Hey everyone,

Just today I noticed that my coolant reservoir was low to nil so I visually checked inside the thing and it looked as though there is a oily residue on the inside and it is dark and smells like oil. On the other hand, my oil looks like I put it in yesterday. I do regualar oil changes w/ synthetic oil since the truck has 160k miles on it. There is no foam in the oil is what I mean.

So my question is... does oil in the antifreeze = antifreeze in oil or is it because I still have a greater pressure one way ?

My worst fear is that my block is cracked. This past winter I found the small hose leading from rad cap to overfill disconnected at overfill reservoir. It was very low and I refilled and have had no problem until now. I live in Lake tahoe so it can get cold.

Thanks for any advice or shared issues.

Doug

 

A bad head gasket can cause combustion byproducts to be blown into the cooling system. The residue creates an oily sheen on the top of the coolant, but is not oil, per se. The best bet for diagnosis of such a situation is a compression test or leak down test. Any time combustion gases get into the cooling system there will be a tendency for the vehicle to overheat, so this can also be a sign of this sort of problem.

A cracked block is very unlikely unless one of the following happened:

1) The cooling system got refilled with plain water, instead of antifreeze and water, and it got really cold out.

2) The truck was run without coolant, and was allowed to overheat really, really badly.

If neither of these things happened to you, I would doubt the block is cracked.

Check the heater control valve and the water pump weep hole. These are two places that can exhibit "hidden" coolant loss.

 

My money is on a bad head gasket. Same thing happened to me with the 93 Caravan I own (I know I'm stupid for buying a junky Dodge that I can't get rid of). Have lots of "oil" in the coolant. Sure enough the head gasket is blown. Typical with those 2.5L junk engines.

Drips Oil Drops Grease Everywhere

My Old Pig Aint Running

 

IMO if you had a cracked block the engine oil would look milky or would would have no coolant in the system depending on the location of the crack (internal vs. external). If it was a head gasket then the res. would be full from the pressure blowing back out of the radiator into the res. Fill up the res. and top off the radiator while cold and then drive it around for a while and when it's toasty hot, pop the hood and see how much fluid is in the res. If it's flowing out the cap then head gasket is shot. Also the radiator cap can weaken with age, check that too.
I'd check around for a loose hose or leaky gasket or even the water pump. If you did overheat the engine you'll need to change the oil and tranny fluid. Sometimes the trans fluid cooler on the radiator can crack and leak and at 160k miles it could be the source of the oily residue. Check the trans fluid as well. But lastly the res. could be cracked and leaking. Do a good look around before start worrying about the big ticket items.