is that the only explanation for oil in the coolant reservoir, or are there other things that may be wrong?

 

Check the oil and see if its looks like a milkshake...if it does than you have a problem.
The oil in the coolant resevior you may be mistaking that with dirt.

-matt

 

does ur car smoke alot?

 

car does not overheat (knock on wood) nor do i notice smoke. These days it's cold so i see the exhaust coming out the back. would it be noticeably smoky?

 

In some head gasket cases oil will mix with the coolant, in other cases coolant will mix with the oil, you might not nescesarily have both. Also, I agree with matty that it could be dirt, I wouldn't be worried unless you see oil floating around when you open the radiator cap, or if you cleaned the reservoir and still see oil. If it is oil, the only possible causes are a blown headgasket, a cracked head, a cracked block, or a junk radiator (<--- if it's an automatic tranny)

 

Would it be possible for oil to enter the coolant system through a faulty waterpump? Just a thought, as I dont have oil in my coolant, so I'm just tossing out ideas.

 

Do you have an automatic? It could be a leak in the transmission cooler and be leaking transmission fluid into the coolant.

 

check ur oil filler cap...if it has a milk chocolatey substance inside it...ur screwed.

 

If you have oil in your coolant, you probably do have a headgasket issue, but you have the better one of the two. You can either have coolant in your oil, or oil in your coolant. OIL IN YOUR COOLANT IS MUCH BETTER. You can still drive your car like this as long as you drain your radiator every so often and check your oil. This way you can drive your car and save money for an engine swap or rebuild.

 

When i worked at a small shop before Toyota I saw a car that had its timing chain too loose that when it it rotated at high speeds, it wore a hole into a coolant passage which in return mixed Oil in with the coolant. I dont know if that helps but i mean our cars have timing chains, and metal griding on metal makes big holes.
2) If u have coolant mixed with TRANS fluid ur trans is over/done/finished/no more.