermostat, air intake hose, throttle gasket, ECT sensor, and fan switch sensor with all Honda parts. And I also replaced coolant, ATF/filter, brake fluid and power steering fluid with all Honda parts/fluids. There's no vacuum leak. I'll appreciate if anyone can point out what else I need to look into. Thanks.

 

Did you perform an idle speed relearn?

 

Yes, I did the PCM idle relearn procedure per the service manual.

 

I think we recently through this but there can be several items. You have eliminated the vacuum to start. I would say the next likelihood is the IACV or throttle body issue. See Below.

 

Have followed the procedures you mentioned. After keeping the RPM at 3000 till the fan comes on, the idle is around 700. According to the manual, there's no issue with the IACV Also the OBD reports the throttle is at about 10% and 90% when it's closed and fully opened respectively.

I read that issues with AT torque converter can also cause high idle, so I replaced the ATF filter and ATF (3-qt refill) and cleaned both AT clutch solenoids even though they weren't dirty at all.

 

Initially I found there were some cracks at the end of the air intake tube, which mates the throttle body, so I replaced the tube, but that didn't fix the issue.

 

Few questions.
1. When you were running the above procedures did you do the whole procedure or did you stop where it says intermittent failure since the idle was 700? You should complete the rest anyway it might lead to the intermittent problem. Also, you mentioned that the idle changes after the car is hot after being driven (which you can do instead of idling at 3k to get it hot), you can turn off the car let it set for a few minutes and restart and run the tests again.
2. What happened when you disconnected the connector and when you covered the hole in the throttle body?
3. Have you cleaned or replaced the PCV valve and hose?
4. When it idled at 700 RPM when you tapped on the pedal did the RPMs start to fluctuate?
5. Any play in the throttle plate when cold or hot? (shaft movement the same?)

 

Didn't do the whole thing and stopped at "YES-Intermittent failure, system is OK at this time"

Didn't do this part since the idle was around 700 after the engine warmed up.

Replaced the PCV value two months ago but didn't replace the hose. Hadn't experienced the high idle issue before the replacement.

When I pressed down the pedal, the RPM went up. When released the pedal, it went back to 700 RPM.

No play.

 

another low commitment item to check: Have you burped your coolant system? best route is a spill proof funnel to get all the air out.

 

Yes, I have done that.

 

Just so I can be clear on the issue, when the idle fluctuates it is after the car is warmed up (by driving). Have you ever recreated it at home? Also you mentioned it started after you replaced the PCV valve what else did you touch at that time? Did you move the throttle body at that time? Have you tried clamping off the PCV hose when the car RPM is changing? Was the PCV OEM or aftermarket? Trying to get more information to think about.

 

Sorry about the confusion. Let me clarify the issue. The high idle speed is only when I start the car cold (ambient temperature can be in 60F), and it normally hovers around 1200 RPM but dips to around 1000 after idling for 3 to 5 minutes. After driving 5 or more miles, the idle finally drops to 700ish. If it's less than 5 miles, the idle speed can fluctuate between 1200 and 2000, sometimes even higher.

​​​​​​Two months ago, I replaced both the valve cover gaskets, spark plugs, the PCV valve, and the throttle gasket with all the OEM parts, and cleaned the IACV at the same time. Yes, I had to remove the throttle body.

About a month ago, I replaced the thermostat, two radiator hoses, the ECT sensor, and the fan switch sensor with all Honda parts, and also flushed coolant (Honda coolant) and burped the radiator.

The initial high idle from a cold start has started about one week before I replaced the thermostat. Then, I replaced the ATF filter, flushed the ATF, and cleaned both the AT clutch solenoids.

 

Ok. I misinterpreted what the problem is, I believed that once the car was warmed up the RPMs were varying. What I get now is that the between cold and hot the RPMs vary any time you stop until the car is hot. This is a new one for me so I need to think of possibilities. I never even looked at mine like this as I mostly get on a highway soon after I start the car. Maybe others have noticed this behavior.

 

I'm not sure how ridiculous this sounds (expect to hear feedback on it) but the IACV has coolant running through it, is anything blocking the coolant? My thought is that the coolant is not flowing and the IACV is not warming up from the coolant but from radiant heat, causing the IACV to behave as if the engine was cooler causing the idle to fluctuate.. If you think this may be a valid thought you can remove the coolant line and start the car (route the line to a container to capture coolant) the coolant should have a constant flow. You might want to cap the coolant connection on the IACV to prevent it from sucking in extra air. As I said this could be pure fantasy on my part but since nobody else has had a suggestion so far ...