Hi,
I feel bad posting yet another idling issue thread but this one has been going on for ages and I just can't find the issue.
Car:2003 S2000 2.0 European model. Garage queen driving from April to October. Has always been garage kept
Location: Netherlands

Symptoms:
On start up, car has a high idle.
When stopping at a stop/traffic light, RPMs drop with the engine fully stopping 50% of the time. When the engine doesn't stop, the car shakes because of the low idle. This can vary to 70-30 or 30-70 but I can't link it to temperature, humidity or anything else. If I am approaching a stop and let the car roll with the clutch pressed to the floor, the engine will NOT stall until the speed indicator reads zero. Another one which is confusing me is to be seen in this video. On that video, the car is stopped on a downhill incline in second gear with the clutch pressed to the floor. You can see the idle is very low. When I release the brake (but keep the clutch to the floor), you can see the idle jumping up.
Accelerating, the car feels completelly normal, revs happily, nice to VTEC, an absolute pleasure to drive!
Done so far:
Injector cleaner in the fuel => No difference
Air filter and spark plug replacement => No difference
ECU reset => No difference
Map whack => No difference
IAC cleaning => No difference
Zip tie on the Map sensor => No difference
Cleaning air intake => No difference
Replaced the TPS => no difference
Maintenance on a yearly basis since 2008 when I bought the car.
Mods: Only an exhaust but that was done when the issue already existed and it did not change anything to the idling issue one way or the other.

So, please let me know if you have any idea of things I could try to fix this :-)

 

Does the battery die when the car is parked? If so then the ECU will always be reset and require you giving it to time to relearn idle. It will take several km's of driving and may cutout/switch off when coming to stops for that time but should go away completely and idle fine. Dips in idle will still occur even if everything else is perfect because the stock ECU does not do a great job of catching idle revs fast enough. Aftermarket ECU/tuned, never happens and is always stable (unless badly tuned). On startup the car will have a high idle by default, that's not a fault. It should lower within a few mins of driving though.

 

Thanks a lot for the reply and for thinking with.

No, the accu is recent and healthy, does not reset the ECU by running empty. During the winter, the car is on trickle charger.

 

Oh ok. We can rule that out then. Is drivability affected in any other way? Can you go WOT easily into VTEC then redline without any breakup or other strange behavior?

 

Does it ever stall in neutral?

You mentioned a lot what idle does in this scenario or that, but common to all scenarios was car was in gear and clutch pressed.

I have a theory we can rule in or out if we know behavior when in neutral.

 

Drivability is perfect! It accelerates nice on WOT or any kind of gaz pedal pressure. In any case, smooth acceleration to VTEC and in VTEC

If I roll to a stop in neutral, the issue is exactly the same as being in gear with the clutch to the floor.

 

Another video to try to help

Situation: The car is in a garage, in neutral, just stopped after a +/- 40 kilometers drive, the fan kicked in and stops roughly 4 seconds in the video. Once the fan has stopped, i give it a bit of rev and it comes back to idle. I then give it another gaz plip and the car almost dies but recoveres itself. I only pres the accelerator in those 2 instances, not any other time.

Let me know if that inspires you to any possible cause.

 

That's what I thought
Now it sounds like air flow to the IACV is an issue. I'd assume your air filter and TB are clean enough? You could always clean the TB and butterfly with some good carb cleaner. Still, those two things won't cause this poor idle speed - the IACV is the primary part that controls this. When you cleaned it did you spin the shaft free? If so did you lube the ends of the shaft with some light oil or even WD40? When the ECU sends a signal to the IACV to open the valve, that motor has to spin quickly and freely enough to do so every time. If the shaft is still sticking then you need to get back in there. Even better if you have a buddy that can lend you their IACV for a test you'll have a sure fire way to know if that is the culprit.

 

Thank you so much to the thinking. Let me check what I can find via via before I order from Honda.

BTW, amazing how many AICV from Aliexpress are being offered online

 

My pleasure

PLEASE don't buy an IACV or any other S2K part from Ali Express! It is shocking how many fake parts there are out there...