Idle surge and hard cold start???

[Wolf1]

OK, I have two problems (hopefully they are related). On cold start-up (like below 20 degrees) it is difficult to start and runs very rich. The other problem is after running at fast idle for a minute or two it starts to slow down and then revs back up. This continues for about two minutes then it idles fine.

I have scanned for codes (none found) and monitored the coolant and air temps (they seem to be accurate). Any thoughts?

It is a 91 TSI.....new motor, new stock turbo, etc...

 

[GTM]

Originally posted by Wolf1

...
and monitored the coolant and air temps (they seem to be accurate). Any thoughts?
...

What do you mean by "monitored" ... watched or actual measurment? Off the top I'd suspect the temp sensor is getting ready to die. It is normal for the rpms to climb as the combustion chamber warms (20-30sec?) and then start tapering to a normal idle... what's different about yours?

Cheers,
GTM

 

[mitsutuner]

It sounds like the rubber O-ring around the BISS (base idle set screw) has aged & is leaking air into the throttle-body. To check it, cold start the car, pop the hood & hold your thumb over the cylinder which the screw is inside of & see if this changes your problem--if so you can replace the O-ring,(home depot motorsports) & then fiddle with the screw until your idle speed is correct.
The best & easiest solution is to plug off the cylinder on top of the screw with rubber cork, or RTVor silicone or bubble gum or (your guess here). Alternatively, you coud just rubber cap the cylinder. This should be done no matter what, because even if you have a new O-ring, sooner or later it will leak.

 

[s7e]

Sounds like the FIAV (fast idle air valve). Its on the bottom of the TB, it has two coolant lines going to it. That is the most common cause of surge on start up. I have problems idling when starting b/c I bypassed the FIAV. It can be done by looking on vfaq.com. Bypass it or get a TB from a junkyard.

 

[Wolf1]

Originally posted by GTM
What do you mean by "monitored" ... watched or actual measurment? Off the top I'd suspect the temp sensor is getting ready to die. It is normal for the rpms to climb as the combustion chamber warms (20-30sec?) and then start tapering to a normal idle... what's different about yours?

Cheers,
GTM

By monitoring the sensors I mean watching the scan data.

The difference with my idle problem is: the idle surges up and down as its warming up.

Thanks...

 

[GTM]

Originally posted by Wolf1
...
The difference with my idle problem is: the idle surges up and down as its warming up.

Ok, did you read the 2 previous posts and have you checked those?

If you decide to adjust the BISS, do yourself a favor and carefully count the number of revolutions including fractions down to the seat. It just needs to be finger tight, then back it out to exactly where it was. Please tell me that it's 3 turns +/- 1/2 of that setting.

Now you can play with the adjustment and at least return it to what was working. What you don't want to do is introduce too many variables while trying to find the problem. I did come across something indicating there was a wax pellet thermostat in at least some of the T/B housings but have not confirmed this nor do I know that it is your problem but it's worth investigating.

Cheers,
GTM

 

[mitsutuner]

Here is the link to the IDLE SURGE FAQ. No mention of the wax pellet, but I know that is a cold-start factor talked about on another web page & I think maybe this link is linked to that link.

http://www.myzero.com/engine/surge.html