83 NA low idle

[M. Murphy]

Hello all, thanks in advance for any help you can provide.
Following moderately extensive maintenance: balance shaft front seals, Gates timing belt, fuel supply line replacement, injector reseal and AOS reseal. The engine idles about 200 rpm.
While disassembled I cleaned things up with carb cleaner including the throttle body.
Prior to the maint. the engine idled normally and reasonable at the start of the test drive. I'm wondering if something is blocking the idle circuit. Any thoughts.

[nightheart]

Stupid question, but did you turn the idle adjustment screw while doing your maintenance? It might just need adjusted out.

[M. Murphy]

No, that was not touched. I am unfamiliar with the the idle mechanism and wondering if something may have blocked it.

[M. Murphy]

Today I pulled the throttle body off cleaned it an made sure the idle circuit was completely clear. Replaced the large O ring at the mounting base and verified all the vacuum lines were in good condition and connected. The engine will idle roughly and at most 500-600 rpm with the idle screw adjusted way out. AFM?

[sausagehacker]

The early 944s use an open-loop "auxiliary air valve" under the intake manifold to regulate idle during cold starts. It is open by default and as the engine heats up, the valve closes. Looks like this.

However the AFM does sound suspicious. Make sure it's plugged in all the way. Test procedure:

https://www.youtube.com/watch?v=4pFxhJO4jXc

[M. Murphy]

Would the AFM potentially cause a low rough idle but run reasonably well above that?

[sausagehacker]

It could if the resistor strip was scratched or otherwise damaged on the area of the track where the wiper rests during idle.

Good info:

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

[M. Murphy]

Pulled the AFM this morning and took a look inside. There are definite tracks on the conductor but at the bottom are very light, maybe indicating light pressure. There is an area that looks like the contacts have spent a good deal of time in.

[M. Murphy]

After re positioning the the AFM contacts the unit was tested and found to have clean unbroken output across the entire range. Re -installed and started. Initially the idle rpm went to about 2500 for a few seconds then dropped to a rough 400. Previously I had backed the idle adjustment out several turns in an attempt to increase the idle speed with no effect.

[M. Murphy]

I have heard that a failed DME temperature sensor will result in the low idle I'm experiencing. Ordered a replacement from Pelican and will report on if it was a fix. Anyone know if a failed throttle position sensor would cause a low idle?

[M. Murphy]

Swapped the DME sensor and that allowed adjustment of the idle speed. Left with the idle still somewhat rough and what I think is belt whine. Tried half a dozen times to adjust the timing and balance shaft belt. Before starting this maintenance the idle was smooth and there no belt whine. As for the rough idle I suspect at least one of the large breather/vacuum developed a crack while things where being moved around.
Any ideas or comments.