98 Neon

My Neon has oil or Tranny fluid leaking in my rad that makes it all yellowish colour and gets quite thick like a milk shake and then causes the car to over heat it does not over heat any other time. What could be the cause of this . There is no anti-freeze in the oil!

Yellow? not more of an Espresso foam look?
if it is browner or tanish color it is motor oil. The heat exchanger is on the low side of the tranny, so if the engine is up to 14-15 psi you should be leaking coolant into the tranny when you shut down. If you are getting water in your tranny, the fluid will look like very light pink milk shake.

And by the way, just because you have water in your oil does not mean that you will have oil in your water (and vice versa) tranny fluid usually goes both ways.

To repair a bad tranny cooler it requires one of two ways. The right way and hillbilly way. 1.) Replace the radiator 2.) Get the biggest auxiliary tranny cooler you can get your hands on, and bypass the transmission around the radiator and through the new cooler. You see those coolers are all-supplemental and not stand alone pieces. If you get a small one and run it by its self, you run the risk of smoking the tranny.

If motor oil is getting into the cooling system, its a head gasket that has gone bad. The original head gaskets were known to fail often, the replacement MLS (multilayer steel) head gaskets are a lot better. But it means pulling the head to fix.
If you do a search, you should find a link to the forum which has pictures on how to see if your headgasket is the replacement MLS type. On some of the 1995-1998 cars this replacement was done under warranty, due to it failing in the first 50,000 miles.

Just more info... I did one that the oil in the water ate hoses and the seals from the heater core not something easy to replace flush it out real good.