Sometimes I feel like I just get lucky…I can't be the only one

Mike from Fond du Lac Instructor Posted   Latest   Edited  
Case Study
Driveability
1998 Buick LeSabre Custom 3.8L (K L36) 4-spd (4T65-E)
P0505 - Idle Control System
High Idle

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?

+13

Chris from Commack

 

Diagnostician
 

I wonder if this is the reason the vehicle was put to sleep

+1

Mike from Fond du Lac

 

Instructor
 

Yep, that's my thought!

0

Sean from Cornville

 

Owner
 

I ran into something similar on a 1990 eagle talon recently. Idle was way too low, and found faulty iacv, with one open winding. The original one was aluminum, and all the replacement ones were plastic. I installed the replacement one, still too low of an idle. I scoped it, and found the ecm not grounding the one circuit that was previously open. Sent the ecm out for repair, reinstalled it, and all of a sudden, the idle was way too high. I could see the ecm commanding it lower, and the idle raising. I reinstalled the original iacv, and monitored the movement while manipulating the idle speed, and sure enough, this thing was moving opposite of what it should. On these Mitsubishi vehicles, it is a 6 wire iacv, with two power wires, and the other four are pulsed to ground. I took apart the old one, to see how it worked, and swapped the four ground wires around. Reinstalled it, and now, it has a perfect idle.

Now, I am plagued with injector issues with this same car.

+2

Mike from Fond du Lac

 

Instructor
 

Awesome find Sean! Amazing how you can fix one thing only to uncover a bunch of others..."ever since" haha!

0

Dean from Albany

 

Owner
 

What is the old saying, I'd rather be lucky than good?

+2

Geoff from Lahaina

 

Diagnostician
 

does bad luck count, Dean? That's all I have! :-)

+3

Dean from Albany

 

Owner
 

Yeah, Geoff, I hear you, I've been there and lately, I think I'm still here,😁

0

Geoff from Lahaina

 

Diagnostician
 

how's this for bad luck....after ten years of telling all my friends I would never ever join Facebook, (cuz it's stupid) I heard about the diagnostic groups on there, so I signed up, and joined a couple groups.....Facebook just disabled my account saying they don't believe I am me.

+1

Eric from Edgerton

   

Owner/Technician
   

I'm not sure I believe I am me, er ah , you are you either. Got any proof? ;-)

Oh and, welcome to FaceBook another great time sink.

0

Geoff from Lahaina

 

Diagnostician
 

The time sink is trying to figure out why my account is disabled. I only signed up for access to the diag groups. Maybe since I didn't link ANY local friends or family that tripped their "fake person sensors. IDK

0

Jaime from Ocala

 

Diagnostician
 

Do you think someone "Reported" you? Got any enemies? LOL

0

Geoff from Lahaina

 

Diagnostician
 

certainly can't figure for what (or how). They don't even alert you or give a reason, you just suddenly get "account disabled" when you try and log in. Possible reasons are listed elsewhere but they are generic.

0

Michael from Atlanta

 

Diagnostician
 

Better to be lucky than good Becker ;)

+2

Geoff from Lahaina

 

Diagnostician
 

Post pics of the carnage next time, That's the "fun" part of posting. Long before I had a camera at work I saw an O2 sensor wired up with MEDICAL tape. LOL-serious!

Great work Mike.

+1

Michael from Clinton

 

Mobile Technician
 

Fantastic! Thank you for sharing!

+1

Bob from East Longmeadow

 

Diagnostician
 

Nice job Mike. Nothing wrong with getting lucky sometimes. Visual checks are an important diagnostic step.

+1

Felix from Miami

 

Diagnostician
 

good to see you here Mike , just last night I was watching your video of motorage about the labscope use in diagnostic, thanks for sharing

+1

Michael from Holt

 

Diagnostician
 

Thats a great find and I have run across that of someone wiring something backwards before. It was a MRE type abs sensor they had put everything new on the car before bringing it too us and I looked up a wiring diagram which is one of the first things I do after a quick scan and data check of which ones not working and I just started laughing when I went to backprobe the connector. I couldnt believe it they were swapped in the connector. So the fix was easy which is how I like them.

0