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Transmission flush honeymoon period?

I have a 2000 Subaru Outback wagon(automatic). At 160K miles, my transmission was hesitating for about 15 seconds going into drive when cold. I have had absolutely no gear slipping though. My transmission has not been regularly flushed, and despite the risks of flushing a high-mileage car, I had it serviced anyway. After the flush, the service man reported no improvement and declared that I needed a new transmission. I was able to drive it out of the shop, but I could tell that it was hesitating even worse so I started shopping for another car.

Two days later, on a completely cold engine, I put my car into drive and it immediately engaged. I drove it around town with smooth shifting. I drove it into the mountains with much improved power. It is now several days later and my car is starting, shifting and driving like a dream! I took it back to the service department for a test drive and they concur! Could my transmission truly be fixed, or is there such a thing as a post-flush honeymoon period? I feel a little bit panicky - like maybe it will be good for a month or two and then crash hard. If I trade it in now it is worth about 3K, if it crashes in a couple months it becomes a donated vehicle.

I truly don’t know the right answer to that question (and I’m sure someone will be willing to point that out) - but here is a guess. Much of what goes on inside of the transmission is controlled by electronic switches (solenoids) that are pressure controlled. In the first 160K the switches got all gunked up from dirty fluid. The flushing may have helped but then it took a couple of days of driving on completely clean fluid to fully clean the solenoids.

If that is reasonable, then I would not expect any problems. The “honeymoon” period doesn’t seem to make sense.

I’ll agree with aa’s theory. If the problem was ONLY delayed shifting, it was a mistake to recommend replacing the transmission. Shift problems can be caused by hydraulic reasons, mechanical reasons, or electrical/electronic reasons. Some of the fixes are outside the transmission. A reputable repair shop would know.

You could drop by a reputable independent transmission shop and have the car scanned for any codes that may be present.

Since gear slipping can sometimes be very subtle you might consider performing a converter stall test. You can do this yourself.
Set the park brake, hold the foot brake, and shift the trans into LOW.
Try to rev the engine very quickly. It should not rev over about 1900 RPM.
Shift into NEUTRAL and wait a few minutes.
Repeat process in SECOND gear.
Again, shift into NEUTRAL and wait a few.
Shift into DRIVE and repeat process. The results should be the same in all gears; the engine should stall out at around 1900 RPM.

If it revs way over 2000 then you probably do have some slippage and you may worry because Subaru transaxles are somewhat pricy.

You should also consider checking the final drive gear oil which is not the same as the transmission fluid. Subaru at times has had some internal seal leakage problems in which the gear oil mixes with the ATF. This dilutes the ATF of course and the hypoid oil is pretty rough on clutches in the transaxle.
Hope some of that helps.

Thank you all for your replies. Regarding the stall test, my engine stalled out in all gears at about 2200 RPM. A higher RPM than expected, but consistant and steady at that number. Is that something to be concerned about?

In general, even if the transmission is doing well right now, how long can it last without a rebuild? I drive about 18K miles a year including city and mountain driving, but I’m a right-lane girl most of the time lately. Thank you all for your help!

Did it stall at 2200 RPM in reverse also? One problem with doing a stall test in all the forward gears is that if first gear is slipping trying the other forward gears will show the same slippage as the transmission always starts in first gear forward. Also, double check with the dealer to ascertain what the exact stall speed should be for your year, model, engine size, and transmission type. If you truely are getting slippage in first, you might be able to keep this car going a while longer if you are gentle on launch. A transmission tech could probably do a line pressure check to diagnosis the condition of the pump, first gear hydraulics, and the health of the seals for the other gears.

Just to be safe, you should probably always carry a fully charged cell phone and keep you AAA membership up to date.

Hope that helps. Good luck

I’d say go with what you have for now. Just change the tranny fluid and tranny filter at about every 30,000 miles or whatever the manufacturer recommends for “severe” service. It’s cheaper, by far, than a rebuilt trans. “Oil is cheaper than metal”—just about any time.