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930 Idling too high after warm up?

My 1979 930 engine cold starts fine and idles down to about 8-900 in a reasonable and correct amount of time. Once I shut the engine down for five or more minutes, and restart it hot, it wants to idle at 2000-2200 rpm. It then takes several minutes of driving to calm it back down. Where should I look for the cure? WUR?

I've had a little experience troubleshooting CIS on a NA car, but nothing on the turbo version. All suggestions will be appreciated!

Drew
Old 11-14-2007, 02:16 PM
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It's likely the Auxilliary Air Regulator.
They are activated by both electricity and engine heat. With age the bimetal strip begins to deteriorate in function and may be slow to react.
When you re-start your car with a warm engine you can have a condition where the engine oil is hot but the top of the engine is cold (more so on a cold day). The hot oil allows the engine to run more freely which exaggerates the high idle from the partially open AAR.
The cure? Yank out the AAR and toss it in the dumpster. You can perform the same function with your right foot.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 11-14-2007, 05:18 PM
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Mine does pretty much exactly the same as you described, except that the maximum I see on the re-start is about 1,800 RPM, dropping off to 1200 within about a minute.

John Walker said in a thread on the 911 forum that the 930 AAR had this specific characteristic (he described it as an "obnoxiously high idle") and that they often used to replace 930 AARs with SC AARs, back in the day.
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Old 11-15-2007, 02:01 AM
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I'm glad to see that I'm not the only one faced with this "characteristic". You hate to ask a question like this just to have everyone say "HUH?".
So, AAR seems to be the culprit.
If I bypass the AAR, should I also bypass the AAV?
The residual effect might be hard starts on cold days or does the WUR deal with cold temp start?

Thanks for the replies.

Drew
Old 11-15-2007, 07:01 AM
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Block off the hoses leading to and from the AAR and see how you like it, completely reversible if you don't.

The WUR will adjust the fuel pressure to compensate for temperature. The AAR does nothing until after the engine starts. You can do the same thing with your right foot.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 11-15-2007, 07:53 AM
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My 79 does the same but only on cold days as rarelyl8 says.
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Old 11-15-2007, 09:22 PM
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"My 1979 930 engine cold starts fine and idles down to about 8-900 in a reasonable and correct amount of time. Once I shut the engine down for five or more minutes, and restart it hot, it wants to idle at 2000-2200 rpm. It then takes several minutes of driving to calm it back down."

For comparison purposes, my car starts cold @ 1000 and stays there to full operating temperature with very little deviation. It is possible if all things are functioning properly. A couple of things in your discription above would concern me. "Reasonable and correct amount of time", the idle should settle instantly regardless of temperature. "several minutes to calm back down", again the idle should stabilize immediately even if it's wrong. I would also suspect throttle linkage and/or false air that could be influencing your idle. You might also want to check your distributor advance, it's possible it could be getting stiff and slow to return @ idle. Small changes in timing @ idle yield large changes. In fact, I think I'd start with the distributor advance, check your initial timing to see if it's wandering due to stiff or sticking advance.
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Old 11-16-2007, 12:39 PM
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