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I've isolated the problem to the secondary vacuum switch. I replaced it with a new one but the car runs better with open vacuum. What gives?
Chris
1991 Taurus SHO stock
Oops yeah I forgot the background. At first the car was surging at idle and had horrible gas mileage. I replaced the IAC and gave it a thorough tune up. I stumbled on what appeared to be the problem. I unplugged the vacuum for the secondary vacuum switch (attached on the back of the rear intake, has a round check valve attached to the same bracket) and noticed the secondarily close! The switch must have kept them open all the time. Now it runs better but I still get the surging unless you unplug the vacuum line in the back of the intake. With Open vacuum it runs better. Is it stuck on the old idle program?