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Can gum and varnish actually hold an aging transmission together?

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Dear Tom and Ray:



I have a 2002 GMC Envoy that just passed 100,000 miles. I have always been pretty conscientious about scheduled and preventative maintenance. The owner's manual recommends changing the transmission and filter at 100,000 miles. I have read and heard advice that if the filter or fluid has not been changed before 50,000 miles, you should not change it, due to varnish and film that would then be released into the new fluid. I don't see any sense in calling the dealer, as they obviously will advise me to change it. I would like a "nonprofit" answer from someone I trust, which is you two car guys. -- Jim

TOM: Change it.

RAY: This is an old myth that we've heard many times -- and probably passed along ourselves, on occasion.

TOM: It goes something like this: When your transmission is really old, the gums and varnishes that have built up over the years are actually holding the transmission together now. And if you change the fluid and drain out all that gunk, the transmission will fall apart.

RAY: But I've got news for you. If varnish is all that's holding your transmission together, there's a rebuild in your near future anyway.

TOM: I think this myth got started because some guy with a really old heap changed his transmission fluid, and a day later the transmission died. Well, the transmission was a heap before he changed the fluid. And the timing was just coincidence. But that didn't stop him from telling everybody he knew not to change the transmission fluid in an old car.

RAY: But that would be like saying: "If you're 95 years old and sick, don't go to the hospital. My grandfather was 95 and sick, and he went to the hospital, and he died!" Well, duh! He was 95 and sick.

TOM: So if your owner's manual calls for a transmission service at 100,000 miles, do it, Jim. In fact, if you plan to keep your car until the bitter end, you can even change it more often than that -- say, every 50,000 miles. And if you feel pain in your chest or shoulder that radiates down your arm, don't be afraid to go to the hospital.
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