RPMs drop accompanied with a whining noise

[coax31]

This forum has been very helpful for me in the past and I appreciate any help provided. I have 1998 Ford Explorer XLT 4.0 SOHC with 211K miles. Last August the engine died at 202K and I had a lower mileage engine installed and it has run fine since. Yesterday I was driving around 70 MPH and the RMPs suddenly dropped by 1-2 K and it was accompanied by a whining/screeching noise for a few seconds and then back to normal, this started to happen 2 or 3 more times so I pulled over and had it towed back to my house, thank god for AAA. The noise seems to be coming from below my feet, the car seemed to maintain its speed through these episodes and no CEL or OD light blinks, and it also seems to be shifting ok it’s an automatic. I was wondering if this is the first sign of a transmission failure or is my torque converter bad or something else. I also noticed some time back some oil on the transmission pan so I added some ATF, maybe a little too much.

 

[RomeovilleIL]

Kind of hard to diagnosis from afar. My first thought, aside from you saying sound came from below, is a frozen or sticking ac compressor/clutch. It would certainly drag on the engine and create serious noise but without changing the drivability and would kick on intermitently (defrost uses the ac). I would think a transmission or tc issue would be immediately felt through the drivetrain, like a lurching.

 

[coax31]

My AC has been blowing hot air for 5 years and I never turn it on anymore, I was running the heater.

 

[Van]

I'd check the idler pulley.

 

[lounge lizard]

and the cat might be plugging up

 

[coax31]

 

[RomeovilleIL]

Watched the video. Speed wasn't affected and did not see a change in engine temp, oil pressure, or voltage when the rmps dragged down. Did not seem to affect your steering either. I am still leaning toward the ac accessory belt based on the sound and drag, but thats just my opinion. Easy and free to test by disconnecting that belt and taking a drive.

 

[coax31]

Watched the video. Speed wasn't affected and did not see a change in engine temp, oil pressure, or voltage when the rmps dragged down. Did not seem to affect your steering either. I am still leaning toward the ac accessory belt based on the sound and drag, but thats just my opinion. Easy and free to test by disconnecting that belt and taking a drive.

You can disconnect that belt and not affect anything else? And drive?

 

[RomeovilleIL]

Sorry, had a momentary brain freeze. Bypassing the ac takes a shorter belt.

When you are driving and this activity starts, have you tried turning off all the hvac? The heater will activate your ac compressor as part of the defrost system. If that is the problem, it should make the drag go away or come back simply by turning it off/on.

If that turns out to be the problem you can unplug the AC clutch control wires and call it done.

I would also take the belt off and try turning the idler and tensioner to make sure there is no binding or wobble.