I have a dodge neon 98 overheating and I am at a loss. New radiator, thermostat, flushed everything out, new reservoir tank. Also replaced the transmission line.. What is happening is there is a coolant leak from somewhere toward the behind of the reservoir tank. Also there is some kind of leak we can't find into the coolant. There is a milky white/tan substance that shows when removing the radiator cap. Oil and transmission fluid show clean. Any ideas?

 

Possible head gasket leak. That can cause overheating as well as the milky substance. Also you may or may not see white smoke coming from the exhaust if there is a head gasket leak but if there is white smoke then pretty much you can be certain it is a head gasket.

Buy a combustion leak tester for around $30 to verify for sure.

If it does turn out to be a head gasket it is going to cost a lot of money for a shop to fix possibly over $1000. But there is another option that is not guaranteed and can come with a risk and that is BlueDevil head gasket sealer.