O2 sensor= bad IAC?

[legacy92ej22t]

I was thinking about something. I've been having issues with my O2 sensor setups for a loooong while now. I used to have the wiring extended to just before the mid-pipe on my modded CES WRX TBE and although it worked for awhile it didn't last long and started causing severe lean issues with my fuel trims. I now have a new 4-wire O2 sensor setup in the proper location (thanks to my new CES Legacy TBE) and the problem is still the same. So here's my thought, could my problem be related to the IAC? I got a code 24 again in r-mode. I've never gotten a CEL for it but it seems to show up whenever I try and use the O2 sensor. Could using the O2 sensor cause the IAC to have a problem that isn't present when running in safe mode without the O2 sensor? Just a thought. Any help or suggestions would be great.

[BAC5.2]

The IAC valves are bitches. My car throws code 24 on a more than daily basis, and the car stalls in neutral so often, you could set a watch to it.

Unfortunately, the 02 sensor is completely independent of the IAC valve. The IAC valve reads ONLY Engine Speed, Vehicle Speed, and Throttle Position, then it gives fuel cut to keep these three variables in check (if the valve is malfunctioning).

You have an FCD, so you wouldn't know if the valve goes bad (no fuel cut). Safe-mode probably prevents the throwing of this code since it has a fail-safe.

The 02 sensor brings the ECU out of safe-mode and thus throws a malfunction code.

[legacy92ej22t]

Ya, I figured they were independent of each other but I'm just grasping at straws now because I'm SUPER frustrated by this. This problem has been going on for about 7 months now!

When the O2 sensor is hooked up my car acts as though it has a severe vacuum leak. Vac drops from a solid -19 inHg at idle without it hooked up to -15 inHg (sometimes all the way to -10 inHg when it stumbles real bad) at idle with it hooked up. Then, the car stumbles going on throttle for a second or two (kinda like the ECU gets a delayed TPS signal or something) then it pulls crazy hard all the way to redline! It's nuts.

I just can't figure out what the problem is. I think it's either the wiring harness side of the O2 sensor wiring or the ECU is bad. I also have a VERY hard time getting codes to clear off my ECU memory so I 'm leaning towards a bad ECU.

[BAC5.2]

Sounds like a good case of a malfunctioning IAC valve. Well, the vac problem anyway.

I wonder if it's possible that the IAC is open a little bit, but the O2 sensor is keeping the idle in check (by comparing exhaust flow with throttle position). This would create a normal idle with a super low vac. When my IAC acts up, I only read around -15 or -16 inHg. But I'm also idling at around 2k. The car doesn't stumble though on acceleration unless I hit fuel cut.

I'd suggest cutting out the FCD and soldering things back to stock. Back off the MBC to stock boost, and see what goes on. Reset the ECU by pulling the battery, and drive normally for a while.

I can't think of anything you probably haven't checked that would cause this. I hope it's not a bad ECU. I also hope it's not a bad IAC, because them bitches are $400 bones at the stealership.

[legacy92ej22t]

When I have the O2 sensor hooked up and have the bad vacuum readings at idle, the car runs soooo rough it almost stalls. Rpm's are normal if not a little low and oscilating.

When I have the O2 sensor unhooked or hooked up and have a cold start (rpm's at around 1.5k) I do usually get around -16 to -15 inHg too until the engine warms up and rpm's fall to around 750 rpm. Once warmed up it's -15 inHg and super rough, at the verge of stalling, idle with O2 sensor hooked up and solid -19 inHg at idle with the O2 sensor unhooked.

I don't believe the FCD has anything to do with this. It's not a boost issue at all. Once I'm on boost the engine will run super, super strong. The problem is mainly at idle and that my fuel trims are getting way leaned out ( like -20 some %). Plus this problem manifested itself BEFORE I installed my FCD.

I think I need stand alone engine management like a nice TEC III. Anyone have $3k I can have?

[BAC5.2]

But the FCD cuts fuel when it malfunctions, so under any conditions it can cut fuel, boost or not.

But the FCD probably isn't the problem (although I remember you saying you were having a problem with it on the highway).

I'd say spray the IAC valve with some brake-parts cleaner. It didn't work for me, but it might work for you. At least that will remove a variable.

My car did run super low idle and bounce off the 500 RPM mark for a little while before the 2k idles and then stalls. Maybe your IAC is simply acting up slightly differently than mine?

If the brake parts cleaner doesn't work, you might have to start saving for the $400 bill at the stealership.

[vrg3]

The FCD Matt has only prevents overboost fuel cut. It's not related to the IAC valve problem.

When you hook up the oxygen sensor and the ECU's fuel trims shoot down to the lower rail, you'd expect that to be a lot like a vacuum leak, wouldn't you? Both situations make you run lean.

"Safe mode" is just a term Matt's using to describe how the ECU behaves without the oxygen sensor. I don't believe it's an actual different mode of operation than normal driving.

I suspect the wiring harness. It might not be a bad idea to replace the one signal wire for the oxygen sensor with a new one going all the way from the sensor to the ECU.