Engine rev up/down when coasting

[Rochev]

2013 Taurus 275k org miles. Transmission still shifts well, however-
This only happens in gear; 5th or 6th, NOT in neutral. 6th happens at 42mph on light accel. It stays in 6th/lockup until about 36mph. When coasting downhill between 30-45mph, the engine revs up and down 200 rpm. It does it fast, and hard enough to feel AND see it on the tach. BUT! It doesn't do it in neutral. Is this a torque converter slipping issue? I've only noticed it while coasting downhill. Coasting downhill between 30-40mph happens a lot in my area. Again, shifting normally for 275k.

 

[idahopotato]

Let me guess. Only does it when cold?✔

 

[Rochev]

Let me guess. Only does it when cold?✔

Nope

 

[idahopotato]

Idle control valve, or whatever they calling it these days

 

[Rochev]

Idle control valve, or whatever they calling it these days

It has an electric throttle body so there's no mechanical device to clean. I have however cleaned that thoroughly. It was almost spotless before I did it.

 

[ripcord]

The money light is off?

 

[idahopotato]

Idle control valve is electronic. Likely has one, but I am not sure about it. Engine light would not go on.

 

[Rochev]

The money light is off?

Lol yes it's off. I vacuum tested it and did a run with FORScan and didn't see anything out of the ordinary. The transmission wrench light isn't on which is great, but it seems to be transmission related.

 

[Rochev]

idahopotato, I'll order a new one as it's easy enough to change. Trying not to throw parts at it though, but when everything looks good it's hard to tell.

 

[idahopotato]

They usually go before 275K

 

[mattd]

That car does not have an iac. Idle speed is determined by throttle body.

It’s likely the converter slipping. Especially if your foot is on the throttle at all (I know you are concerned when coasting). Post a video if you can. If it has a 6F50 the TCM valve bore wears for the TCC and eventually it will burn the converter clutch.

You may need a scan tool to view TCM data for converter slip RPM vs TCC duty cycle

The car is not AWD correct?

 

[OVERKILL]

idahopotato, I'll order a new one as it's easy enough to change. Trying not to throw parts at it though, but when everything looks good it's hard to tell.

Don't throw parts at it, mattd has provided some excellent insight, you'll need to read that trans code and go from there.

 

[another Todd]

At what RPM is this happening? Modern fuel injected engines shut off the injectors when coasting, until the RPM drops to a certain level then re-activates them. Not sure if you can see this on the tach but you can sure feel it, it kind of feels like you are going downhill with light brake pressure then release the brakes...it gives a little surge feeling, but only once as RPMs drop like when coming to a stop. AC kicking on and off is another cause for RPM bumping though it wouldn't be rapid. In your case it sounds like an observation rather than a real problem, I wouldn't start throwing parts at it.

 

[Rochev]

FWD. Unlike the twin turbo's, it's port injected. It only does it when I'm completely off the throttle and going downhill, but not steep. Depending on speed (30-45mph) the rpm's are anywhere from 800-1300. The tach gauge is digital so it's more accurate, otherwise I don't think I'd see it bouncing. OD shifts quick and on time. Converter locks up smooth basically right after the first time it shifts into 6th. I'll run forscan and check all trans parameters again, hopefully everything is normal. It does NOT do it in neutral which has me anxious. The idle is jumpy on warm idle as well, which is why I was hoping it's the TB.

 

[mattd]

After that description it is possible it could be a TB issue..you’re probably going to need some diagnostic software with live data to see what the ECM is commanding vs actualnfor throttle position. I’d still pull TCM codes though just to make sure..

 

[circuitsmith]

Sounds like a case of surging.
Make sure coolant temp sensor is reading correctly, engine is reaching proper temp, fuel trims are not borderline.
Last time I saw a FI engine surging it was because the engine was too cool: someone removed the thermostat.

 

[another Todd]

Could it be the torque converter locking and unlocking? It doesn't do it in neutral rules out some stuff.

 

[meep]

My guess is also TC, since you can feel it. If the TC was locked, the rpms wouldn’t be able to surge. If it was completely unlocked, you wouldn’t feel it at all during coasting.

 

[LeakySeals]

When is the last time you change the transmission fluid? What does it look like on a white paper towel?

 

[Rochev]

Fluid is changed every 25k. Update:

Started it to move into the garage for the incoming snow storm. BANG when it fired up. Put it in drive after a minute, BANG, no drive. Wrench light came on, whining noise from transmission. BANG in reverse but it worked. Had to back down my street to the other end of the driveway and back into the garage. The return line from the radiator is leaking and everything is wet. It must have started leaking not long ago because it's never left a mark on the floor. I'm assuming the transmission burned out because it lost all the fluid. All I can do now is drain what's left and refill, and hope it still functions. But with the burnt smell and what happened I'm not expecting much.