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Rough/Slow Idle - Edit 200 1991

posted by  Wolfe111 on  Mon Apr 5 09:12 CST 2010
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last visit: Thu Jan 12 13:24 CST 2012
[RELATED]

My 91 240 manual trans has had a rough idle for as long as I've owned the car, about a year.
Can anyone explain why when the vacuum line to the fuel pressure regulator is pulled, the idle speeds up and smooths out? And then, when the line is plugged again with a thumb it goes back to rough again?

I've been switching or replacing parts trying to solve it:
All the usual parts--plugs, filters, etc.
Replaced intake manifold gasket
switched the MAS, the Idle Air Control Valve, + all the hoses to and from.

Thanks

Edit:
Decided the above points to the IAC circuit. Removed the IAC connector and measured 12V.
Removed the IAC valve, applied 12V, valve snaps open.

Should be ok. Plugged the connector back into the IAC, no response. IAC does not 'flutter', does not open.

Wiring diagram shows one side going to a common with the injectors and the other side going to the control unit.

Is it possible the control unit is not allowing sufficient current to trigger the IAC?





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