Why would there be transmission fluid in the water overflow reservoir?

Question

The car is a '98 Buick Lesabre with the 3800 engine and about 170k miles. It appears as though some oil was in the radiator, but I haven't seen any signs of coolant in the oil. The water pump recently went bad, so I'm not sure how much oil was in the radiator because all the coolant leaked out; I just found some on the inside of the cap. When the water pump went the serpentine belt came off. I drove it to the mechanic and they said I needed a new pump, belt and a flush. They didn't mention any other problems. I drove home and haven't started it since. It didn't overheat at any point.

Do I have a blown head gasket? If not, why would oil be leaking into the coolant, but no coolant leaking into the oil?

Answer 1

The 3.8 Natural Aspirated Engine was a rather solid design and were rarely known to blow head gaskets. Their weak point seemed to be the plastic Intake Manifold. Here are the three possible causes to your issue.

1. If you are 100% sure that it's oil in the coolant then remove the intake manifold and check the Intake Manifold Gaskets upper and lower for any signs of seepage. Be sure to inspect the Intake Manifold for any cracks or signs of it leaking. If this is the case, this is how oil would find it's way into the coolant and not vise-verse. Here is a picture of the intake manifold for reference. This is a very common issue with these motors. And honestly I'm surprised that with the mileage your vehicle has it hasn't been done yet.

2. Make sure it's not transmission fluid. If you have a look at the radiator and you'll notice the radiator has two metal pressure lines on the same side one near the top and on near the bottom of the assembly. These lines come from the transmission to the transmission cooler that is integrated into the radiator. If it was transmission fluid then the vehicle will have shifting issues because the coolant would also get into the transmission causing the clutches and torque converter to overheat.

3. If the oil and transmission levels are not dropping then what's going on? Double check the substance in question, if you realize that it is nothing more than sludgy build-up in the cooling system (you will notice it on the bottom of the radiator cap and in the coolant overflow tank) and the vehicle has a Dexcool brand coolant in it, this is caused by air being introduced to the coolant system. Possibly when they changed the water pump. Here is a photo of a the inside of a radiator that had Dexcool in it. You'll notice the substance when mixed with coolant could resemble oil to the untrained eye.
   _{(source: sancarlosradiator.com)}

Answer 2

As a mechanic with many years of experience there are only a couple of ways engine oil can enter the coolant system

1. Someone added oil to the coolant overflow tank or radiator.
2. A crack in between an oil gallery and a coolant passage either in the engine block or the cylinder head(s).

There could be some other reasons but it has been my experience that if you have oil in the water you have a serious problem.

Answer 3

It's likely that the oil cooler is leaking if the vehicle is equipped will have one. Look for metal lines running into the radiator tank. If you have an automatic transmission there will be two sets of lines on each tank of the radiator.

Answer 4

It seems unlikely that you have a blown head gasket if you don't have milky oil. In fact it's fairly common to see some trace of oil on your radiator cap. A little bit of oil works its way in there, especially if you replace parts of the cooling system. You should be safe to just keep an eye on it for now.

Answer 5

Oil pressure is higher than the coolant pressure, so oil gets into coolant through a crack in the cylinder head. Most, if not all heads that have pressurized oil in them have camshafts in the head, which the 98 3.8 Buick V6 does not have.