Sometimes I feel like I just get lucky…I can't be the only one
Hey guys, wanted to share a car I worked on this past Friday. This is my first posted case study so go easy on me :)
Car came in with a high idle, 2000+ rpm. Owner just bought knowing it had some issues. Car has been sitting for "years". Read codes, 8 codes related to just about every sensor that could be unplugged....cleared them out and got a consistent P0505.
Alright, so looking like an idle air issue. Data PID shows 0 counts at idle...weird. Vacuum leak? Something allowing this engine to breathe without the IAC at closed throttle? Smoked the intake, no leaks. No propane handy so I grabbed the can of carb clean. No change in trims while spraying common leak points. Let a vacuum hose sniff a little carb clean and see a perfect reaction from 02's and FT's. By the way, fuel trims are otherwise perfect (probably just wasted 5 minutes looking for a vacuum leak). Attempt the idle relearn, no change.
So trims are spot on, but IAC counts at 0....weird. Haul the scope out...should have screen grabbed the capture...
PCM is controlling the stepper motor...removed the IAC and cleaned the pintle and mounting hole. Reinstalled and idle is low for about 2 seconds and it slowly starts creeping up, higher and higher, 2500rpm +. Attempt to control the IAC with the scanner, nothing happens... but the IAC does react on the scope...hmm
SO now i'm really scratching my head....what am I missing....Ft's are great. IAC is showing closed...this motor should not be idling, let alone a high idle. Throttle plate is confirmed that it's closed
For whatever reason at this point I looked at the wiring leading to the IAC. Everything looked ok, no new tape, no evidence of anyone being in there...I did do a quick visual early on in the process and didn't see anything obvious...
Here's where the luck comes in...
I decided to pull back the tape/corrugated protection....woah someone was in here, obviously a while ago because the tape wasn't new. The wires are twisted and taped (are you kidding me). But, I had control from the PCM...
Upon inspection of the wire colors I find the problem. Someone, looks like a while ago, wired the power wires going to the IAC backwards. So, the PCM tries to close the valve it's actually opening, and vice versa. So, the harder the PCM tries to bring the idle down the higher the idle gets. The blu/wht wire and grn/white wire were swapped. To the defense of whomever worked on this thing...the 2 wire colors were identical at the pigtail. The blue had faded to the same shade as the green wire. I had to peel the harness apart 3 feet back to find the true wire color.
I am not sure if it was luck that led me to the harness but whatever it was, the cars fixed, customers happy and I can put another one in the memory bank that had me saying "what the $%$#."
What would you have done different?