BMW Mystery

  • 2001 BMW 325I
  • 6 CYL
  • 2WD
  • 210,000 MILES
My car has 210,000 miles on it. The other morning, my transmission warning icon came on when I went into "D" drive. I slowly drove the car to a local mechanic. He said there was a leak in the cooling system and transmission fluid had leaked and mixed with the coolant system.

He recommended a new radiator and some hoses and adding new transmission fluid as well.

I went to pick up the car to take it home to ponder this.

When I drove the car home the transmission warning icon did not go on the car drove okay. A week later, I started the car and it was still okay.

If his diagnosis was right, I do not see how the car could be running okay again when we did not do anything.


Advice on where to go from here to get to the bottom of this without spending a lot of money.
Thursday, August 18th, 2016 AT 11:59 AM

1 Reply

  • 33,788 POSTS
Automatic transmissions needs a fluid to apply the clutch packs and move the various valves. That will occur regardless if the fluid is transmission fluid, water and antifreeze, or apple juice. Transmission fluid is the only one of those that will lubricate bearings and moving parts, so while it might seem to be working okay for now, the water is promoting corrosion of metal parts and the antifreeze will slowly dissolve the fiber clutch plates. Water can make the plates stick too well resulting in harsh engagement. Water boils at 212 degrees F. Transmission fluid can get hotter than that. Steam is a vapor and vapors can be compressed just like air can. Air in the transmission's control passages will kill a transmission by compressing under pressure rather than applying the clutch packs solidly. That results in slippage that you may not even notice right away, but it will continue to get worse.

Transmission fluid, (or engine oil) in the cooling system will rot the rubber heater and radiator hoses from the inside. That can take months to show up as a soft, burst hose. Just because you don't notice anything unusual right now, there can be damage occurring that will show up later as a sudden failure.

This type of fluid mixing occurs when the transmission cooler is built into the radiator. Some cars use separate external transmission coolers, but most use coolers inside the radiator. Those internal coolers can corrode through, especially on cars that don't get the coolant replaced every two years. The water pump lubricant and corrosion inhibitor additives in antifreeze wear out in about two years; that's why it needs to be replaced. Acids build up in the coolant when those additives become depleted.
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Thursday, August 18th, 2016 AT 11:23 PM

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