Several months ago, I purchased a used 2001 F150 4x4 with an automatic transmission. It has 90K miles. The truck usually runs about 1500 rpm's at 55 mph. I have recently noticed that when going down a steep hill and letting off the accelerator, the rpm's don't drop to idle as I would expect.

Further, if I shift the transmission to neutral in the same scenario, the rpm's actually climb to ~2000 rpm's.

I assume that something is wrong, and I'm going to have to take it to a shop to have it fixed, but I'd like to have an idea of the problem before going in.

Any suggestions?

 

Both conditions are normal.

When you are going down hill and take your foot off the accelerator, the torque converter unlocks and you are now in 4th gear, using engine braking.

Also: When you put the transmission in neutral at 55 MPH, the RPMs rise because if they were to drop down to 850 RPMs (Normal idle speed) you'd lose traction with the rear wheels when reengaging the transmission on a wet or snow covered road. Ford programed the transmission controls to do this because they know that people will use neutral on long, moderately steep grades to save fuel.