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rpm drop annoyance
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Posted by Monster (more from Monster) on Sat, 27 Apr 2002 12:57:53 Share Post by Email
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I'm having a frustrating problem with the idle on my car now. I suspect I need to replace the AIC, but before I do that, I want to exhaust all other likely causes. Your opinions are welcome.

Here's the situation: The car idles smoothly and at proper RPM. I've checked for vacuum leaks (including looking at that hidden port reported by BigSage in a recent post!) and the system is tight. When I kick on the A/C, the idle bumps upward a little and settles back immediately to normal, indicating the A/C idle compensation function is working as it should. Vacuum at idle is strong, further confirming lack of leaks. I have cleaned the throttle body and cleaned/lubed the AIC valve. The engine runs smoothly, accelerates strongly, revs freely, gets terrific mileage, etc. -- it's working better than new.

The only problem is when I'm slowing to a stop, and put my foot on the clutch with RPMs below 2000, with the A/C on. In this particular situation, the throttle is fully closed (3 - 5 seconds after releasing the gas pedal, the dashpot has done its job and eased the throttle fully closed.) The engine is essentially being driven by the transmission and the fuel is cut off. When I press on the clutch pedal, the RPMs drop rapidly, falling below 850 RPM before recovering. With the A/C off, the engine shudders but recovers (it still drops too far). With the A/C on, it often (not always) does not recover in time and stalls out.

I can re-engage the clutch and restart the engine while I'm rolling, but this is extremely frustrating. In city traffic, I've now resorted to clicking the A/C off as I approach traffic lights. Once I'm idling at the light, I click it back on. It's an annoyance, and probably not good for the A/C compressor either, turning it off and on repeatedly.

The only thing I can figure from this is that the AIC valve isn't reacting fast enough to recover the idle when the throttle is closed and RPMs are dropping. If I "blip" the throttle, it doesn't stall, as the idle dashpot gradually eases the throttle closed the way it's supposed to. It will hold the idle nicely, but not "catch" it in time when it is dropping with the throttle fully closed. Is there anything else which can cause this problem and not give any other symptoms? Any other checks I can perform to isolate other causes?

Thanks for your suggestions!
- = M = -
'87 900T 174K

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