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Replaced everything that could be replaced per the customer.
Warm start idle is 675. After a few seconds drops to half that. Idle is steady. New idle air valve. Don't think it's the new air valve. But anything's possible.
Kevin
Last edited by Kevin in Atlanta; Nov 23, 2025 at 05:47 PM.
Reason: Figured it out
Can you be more specific on what the warm idle RPM is? As I ready your post it sounds like you are saying the idle stabilizes at around 400RPM can you confirm this? If so this seems too low, as in not enough air is getting through the ISV route.
Can you confirm which brand ISV you have? There have been several documented instances that demonstrate the LOWE is not a suitable replacement. I have at least one data point myself.
The low idle does not sound like air leak ro throttle plate miss alsignment.
You could connect the SharkTuner ECU from your GTS on this car to aid in the diagnostics. It would help rule out an issue with the ECU. It would allow you to observe the functioning of the Temp II values, determine if the car is able to maintain proper AFR via the O2 adjust to help rule out fueling issues, and it would allow you to manipulate the ISV opening so these if that fixes the idle speed (if so then the ISV is not providing enough opening relative to the signal from the ECU. I suspect this is the case. The sharktuner would also allow you to validate the Temp sensor as well as the TPS.
Stan suggested that an aging MAF specification could be driving low idle is also a good on. You could try unplugging the MAF or swaping it out with a known good one is a simple enough test.
When you did the top end refresh, did you put additional sealant on the manifold side caps? That is a place that can develop vacuum leaks espcially after powder coating. @Mrmerlin turned me onto that one. My focus on idle vacuum is around the vacuum feed to the FPR, do you have enough.
When you did the top end refresh, did you put additional sealant on the manifold side caps? That is a place that can develop vacuum leaks espcially after powder coating. @Mrmerlin turned me onto that one. My focus on idle vacuum is around the vacuum feed to the FPR, do you have enough.
Vacuum - Something else I can look at.
I always inspect the manifold mating surfaces after powder coating. My guys are pretty meticulous.
But, your mention does not fit the 20 second from idle to 300 RPM.
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Idle drop 89 S4 - Solved - sort of.
