I'll be helping a family member with their GTI's idle problem. The problem, as reported to me, is:
Owner said:
Car wont idle. If I disconnect the coolant temp sensor, the car will idle. I replaced the sensor, the car still wont idle. I think it's the electronics in the ECU for that sensor.
Ok, logical conclusion based on the troubleshooting, but I aint buying it. This is what I think happens: By disconnecting the temp sensor you've put the ECU into a condition where it's running a richer mixture to compensate for the lack of sensor input and thus the car idles.
So moving along, I was going to head out to the car with some brake cleaner and clean out the throttle body, IAC, and whatever else strikes my fancy but I really came here to ask:
Is there any model specific info that'd be beneficial to me before I head out that way?
Thanks Ya'll. 
PS
I have no idea what brought the person in question to disconnect the coolant temp sensor.
next odd question: The owner refers to the car as a "Mark 2". Wouldn't it be a "Mark 3" at 1992?
That is all
92 would still be a mk2.
No idea on the actual problem. 
I'm not familiar with MK2s enough to really have an opinion here but vwvortex is your best place to ask.
Here's a link to the the MK2 forum:
Vwvortex MK2 Forum
And there are also engine specific forums. Assuming this is a 16v motor (tubular intake manifold runners crossing over the valve cover) then this is where you want to look too:
16v Forum
Thanks for the links. Looks like the two most likely culprits are the IAC and an "Idle Stabilization Valve". Also looks like brake cleaner is a "no-no".
Contradiction wrote:
I'm not familiar with MK2s enough to really have an opinion here but vwvortex is your best place to ask.
Take this with a grain or five of salt. You might get a good answer to your question, but you may also be cursed at, called a n00b and/or told to stance your car.
The IAC and the ISV are the same thing.
If it's the 8v, it will be a digifant 2 system, and those ECUs can be kind of flaky. Usually when they go, they don't mess around. Pull the cover off of the ECU and if it smells like burnt electronics, replace it. If it doesn't, you might also check the AFM's resistance to make sure it's reading correctly.
If it's a 16v, it will be CIS-M, and those are VERY sensitive to vacuum leaks. Don't adjust anything (idle, fuel mixture, etc.), you'll probably screw it up worse if you do.
Brett: I've heard the horror stories, that's why I came here
16v: Thanks for the heads up. In the reading I've been doing it sounded like they were two separate valves, now I know. And a definite "thanks for the heads up" about the adjustments. I had "twist the idle adjustment screw" as a backup plan.
Now to find the AFM values.
Brett_Murphy wrote:
Contradiction wrote:
I'm not familiar with MK2s enough to really have an opinion here but vwvortex is your best place to ask.
Take this with a grain or five of salt. You might get a good answer to your question, but you may also be cursed at, called a n00b and/or told to stance your car.
True. Depends by forum and car generation. I haven't spent any time on the MK2 forums but at least on the MK1 forums people are well informed and posting useful content and helpful answers. On the other hand the MK4 forums and on are probably filled with idiots asking how to frame notch their beat up jetta so they can get "crazy low" with a set of $350 Raceland coilovers. Or just air bagging and putting stretched tires on their "hella flush" wheels so they can get "stanced". I can't wait until this stance fad is over.
Vacuum leak(s) would be my guess. As the the Vortex, there are also engine specific forums and those are, for the most part, helpful.
I worked on a VW once, I don't remember the year, that wouldn't idle. Instead of an IAC I found a small motor that opened the throttle plate a little. I got it drivable by adding a ty-wrap to the linkage. The actual problem was that a garage over filled it with oil and oil came out of the breather and messed up the MAF. Proper oil level and MAF cleaner eliminated the need for the ty-wrap.
Holy crap! The oil got the MAF sensor!?
Air leak(s) or bad idle motor is the first items to check.
Getting it to idle with the ECT unplugged makes me think air leak. Unplugging the ECT makes the car go full rich, which would compensate for an air leak (lean condition). Hopefully it's not the idle motor, that sucker ain't cheap!
Check the air intake boot carefully. Remove it and flex it while looking for cracks in all the bellows.
I'll try to run the motor in and out while it's off the engine, I also found some resistance values for it I can check. The boot should be easy to see if it's good, and the guy who owns the car smokes so vac leaks should be easily located with a cigarette. Looks like I've got my list
Thanks everyone!
Make sure all of your grounds are sparkly clean as well. My '92 GLI ran a hell of a lot better just from cleaning the grounds, CIS-M doesn't like dirty grounds.
Also, +1 on the recommendation to never, ever adjust the mixture screws. The fuel disty screw has an anti-tamper plate installed for a reason...
Hungary Bill wrote:
Brett: I've heard the horror stories, that's why I came here
16v: Thanks for the heads up. In the reading I've been doing it sounded like they were two separate valves, now I know. And a definite "thanks for the heads up" about the adjustments. I had "twist the idle adjustment screw" as a backup plan.
Now to find the AFM values.
If it is the 8v, you can mess with the idle screw a bit and not mess things up. Also, at least on the older 8v's, the o-ring on the idle screw can get hard and flat, causing a vacuum leak, and making the screw VERY easy to turn, so the vibration itself can cause it to self-adjust, and not in a good way.
SlickDizzy wrote:
Also, +1 on the recommendation to never, ever adjust the mixture screws. The fuel disty screw has an anti-tamper plate installed for a reason...
Seriously. When I used to work on a lot of them, I would always immediately get depressed when I saw that the plug had been knocked out on a CIS-M car.
Just got a text back. It is an 8v (so squirshed idle adjustment o-ring goes on the "places to check for vacuum leaks" list)
Good times
docwyte wrote:
Check the air intake boot carefully. Remove it and flex it while looking for cracks in all the bellows.
This is what I found on der rally car.
https://www.youtube.com/watch?v=gwqP8MVcvQU
Skip to 1:09
holy smokes! That's quite a vac leak.
I'll have the car tomorrow at 10am. If you haven't heard from me by Sunday evening, call my wife. Tell her it wasn't her fault.
So the check went as follows:
arrived and found out that the IACV had been cleaned and changed already, but I took a resistance reading with my meter (4 ohms). Intake elbow had been replaced with a solid plastic unit, and everything looked secure. So I started the car and the idle was kind of rough.
Got a cigarette from the owner and ran it near everything I could. No leaks.
So I cleaned the throttle body, fired her up, and she's as smooth as butter now
Come to find out this had been troubleshot by friends, taken to a shop (for a week), and then had an IACV thrown at it.
I figure 20-minutes isn't a bad timeframe to troubleshoot and solve an unfixable gripe
Thanks everyone, for making me look daaaaaaang good.
