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Discussion Starter #1 (Edited)
Hello. When the MKS is in 5th/overdrive and cruising at a steady speed if I slightly depress the accelerator there is a slip/jerking motion when it downshifts and comes out of overdrive. It also does this when slightly accelerating from cruising speed without it shifting down. Could a transmission flush possibly fix this or am I looking at a more serious mechanical issue? The car has 90k miles on it and have not flushed the ATF fluid yet.
 

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^^^^ +1

I know Ford says the fluid is good for 100K+ miles, but IMO that is total BS. Change the fluid for sure. Fresh fluid may eliminate the problem. If this is the 6 speed tranny, the filter cannot be changed without dropping the tranny and splitting the tranny case.
 

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Like rdanville said, Don't count out a misfire. I dealt with what I thought was a transmission slip in my 2008 Taurus (3.5) for over a year. After a lot of research I ended up changing the plugs and COPs and she's been running perfect ever since. Believe it or not, I've read that my problem could have just been spark plugs but I purchased a complete kit so cheap it was worth changing both the plugs and the coils. There's a Motorcraft kit out there for the 3.5 that comes with 6 plugs, 6 COPS and UI gaskets for around $150.00.

My symptoms were only on the highway when I would try to accelerate without dropping down a gear. If I got the rpms up by hitting the gas and dropping down a gear it would seem fine. So, under load above 50 mph it really felt like a trans slip and I was 99.9% convinced that it was.
 

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Discussion Starter #5 (Edited)
I know Ford says the fluid is good for 100K+ miles, but IMO that is total BS. Change the fluid for sure. Fresh fluid may eliminate the problem. If this is the 6 speed tranny, the filter cannot be changed without dropping the tranny and splitting the tranny case.
Thank you, that is what I had just read that Ford says 100k but most professionals say 50k. That filter you mention. Is that a necessity/recommended to replace also? Around how much $$ are we talking to drop and open the transmission? I don't see it just giving out mid-drive. It is relatively minor now and I'm more apt to maybe just do a fluid change and get rid of when the symptoms become more serious. Last, going slightly off-topic but for my car I was told to do 3 drains/fills. Is this what you recommend or is that a bit overboard (looking at $90 for 9qts since I have to use proprietary fluid).

Yes, 6-speed, this is my father's I forgot that they aren't 5. Looks like it is the 6F50 and 6F55 was for the turbo.

My symptoms were only on the highway when I would try to accelerate without dropping down a gear. If I got the rpms up by hitting the gas and dropping down a gear it would seem fine. So, under load above 50 mph it really felt like a trans slip and I was 99.9% convinced that it was.
I was about to say that acceleration without downshift is perfectly smooth. At the same time, if I floor it at 60 I sense no slip. I need to test it again when I'm back over because I think you are right now that I think about it. There is jerkiness coming lightly out of overdrive but I think it is also there even without dropping out of OD under light load. I need to check when plugs were changed last anyway. Cops are the coils and UI gasket is the head gasket?

Edit: Regarding testing it again. It absolutely is jerking under light load without the downshift.
Edit2: Regarding 3 drain and fills tracked down a reasonable answer:
One drain and fill gives you 43% new fluid. Second D&F leaves you with 68% new fluid. Third D&F: 82% new fluid
 

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Not worth the likely $1000+ to remove the tranny, split the case, and change the filter IMO. The filter has a fairly large surface area, so should last longer than the easily replaceable filters in older transmissions.

I agree with changing what fluid comes out a little under half the total fluid), refilling, driving it for a day or week, and changing the fluid at least once more.

A good scan tool that will show Mode 6 data in real time will allow you to monitor each cylinder for excessive missfires while driving to rule out a missfire.
 

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May also check that the axle nuts are tight. What atf does the owners manual spec? Probably available at Walmart or parts stores a lot cheaper than $10/qt.
 

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Discussion Starter #9
May also check that the axle nuts are tight. What atf does the owners manual spec? Probably available at Walmart or parts stores a lot cheaper than $10/qt.
I will check those. I'm not sure what ATF the owner's manual suggests (mercon, dextron) need to check that but am sure it is less than the the other stuff I have to buy for my car. I just found out though that Valvoline MaxLife is a viable alternative for that and is more reasonably priced.
 

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Looking for your opinion on mileage to change the ATF. The manual on my 17 says 150K, I find that to be excessive personally. I am approach 40K and thought it should be about time. Experience in my mind is the best to follow.
 
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