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1983 944
 
Join Date: May 2017
Location: San Antonio, TX
Posts: 27
'83 944 Suddenly Erratic Idle

I have an 83 944 that Ive had since 84 and I do most of my own work on it. At 170,000 miles, it has never run better. About 3 months ago, I replaced the spark plugs (Bosch Platinum+4) and plug wires (Clewitt Engineering), distributor cap and rotor (Beck Arnley), and air filter (K&N). I borescoped the valves and pistons (through the spark plug holes) and theyre all clean, with just a thin layer of carbon.

About 2 weeks ago, the idle went wonky. On any given drive, it will idle rock solid at one stop and vary by +150RPM at the next stop. It may idle perfectly for several stops and then idle wonky for several stops. Ive not discerned any pattern. Off idle, it runs perfectly.

Any ideas?
Old 12-01-2017, 09:21 AM
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does it happen only when warm, only cold, or both?

if only warm, how old is the O2 sensor ?
Old 12-01-2017, 09:59 AM
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1983 944
 
Join Date: May 2017
Location: San Antonio, TX
Posts: 27
It happens with the engine cold or warm. Ambient temps here in San Antonio, TX have been from the low-50s to upper-70s since this issue appeared.

I’m not sure how old the O2 sensor is; I estimate at least 7 years, and maybe as much as 15.
Old 12-01-2017, 10:10 AM
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try unplugging the O2 sensor and see how it behaves
Old 12-01-2017, 12:33 PM
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1983 944
 
Join Date: May 2017
Location: San Antonio, TX
Posts: 27
UPDATE: I just returned from driving about 25 miles on the freeway and in stop-and-go rush-hour traffic. It idled normally most of the time, but varied at a few stops. Just for grins, while the idle was varying, I turned on the AC and it idled rock steady. As soon as I turned the AC off, the idle varied again.
Old 12-01-2017, 02:13 PM
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1983 944
 
Join Date: May 2017
Location: San Antonio, TX
Posts: 27
Quote:
Originally Posted by v2rocket_aka944 View Post
try unplugging the o2 sensor and see how it behaves
ok
Old 12-01-2017, 02:14 PM
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Not sure how this might relate to AC...but another easy test is to unplug the ICV (on my 85.5 its located under the intake manifold...not sure about '83?). If you unplug and nothing changes, it could be the ICV.
Old 12-04-2017, 06:50 AM
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83 doesnt have an ICV, it has an "auxiliary air valve" that is only used during engine warmup to bypass the throttle plate.
when the AC turns on there is ANOTHER "supplementary air valve" mounted behind the airbox that allows even more air to bypass the throttle plate, to help stabilize against the load of the AC compressor at idle.
Old 12-04-2017, 09:54 AM
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Turning on the AC will bump up the idle speed. That could take it out of the range of what is causing this to happen.
Maybe a vacuum leak? Try disconnecting one of the hoses while it is happening. If no change,...
Old 12-12-2017, 09:23 AM
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1983 944
 
Join Date: May 2017
Location: San Antonio, TX
Posts: 27
I disconnected the O2 sensor and there was no change in the erratic idle. It was time to replace the O2 sensor and fuel filter anyway, so I did so with no effect on the erratic idle. When it idles erratically, it drops about 150 RPM then immediately bounces back up to about 50 RPM above base idle, and repeats. The only pattern I’ve noticed is that idles normally when cold (presumably when the aux air valve is open) and does not idle erratically until warmed up, but, as I mentioned initially, at one stop it will idle normally and the next not, with no discernable pattern. Ambient temps have been as low as the 30s since I first posted, with no change in the erratic idle behavior. I’ve tried turning on the AC when it was idling erratically, and, most of the time, it smooths out, but sometimes not. Last summer, I adjusted the base idle up to about 1050 RPM, as the AC has always dragged the idle down to 800-900 RPM, so, when the AC’s on, it will idle to spec of 950 RPM. I have been unable to catch it idling erratically in my driveway to get any clues, including looking for a vacuum leak.

Last edited by AC130Tron; 12-17-2017 at 05:30 PM..
Old 12-17-2017, 05:21 PM
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Check that your throttle body is closing completely and is not binding at any point. Check that your AFM has a nice smooth resistance (ohms) when rotating the barn door.
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Old 12-18-2017, 03:43 AM
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