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I just had the transmission cooler line in my radiator fail and it created one hell of a mess. Transmission fluid was in the cooling system and the dexcool was in the transmission. I did some google research and it seems Toyota had radiator issues with both the '99 4Runner and the 2000 Tundra. The failure point seems to be the drivers side transmission fitting on the radiator. The 4Runner apparently had an issue with dissimilar metals being used on the fittings causing corrosion and eventually failure. I would strongly suggest that for the cost of around $200 dollars you can replace your radiator and alleviate this problem. I drained the radiator and bypassed the transmission cooler in the radiator and ran the lines through the external cooler. I then ran Simple Green through the radiator and flushed 6 times with this mixture. I then removed the radiator and decided to go all out and replace the timing belt and water pump. Before I reassembled I thoroughly flushed the coolant system and block. The transmission was the tricky part. The fluid had the look of a Strawberry Milkshake. I have heard of too many horror stories of transmission flushes and subsequent failures, but I knew that the majority of the fluid was in the torque converter. I did some research and decided I would try a manual fluid exchange. The Transmission apparently holds about 16 quarts and it took 9 gallons on transmission fluid to flush all of the glycol out of the system. I then added a pint of SeaFoam Transmission additive and my truck is running absolutely fabulous. Hopefully this will help someone in the future. This is more common a problem then we realize and it has nothing to do with poor maintenance habits. This is unfortunately another Toyota screw up that will cost someone their transmission. I luckily was not driving when the failure occurred. That is the only thing that saved me from destroying my transmission.
 

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Did you say you were using DexCool? If so, that was the problem. It is not compatible with Toyota or Honda components. My local mechanic had flushed my cooling system with that stuff a few years ago, and it resulted in a new radiator, new water pump, and a new thermostat manifold (gasket was leaking). Here's an old but good discussion about the different types of coolants. The bottom line is to use only Toyota Red coolant, or you're asking for all kinds of trouble.

Motor Magazine Coolants Article - Consumer Maintenance - Automotive Aftermarket | Gates Corporation
 

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2015 Toyota Tundra DC SR5 5.7L
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Did you say you were using DexCool? If so, that was the problem. It is not compatible with Toyota or Honda components. My local mechanic had flushed my cooling system with that stuff a few years ago, and it resulted in a new radiator, new water pump, and a new thermostat manifold (gasket was leaking). Here's an old but good discussion about the different types of coolants. The bottom line is to use only Toyota Red coolant, or you're asking for all kinds of trouble.

Motor Magazine Coolants Article - Consumer Maintenance - Automotive Aftermarket | Gates Corporation
Yep. The old DexCool formulation is now the subject of an ongoing class-action lawsuit for just these types of incidents.
 
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