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Questions and Answers

Hot Transmission

Q. Car problem: Automatic Transmission leaks one or two ounces of transmission fluid when engine is shut off after the car has been driven. The harder the car is driven and the hotter the Trans gets the more leaks out. One or Two ounces being the most. Short Trips with the engine not so warm maybe only a couple of table spoons.

The Car: 1991 Toyota Supra. Electronically controlled Automatic transmission with Overdrive which acts as a 4th gear. Mileage is 65,000 miles. Trans fluid and filter last changed 4 years ago at 25,000 miles. This is NOT the turbo model. Engine is 6 cylinder in line, 24 valve double overhead cam. I believe the design incorporates a built in transmission cooler.

Onset of symptoms: The transmission fluid has been about one fourth of a pint over full for the past couple of years. During the past several months I had noticed that the car would feel a little rough and kind of shudder slightly as it went from stopped to say 5 mph. No other symptoms and it did not do it every time. I first noticed a problem when driving the car on the German Autobahns which have no speed limit. The first two days I drove between 85 and 95 mph for 4 or 5 hours each day. The last day I drove between 95 mph and 115 mph. There was a lot of up and down hill as we were driving thru the mountains. I stopped for gas and after I turned the ignition off a cloud of white smoke started from under the hood. I later discovered the trans fluid was dropping onto the hot exhaust pipe. Most of it seems to be on the same side as the filler tube. I thought perhaps the trans was overfull so did not add any fluid. I waited until the dipstick showed it was one and a half pints low on fluid. This seemed to slow but not stop the leakage of fluid. It took about 800 miles to go from full to 1.5 pints low. I then filled the transmission to the full mark and the symptoms remained the same. Other than the oil leak the transmission seems to operate normally. It does not seem that the Transmission is overly hot nor does the fluid smell burned. Engine temperature and all other systems operated within normal parameters. Car was being operated in appropriate gear ranges. Normally loaded.

My diagnosis: I put the car on a lift. It looks like the fluid is coming out the top of the filler tube/dip stick spout. It seems to be only on the one side of the car. There is no leakage when the car is sitting running or not running. The Toyota factory manual lists trouble shooting but does not list these symptoms. The Chilton manual for this car lists "clogged vent tube", or "clogged cooling lines" as possible causes for this problem. I am not sure whether they are referring specifically to this model or simply auto trans in general. I have been unable to find evidence of a vent tube.

There are a series of diagnostic electrical checks which I can make to determine whether the computer has registered a fault in the electronic system. I have yet to do this. I intend to drop the pan, change the fluid and filter and look for any abnormal particles in the pan. I have considered using an air chuck to pressurize the transmission thru the filler tube in an attempt to blow out the vent if there is one. I hesitate to do this for fear of causing damage. I will certainly disconnect the cooling lines and blow them out with compressed air to assure they are free and normal. Beyond this I do not have any ideas. I feel it is likely something simple as the transmission acts normal in every other way.

I would be most grateful for any ideas.


A. What I would say is happening here Mark is the transmission fluid is getting way too hot and is backing out the filler tube. The vent is not very large and not readily visible from underneath. You have to feel along the top of the transmission to locate it.

Any car with an automatic transmission has a transmission fluid cooler. It is part of the radiator, usually on one side or along the bottom. Chilton is right on as far as the probable causes. As the transmission fluid gets hot it expands and the excess air has to go somewhere. Hence the vent. If it is clogged, it will back fluid up the dipstick tube. Like wise if the transmission cooler is not doing it's job. Since you are driving the car faster than normal and at a sustained length of time, the stock fluid cooler may not be up to the job. You should install an auxiliary transmission fluid cooler to increase the cooling capacity of the system. You can get a kit for it at any auto parts store. As far as the vent goes, I have never had to take one out so I'm not really sure how it is removed.

As far as pressurizing the transmission to clear the vent, I'm not real sure if this is a wise thing to do. I would check with a transmission shop on the best way to check and clear the vent.

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