my a6 block and z6 head combo motor i built sometimes idles fine. but other times it goes from 900-1500 up and down over and over. timing is right and there is no air in coolant. my IAC is new.. when it starts idling funny if i turn on head lights and either heater or cold air it goes down to normal , it may fluctuate a lil but not as bad. i had the iac bypassed but since its like 14 degrees in the morn the car just keeps shutting off until its warmed up. so i just put it back to normal. does anyone have any idea why this can be? no check engine codes at all.

 

ya i have a car that jumps up and down from 800-to-1500...havnt figured it out yet...done all the usuall stuff

 

ya i just cant figure it out. even before i did the swap my stock dx motor and ecu started randomly doing this. its annoying. i dunno if its the iac or the injectors cause even when i had the iac bypassed it did it a few random times so that would mean its the injectors ..