Mercury Mountaineer High Idle and No OD

[tom slick]

My FIL's 2001 Mountaineer, 4.6l AWD will idle at 2500-3000 and will not shift into OD when it has sat in near 0*f temps. This condition will continue when the engine is fully warmed up, even after 20 miles. The transmission will shift into OD if you push the OD button repeatedly once things are warmed up. Both only happen at the same time.

If it sits in the garage where it isn't as cold driveability is normal. It doesn't throw any codes when cold or hot.

I'm thinking it has a sensor acting up, maybe water in something that freezes and makes the sensor act up. I wonder if the two issues directly connected to each other?

 

[brianl703]

I've had a flaky throttle position sensor cause a high idle on a Ford before. I suppose that could also cause it to not shift into overdrive because it thinks the throttle is pushed further than it really is.

 

[artificialist]

Is the coolant temp sensor accurate? A coolant temp sensor can fail at a specific temperature, and the engine computer can't trigger a check engine light in those conditions. Many vehicles also will stay out of overdrive at low coolant temperatures in order to warm the engine more quickly.

Check for vacuum leaks. Sometimes a vacuum leak will cause a high idle if the amount of fuel injected matches the increase in airflow.

Mass air sensor inaccuracies can make the ECU assume that the throttle is wide open, and therefore it shouldn't shift into overdrive.

 

[punisher]

TFT sensor. I know the torque converter won't lock in when the trans is cold, maybe effects OD as well. Might be normal op.

"Throw codes".........does that mean no ck eng light, or no codes when using a scanner?

You get to the point where you need a scanner with data function to monitor TP. MAF, ACT, ECT, TOT/TFT, IAC%, and see whether the PCM is commanding that idle speed due to a faulty sensor input or if something is sticking.