My wifes 1986 740 turbo was running fine until recently. The engine was out of a 1984 760 turbo which I managed to make work with the 86 740, which I bought with melted pistons. The idle while driving it went up to about 2500 rpm, while it is hot. When cold the idle is at about 1500. This is with the air screw on the throttle body turned all the way in. I disconnected the temperature sensor on the block which causes the car to go even faster. While the car is hot it will not restart without this attached. I also disconnected and shorted the radiator temp sensor which had no effect. Does anyone out there know how I can find the bad sensor? Please HELP!

Did you clean the throttle body and tighten the screws holding the throttle plate to the center pin inside. Mine would stick open until cleaned and adjusted, and simulate a fast idle. Not fun on exit ramps.

Thanks for the help. I will try these suggestions, Thanks again

Use the idle air motor from the 1986. I think the coolant temperature sensor is the same, but I cannot verify. Make sure the linkage is not keeping the throttle valve open. Pinch the hose off from the idle air motor to the throttle body. You should be able to get the car to idle at 725 rpm with the idle speed adjustment. There is lots in the 700/900 FAQ on adjusting the idle speed.

COULD BE THE IEC (IDLE ENRICHMENT CONTROL) IF YOUR CAR IS SO EQUIPED. LOTS OF GOOD INFO ON THIS IN THE TURBO TECH TIPS. CAN BE REBUILT IF YOU'RE HANDY AND ADVENTUROUS OR EASILY REPLACED.