Search Corolland!

By saturnhasrings, November 25, 2015

This seems to be a complicated issue which a new friend at the dealership thinks is software related rather than parts.

I have a 2003 Pontiac Vibe GT with 2ZZGE engine. This past weekend I cleaned the throttle body and IACV (idle air control valve). I did a complete throttle body removal and also removed the IACV from the throttle body. Everything went perfectly and was cleaned to new condition.

Exactly what I did:

PROBLEM: when I performed the clean I made sure to disconnect the negative terminal and I was disconnected for at least 4 hours (I was also replacing the oil pan gasket). When I finally started the car it idled at 2000 rpm. Once I actually drove it and then clutched to return to idle or put it in neutral the idle was 3000 rpm and a quick gas pedal flick would cause it to drop down and begin an idle search where it surges the rpm from 1500 - 2100. Sometimes this doesn't stop till I put it in gear and drive again or sometimes it finds idle at about 2000 rpm. I can also tell that while the vehicle is driving that there is throttle without my foot on the pedal.

I spoke with my new dealer friend and pulled my EFI fuse and relay for 1 hour with no change, so I left my car overnight with the EFI fuse pulled and still no change. I confirmed that when the surging is going on, that the throttle butterfly is fully closed and not moving. This means that the IACV is indeed working and is what's causing the surge. Like I said, the IACV is mechanically moving but it refuses to relearn proper idle.

The only thing I can find that is pretty much exactly what's going on with my car is this video were a guy has an MG that surges just like mine and he does a manual idle relearn through a pedal press/ignition on/off coded process. I've asked the dealership if there is a manual idle relearn override and they've advised that it's supposed to automatically relearn idle after ECM or EFI was not powered for at least 1 hour.

MG video:


Just to be clear:

Throttle body is perfectly clean and butterfly is not restricted or not closing

TPS (throttle position sensor) was not adjusted or changed in anyway during cleaning

No engine codes or engine lights are present

Throttle cable is fine and tension is good

Idle surge has plagued many of the this particular family of engines. Not just the high performance 2ZZ-GE, but also the efficiency tuned 1ZZ-FE. Originally thought to be a software issue, it was eventually tracked down to a more common culprit: a damaged intake manifold gasket.

Any leaks in there will cause the idle to surge, as this is additional air that has not been metered by the MAF sensor upstream. Same with the throttlebody gasket - leaks there can cause idle surging.

True, it does take a little bit of time for the ECM to relearn new parameters when it is reset or had power pulled from it - but the actual idle is preset, both in software and physically (cabled throttle).

Since you've already mentioned that the TB is clean, IAC seems to be functioning, cable is clear and not too tight, TPS wasn't messed with - that pretty much leaves a vacuum leak as a most likely culprit. Double check all the vacuum hoses, hose unions, and induction system gaskets. A high idle could also be caused by a malfunctioning upstream O2 sensor, exhaust leak, bad MAF sensor, or bad PCV (actually pretty common). Software or ECM would be at the bottom of my list unless the ECM was modified in anyway (piggyback or standalone EMS addition, any resistor box mods, etc.).

So I finally purchased a new IACV and installed it to find that it made my problem even worse. I've since checked a couple of other threads that advise air in the coolant system can cause surging idle and I've confirmed this isn't the issue either.

Here's what is most confusing about the problem: with the car running or off, before or after an ECU reset (tried all) I can completely unplug the IACV (my new one or old OEM one) and it makes ZERO difference to the idle.

to follow up: initial cold start causes a surging idle between 2500 - 3000 (seriously risking engine damage as it hits 3000 rpm on start up in -20 C)

When warm it calms down to a surging 1500-2000

never gets below 1500 rpm


Currently get OBD code P050 "Idle air control system" (clearing the code or resetting ecu does nothing)

Toyota tech spent 2 hours checking the system, adjusting the throttle close nut and could only come up with IACV

Replaced IACV with no change

Tested theory on air in coolant system and was able to bleed a few air bubbles for a very minimal improvement (100 rpm decrease or so?)

I honestly have no idea where to go from here and I really can't understand how the car reacts EXACTLY the same when the IACV is plugged in or unplugged... Is my PFC causing this entire issue?

absolutely any help would be amazing!!!


Sounds like you've covered most of your bases on this - technician spending that much time on it and you've done due diligence in swapping parts / diagnosing as much as you can. I agree - this is sounding more like an electrical / ECM issue.

Higher idle speed or idle surging is usually a sign of a bad or non-functioning IACV. Since you disconnected it and it made no difference - this leads more weight to it pointing to an electrical issue.

If the tech has access to a Techstream tool - they should be able to manually command the IACV to open/close - which should make a marked difference in idle speed. This will also verify if the ECM can actually move the IACV.

I'd rule out the simple stuff first - ie, check the connector to the IACV and make sure the wires aren't pulled out / damaged, no corrosion on the connector. Trace it back to the main wiring loom, make sure there are no cuts or other damage to wire. Also check that the IACV is making good contact with the throttlebody - might look like it is sealed, but could be leaking air around there. Can try spraying something around it and the throttlebody while it is idling - see if it changes the idle speed at all. If it does, then you've found a leak. If you have a multimeter and copy of the wiring diagram handy (technician should be able to print this out for you) - you can back probe the connector all the way to the ECM - see if there are any wires that are broken inside the jacket.

If all that checks out - possible that the ECM is bad. I've seen some weird cases were the functionality of the ECM wasn't completely compromised - some stuff still worked, but due to a cracked board - there was intermittent issues that kept pointing away form the ECM.

ECM swap is not a cheap fix - may be able to score one cheap from Ebay or Craigslist - but it may not fix what you have. These intermittent electrical stuff is tough to diagnose.

Topic List: Go to Toyota Corolla, Chevy Prizm (1998-2008)