Higher idle when raining?

[m3tal]

Noticed this recently. Seems like whenever its raining out/high humidity, if I have the defroster on and any fan speed, my idle will hover around 14-1700 at a stop, then gradually drop down to around 900 after about 10-15 seconds. Especially noticeable in stop and go traffic, because the cycle repeats at every light I stop at. High idle, dropping down to normal within 10-15 secs. Anyone else experienced this or have a good reason for why its happening?

[Skym]

ECU can make engine idle at around 1500rpm when engine is hot or cold. It means MAF is faulty and usually have a MAF error code on ECU. Or it could be the silicon seal on square part of MAF has failed and MAF is intermittently failing, then not failing. Check ECU for MAF error code.

But ECU has kick down tables for manual and auto. When TPS touches idle contact, ECU switches to these tables and ignition timing is changed, rpm rises then settles. Sometimes it's affected by airleaks (too much air to idle valves, MAF, etc), so check for airleaks around MAF, throttlebody (where air is fed to idle valves), etc.

In some cases it can mean idle valve or around throttle plate inside throttlebody needs cleaning due to sticking (oil deposits from excessive blowby or blown oilseal on compressor side of turbo in the past).

[m3tal]

Last I checked, had no ECU errors, and I'll check the other stuff, but like I said, it only happens solely when its raining out. When the air is dry, this doesn't happen.

[markc32]

idle steps working but having a hard time...IACV when it tries to compensate for more engine load....when u put load on the engine idles LOW so thats when the IACV comes in....it should not take 10-15 sec for idle to go back to normal but check things out

[nelsonmxmarc]

AACV effects high rpm due to defroster steeling a lot of power from the motor. It's a large capacity system to use, as the wire i pulled from that unit must have been at least 12 gauge, meaning rather high current. So yes, your idle will be higher. This is due to the engine having to work harder for it to power the unit, thus letting more air in, thus increasing fuel tables and etc raising rpm. When you turn it off, like the AC, you'll notice the rpm level off differently. Newer cars are better attuned to these sensitivities; changing AACV will help in most cases but its not something to fret over.

other things to take into account is air quality. Wet, cold air is damp and dense. Its not quality air, and the MAF reads these characteristics.. Takes a while to figure itself out sometimes.