Rep Power: 267

Can a coolant leak cause the engine to run at high RPM's? My friends 2003 Hyundai is shooting up to 1400 rpm, engine runs smooth though. If I disconnect the IAC plug, it goes to 700 which is normal. He already replaced the IAC so it's not that, but he has a large coolant leak somewhere. In fact when we checked the reservoir, it was empty. From what I recall, air in the coolant causes some sort of false reading... but I'm not sure which part takes the reading. Is it the IAC valve since coolant flows in there?

 
Rep Power: 812

its possible. the ECT (engine coolant temp) sensor is in the coolant path and is responsible for a long list of functions including how the engine runs (air/fuel). if its not in the coolant due to air pockets, you could have false readings like the ecu thinking the car is still "cold" and raising the idle accordingly. this overrides/controls the iac valve until the engine is warmed up (when the ect determines the coolant is warm). by unplugging iac, you remove the ect sensor's control over how much it opens, and it defaults to the normal position. i think you should tell him to fix the coolant problem. air is never good and can cause overheating too.

 
Rep Power: 267

Yes I've made it top priority to fix that. I do recall that I was not only having high idle rpm on my civic due to a large coolant leak, but idle surge. Much appreciated gearbox!