Oil in cooling system

[rgaidies]

1992 Carver CPMY 43 Twin Cummins diesel 300 HP
when summerizing vessel tech found oil in the cooling reservoir.
I've been wicking out the oil with bilge pads rolled up stuck into the reservoir.
Just wondering if this is possible that the cooling reservoir would have oil in it and how.
Or is it likely that the previous owner mistakenly put engine oil into the wrong location as the openings are close to each other.
Visibly the oil is not dirty "but" I had the oil changed recently as well.
This issue has me concerned but possibly just an error?
Any feedback would be appreciated.

[Tireless]

I had a mechanic tell me that when he tested my coolant reservoir for winterization that it wouldn't test because he believed there was oil there. He stated that it could be a head gasket issue. I looked into it more and learned that there were also other possibilities as well, like a leak at the oil cooler, etc. I was freaking out.

The other possibility, and what I believe was more likely, is what you mentioned. I believe that someone had put an oil top up in the reservoir, or used a dirty funnel to top up the coolant. When I purchased the boat I had an oil analysis done and it was fine. I was of the view that if oil was coming into the coolant from the oil side, the fluid transfer would go both ways. In addition, I was told it was clean oil. If it came from the engine, wouldn't it be darker as the diesels will stain oil fast?

I also found that there was a brownish film on the bottom of the reservoir that I cleaned out. This was just old coolant residue.

I myself never saw any oil in the reservoir and only took the word of the mechanic. When I advise him that I was going to have the dealership come and examine the issue, he advised that it was probably oil put in the reservoir by mistake. ???

I guess you could have a coolant pressure test done. That could assist in locating a leak somewhere. I am not an expert in this and there are experts on this forum that can better advise., Just thought I would tell my story in this area as I also believe it was a contamination issue as well.

Greg

[RobGunn]

Oil in the coolant is more than likely a head gasket issue. Bigger issue to worry about is if it goes the other way and you have Coolant in the oil. This will contaminate the oil and eventually cause the oil to not lubricate the engine correctly and cause the engine to wear out prematurely. Now you have a $50 head gasket cause $5000 engine damage. Look for a white milk-like film on the dip-stick and on the bottom of the oil fill cap.

[Viper]

Source of the post......I looked into it more and learned that there were also other possibilities as well, like a leak at the oil cooler,...

The oil coolers are on a completely separate cooling circuit. They are raw water cooled so no coolant flows through them that can mix with oil in a cooler failure.

There would be other symptoms if there were a blown head gasket. Certainly a cooling system pressure test would reveal a leak if one existed. Rob's suggestion about looking for a milky/creamy film is a common symptom and tell tale sign of a coolant system leak under the right conditions. A blown head gasket in the right spot to mix with coolant would also cause the engine to pressurize the heat exchanger and blow out the coolant into the reservoir or bilge depending on design. You'd also lose coolant into the cylinders in a static state. That would burn and exit the exhaust as white smoke which isn't the easiest thing to distinguish the difference from cold startup exhaust of a diesel application. As noted, coolant in the oil can cause a catastrophic engine failure, and water or coolant in the cylinders during the combustion process is not good for injectors either.

You may want to consider the age of the coolant, it's not uncommon for old coolant to discolour and be mistaken for something else.

Keep us posted.