1G Another Idle surge post!

[Masterbox]

91 eagle talon TSI stock.

had idle surge until warm up and then it operated normally.
did a bit of research and replaced the IAC.

with the engine warm and off, i screwed the BISS all the way in and then back out 2 full turns.
Started the engine up after reconnecting the battery.
Screwed the BISS back in slowly until idled down to 900.
Biss is all the way in at this point so i backed it off a bit.

Basic idle behavior now:

Cold start,
revs up to 3k
comes back down to around 1800
creeps back up very slowly to 2400 and then surges quickly between 2200 and 2400 rpm.

Once engine warms, idle surge slows down and levels out at 900rpm, sometimes 1k.

When driving around at operating temperature and coming to a stop, the car will surge slowly from 1200 to 1500 and then level out at 1000rpm after about 15 seconds.

It often wont idle crawl in gear without surging.

--------------

I dont have any of the logging tools at my disposal other than a multi meter. I dont really know how to check for vac leaks yet
Id really like to fix this idle surge problem.

Any guesses as to what might cause the behavior?
I am particularly interested in the cause of the fast idle surge while warming up as compared to the much much slower idle surge when warm.

 

[jed344]

Boost leak test first.

 

[Dhan]

Gonna need to learn how to check for vacuum leaks and do a boost leak test (BLT) like jed mentioned.
Idle surge is probably changing when it's warm versus cold because the motor and parts expand as they heat up. That expansion pushes all the parts closer together and better seals things.

The objective of each test is pretty much the same thing- find and fix leaks. The process is different though.

Boost leak test is where you pressurize the intake system with air (usually about 10 psi higher than your maximum boost level) and track down where there are leaks. Can use your ear and a spray bottle with a little water/soap and watch for bubbles.
Vacuum leak test is where you have the engine running and are going over the motor with a fine tooth comb and hunting down leaks. Listening for leaks, checking vacuum hoses, spraying suspect areas with your soap/water mixture, etc.

Start with the BLT. Could be a simple air leak.

Edit: Google "Terry's Talon idle surge" if you havent already. That guide has been the go-to for idle surge issues.

 

[pauleyman]

91 eagle talon TSI stock.

had idle surge until warm up and then it operated normally.
did a bit of research and replaced the IAC.

with the engine warm and off, i screwed the BISS all the way in and then back out 2 full turns.
Started the engine up after reconnecting the battery.
Screwed the BISS back in slowly until idled down to 900.
Biss is all the way in at this point so i backed it off a bit.

Basic idle behavior now:

Cold start,
revs up to 3k
comes back down to around 1800
creeps back up very slowly to 2400 and then surges quickly between 2200 and 2400 rpm.

Once engine warms, idle surge slows down and levels out at 900rpm, sometimes 1k.

When driving around at operating temperature and coming to a stop, the car will surge slowly from 1200 to 1500 and then level out at 1000rpm after about 15 seconds.

It often wont idle crawl in gear without surging.

--------------

I dont have any of the logging tools at my disposal other than a multi meter. I dont really know how to check for vac leaks yet
Id really like to fix this idle surge problem.

Any guesses as to what might cause the behavior?
I am particularly interested in the cause of the fast idle surge while warming up as compared to the much much slower idle surge when warm.

Is this a new problem? You may have more than one thing wrong. For starters it's not possible to idle above 1500 while sitting still. Idle position switch kills that. So if you're getting idle above 1500 you have an ips not working or the throttle isn't really closed suggesting a possible cable issue or throttle plate/shaft issue.

 

[dwb]

Make sure you set the idle per the vfaq: http://www.vfaq.com/mods/BISS-1G.html
Once that is done and if you still have surge or high idle issues, your problem is too much air bypassing the throttle plate. That could be numerous things - faulty FIAV (very likely), boost leak (also very likely, it's imperative to do a boost leak check), throttle plate not closing fully (cable too tight or misadjusted idle switch), or your ISC is faulty (not likely if the replacement was known good, but what about the ISC o-ring?).

Are you seeing a check engine light by chance?