1G Idle surge

Happys_TSI · Mar 3, 2019

i have a 1990 talon tsi 2g maf 550cc injectors fpr set to 37psi. I have idle surge about 7 mins after running. Up until then it idles fine afr raising from 11.1 consistantly to 14.3 until it starts surging it jumps from 15.8-18.6 and back. I have DSM Link V3 and logging I have consistant up and down o2 readings during the surge. When I unplug the iac and start it it doesn’t surge. Just random drops in in afr from 14.7-14.0 then back to normal. I’ve set the biss screw before never made a change. Fiav blocked off. Recently tried adjusting biss it bottomed out now it won’t turn either direction. But now it surges a lot lower but idle without iac stays around 1000 rpm. I have no clue what is causing it it’s not my ecu. Every ecu I’ve put in I have a surge issue and the iac is brand new and tested. I’ve been thru all the idle trouble shoot pages. Did boost leak test and smoke tested Vc lines all checked out. I tried to ignore the surge but it’s annoying and I need the iac. Any help is appreciated.

Twinturbo120 · Mar 3, 2019

Make sure your TPS is adjusted correctly and reading .63 volts at zero throttle and 5 volts at 100 percent throttle.

You need to adjust the BISS screw with the timing and diagnostic pins grounded. You can do this with link.
Once you have done that, you can adjust the BISS. Keep on eye on the ISC position until it measures 30, and you are good to go.

Hope this helps.

Happys_TSI · Mar 3, 2019

Twinturbo120 said:
Make sure your TPS is adjusted correctly and reading .63 volts at zero throttle and 5 volts at 100 percent throttle.

You need to adjust the BISS screw with the timing and diagnostic pins grounded. You can do this with link.
Once you have done that, you can adjust the BISS. Keep on eye on the ISC position until it measures 30, and you are good to go.

Hope this helps.

The biss screw won’t move. I turned it and it bottomed out then tried to back it up and it didn’t move. Slightly strippped the top out too. But I’ll try the tps. Is the voltage different for a 1990?

Twinturbo120 · Mar 3, 2019

Happys_TSI said:
The biss screw won’t move. I turned it and it bottomed out then tried to back it up and it didn’t move. Slightly strippped the top out too. But I’ll try the tps. Is the voltage different for a 1990?

Same voltage for the 90.

Sounds like you need a new BISS screw. You should get a new one, they are pretty cheap and will help
with the headache, you are having now LOL

motomattx · Mar 3, 2019

It also sounds like you could have a vacuum leak somewhere, your fiav blockoff could even be leaking.

Killa_dsm · Mar 3, 2019

The o2 voltage going up and down is normal. I would first change out the biss screw, since you're saying its stuck. Then make sure there are no vacuum leaks.

Follow the instructions on this thread to adjust your biss screw and ISC position.
https://www.dsmtuners.com/threads/biss-adjustment-iac-position.321158/

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