Hi. I've had a '92 Mercury Cougar for a few months, and it always seemed to idle a bit high. Last week it seemed to get much worse, and it now idles very high, at 2500 RPM in Park and 1200 RPM in Drive with brakes on. I can drive 35-40 mph without giving gas. My mechanic checked thoroughly for vacuum leaks (lowered rpm for testing in some nifty way and carefully checked everywhere, including removing to intake manifold to check the gasket. No leak found. Sensors all test OK. Codes not showing anything (though Check Engine has come on then off a few times). The injectors, however, are showing very high voltage, so he thought brain box much be faulty. I got another for $30 from the junkyard: no change. Of course it's possible that the "new" one is bad in the exact same way . . . I don't want to drive it much, because it's bad on gas, it's stressing my brakes out and I heard it could blow a head gasket, never a strong point on these cars (Ford 3.8 V6) anyway. Can anyone help? Thanks.

Have you checked the throttle position sensor. That could definately account for all of those problems.

Have you checked the throttle position sensor. That could definately account for all of those problems.

Thanks, (Mario? Sarah?) I'll look into that, but wouldn't that have showed up on the scan?

Try replacing the idle air control valve. I have a 1989 Mercury Cougar that had the same symptoms. I went to the dealer to get raped for $250 when i couldnt done it myself. You burning your brakes up trying to keep your car still?

OBD1s don't always show trouble codes when they are supposed to. I'd suspect the throttle position sensor or Idle Air Control Valve if there is no vacum leaks.

Id go for the idle air control valve. If it were a blow head youd have other problems. It could be the TPS but the O2 sensors will compensate for some of the false readings from the TPS.