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Questions and Answers

Hydroplaning Explained

Q. I have a '91 Coupe DeVille with 4.9 Liter V8, ABS, Fuel Injected, A/C, P/S, 64,000 miles.

I live in the South and we often have torrential downpours during the summer months. When it rains very hard, the car starts to "roar" very loudly. It sounds like the engine is racing with no load at all . . . as if it were completely disconnected from the transmission! It does maintain speed but probably won't accelerate. (Due to the heavy downpour and resulting lack of visibility, I have to slow down anyway.) As soon as the rain slacks off a little, everything returns to normal.

I've had a dealer look at it and was told that there is nothing wrong with it. The service manager said, "There is absolutely nothing that could cause the problem you describe." He speculated that the front tires could be hydroplaning. I've since replaced all the tires (they were due for replacement anyway) and the problem persists.

This problem only occurs when traveling at highway speeds and only in very heavy rain. Normal rainfall has never caused the problem.

I can only guess that perhaps "something" getting wet is causing the transmission to shift all the way down to first or second gear. But instead of slowing down rapidly when I let off the accelerator, the car seems almost to be coasting or free-wheeling.

Taking the car back to the dealer is a waste of time because they only connect it to the computer, drive it around the block, and say they cannot duplicate the problem.

Have you any ideas?
Thank you for any insight you can provide.
Don

A. For those who do not know what "Hydroplaning" is, let me explain it.

When it rains a layer of water forms on the road. As a car moves down the road it will actually ride on the top of this layer of water instead of contacting the road surface. The effect is like riding on a sheet of ice since there is no traction and loss of control is very possible. The faster the vehicle is moving, the more pronounced is the hydroplaning. Vehicle weight and tire condition can reduce the effect, but not eliminate it. Vehicles will begin to hydroplane at about 35 to 40 mph for an average size car. Smaller cars will start to hydroplane at lower speeds and heavier cars at higher speeds.

The hydroplaning problem is even further compounded by the normal accumulation of oil on the road. The oil comes from cars leaking on the road surface and floating to the top of this layer of water. The oil layer will wash off which is why the first 10 or 15 minutes of a rain is the most dangerous.

I strongly suspect the dealer is giving you good information here. I don't see any mechanical problem that could cause the symptoms you describe. There is nothing on the transmission that will malfunction when wet and likewise the engine. Since it seems to happen more so in a heavy rain than a light rain is a good indication that the car is, indeed, hydroplaning. There are special tires that will decrease the hydroplaning, but will not eliminate it.

When my wife was sick and I had to get to the drug store to get a prescription filled. It was my first, personal, experience with hydroplaning. It was raining, not too hard, and I was in a rush. My van lost traction and I was all over the road. I did a couple of 360's and wound up smacking a high curb. It broke my left rear wheel and bent the left axle. I was only going 45 mph when it happened. Luck was with me that day because even though it was a Sunday afternoon, I was the only car on the four lane road. I had my two sons with me and it could have been a real tragedy. The danger is real and only by slowing down will you increase your safety factor.

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