So my ep3 all stock with 197000 miles and running strong has a strange idle issue. First thing in the morning it has a high idle around 2k rpms. Once hot, at stop lights idle goes to 1200 to 1500 rpms for a little bit then down to normal range until I step in the gas it starts doing the same all over. I checked everywhere for vacuum leaks, there are no cels, I removed and cleaned iacv which seemed fine and replaced it’s gasket albeit it was not that dirty and it moved freely, I also cleaned TB and replaced TB gasket. Valves were adjusted. I cleaned reusable air filter, checked pcv operation, and installed new oem spark plugs, and cjhecked coolant level--no bubbles . I noticed that if I turn off ac at stoplight idle runs higher by at least 500 rpms and when I turn on ac idle goes down to the 1200-1500 rpms so I am thinking maybe the iacv is an issue despite looking fine? I do not know if this is related but I disconnected the throttle and cruise control cables when I took TB out, put them back and seem fine but now my cruise control does not work, not even the dashboard light when you turn on cruise control. And BTW I also did the idle relearn and it made no difference.

 

Idle should be at 750 +-50 rpm. Are you throwing any codes? Check pages 11-130 and 11-138 in the service manual

 

No codes

 

Seems like you went for most idle issues. Did you check your throttle position sensor? Possible chances of relays and/or fuses? Also a basic tune up helps. Could also be a chance of it taking in too much air than it should but all of the cases I mentioned should throw CEL codes.

 

Second the TPS. Check voltage, sould be 0.5 - 4.5 closed/open

 

Well, it seems to be the thermal by pass valve allowing too much air, I did the vacuum tests and it failed. I ordered a new one. Now my cruise control stopped working!

 

Ok so I replaced the by-pass thermal valve and the up and down idle is gone! There is now a slight uptick in idle at stoplights that lasts a couple of seconds at around 1100 RPMs then quickly settles down to the normal 750 RPMs, I rechecked coolant level and bled the system for 40 minutes to make sure no more bubbles came out. All vacuum lines are good and IACV is working fine. Not sure what is causing the slight idle surge.