Oil in Coolant. Volvo Tamd74 EDC

Previously I noticed I had been losing a little bit of coolant on my port engine. Expansion tank was empty and I've probably put a quart of coolant in since a cooling system rydlyme flush/refill performed by yard at the end of last seasons (maybe 40hrs ago).

So I was checking things over and popped the coolant cap on top of the engine to check the level there since expansion tank was empty. I found it covered in oil. I dipped my finger in and got to coolant pretty quick. So estimating 2-3 ounces of oil that settled at top.

As I understand it oil in coolant could be one of 3 things... head gasket, cracked head, or oil cooler.

I change my own oil at the end of each season and have never noticed anything odd about the used oil. But I pump it into buckets and don't always see it that closely. The yard JUST changed the oil as part of an exhaust elbow service and didn't say anything about the oil looking odd but not sure how carefully they looked either. And I'm not losing much coolant so not sure how noticeable it would be in the oil (7+ gallons). I would also note I've never had a problem with temps.

My best guess is and I am hoping it's the oil cooler, as I think that would be cheaper to replace than tearing down the top end of the motor. Seems like that is the most likely culprit given the quantities I seem to be dealing with.

I welcome any thoughts and armchair wisdom on the issue and the diagnostic process. I don't want to just change the oil cooler out as it's a pricey part so not sure what the process is. At $125/hr I'm hoping it's not "take stuff apart until we find it".

My volvo engine oil cooler interfaces with seawater. An oil cooler failure on my engine would result in oil loss to the sea. Are you sure your oil is cooled by engine coolant?

obthomas said:

My volvo engine oil cooler interfaces with seawater. An oil cooler failure on my engine would result in oil loss to the sea. Are you sure your oil is cooled by engine coolant?

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Yes, that is my understanding. And, unlike my gear oil cooler, it does not have a zinc which would imply to me no sea water.

I don’t like guessing at these questions. I like data. First thing I would due is test form hydrocarbons on the coolant side. If thee are no hydrocarbons then it’s an oil cooler issue. If there are hydrocarbons present then we know it’s a head issue.

oil cooler is raw water on mine. If yours is fresh water then good guess.
Have you checked around the coolant circ pump for the coolant drip? If that pump wears the shaft seal could allow a bit of oil into coolant.

tiltrider1 said:

I don’t like guessing at these questions. I like data. First thing I would due is test form hydrocarbons on the coolant side. If thee are no hydrocarbons then it’s an oil cooler issue. If there are hydrocarbons present then we know it’s a head issue.

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I appreciate your reply but not clear on the advice. There's no doubt the coolant will test positive for oil. I could see it in the coolant. 2-3 ounces at top of tank.

I could certainly test the oil for coolant. In fact, I have test kits on the way. But couldn't a leaky oil cooler still let coolant into the oil after shutdown?

Soo-Valley said:

oil cooler is raw water on mine. If yours is fresh water then good guess.
Have you checked around the coolant circ pump for the coolant drip? If that pump wears the shaft seal could allow a bit of oil into coolant.

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Thanks, had not considered circ pump.

Edit: To add that I have not detected any coolant under the engine.

I'm pretty sure the 74 has a coolant/oil heat exchanger. If it is easily removed, I would pull that and pressure test it on the bench.

You can pressure test with air on the oil side and submerge unit in a bucket and watch for bubbles.

If not too expensive, you could just replace the cooler and flush out the coolant system to get the oil out. If you see two ounces in the tank, there is more in there hiding.

I have seen oil coolers leak oil under pressure without having the coolant go into the oil. A crack can open up under oil pressure and close when oil pressure is not there.

Coolant circ pump is belt driven, that won't do it.

The head gaskets could, but a pain to take heads off. Lots of hours.

Has engine ever overheated?

Ski in NC said:

I'm pretty sure the 74 has a coolant/oil heat exchanger. If it is easily removed, I would pull that and pressure test it with oil under pressure on the oil side. Pressure test on the bench. Leave oil pressure on it for a long time and see if oil shows up on the coolant side.

This will require fabricating a test rig capable of providing oil at like 80psi. At $125/hr that can get pricey.

If not too expensive, you could just replace the cooler and flush out the coolant system to get the oil out. If you see two ounces in the tank, there is more in there hiding.

Coolant circ pump is belt driven, that won't do it.

The head gaskets could, but a pain to take heads off. Lots of hours.

Has engine ever overheated?

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Ski... thanks for the reply, was hoping you'd chime in.

No, never had any temp issues in 6 season.
Access to oil cooler is poor. This is port side and oil cooler is on port, up nice and close to fuel tank. Unfortunately, cooler is a $2600 part so not wild about replacing it for the heck of it. But sounds like pulling and pressure testing may be best bet.

I am losing coolant. Again, about a quart since system was serviced last August.

Very unlikely that it’s a head gasket problem....both the oil passages and the coolant passages in the head gasket are O ringed. The oil would have to go out one O ring and in another....

Possible to be a cracked head, but also unlikely. Coolant might leak into the oil via a crack in the top of the head, and this might be seen by pulling the valve covers. And oil could leak into the coolant via a crack in the oil passage to the rocker arm shaft, but very rare.

Most likely it’s the oil cooler. It’s a plate type cooler clamped to the block and cooled with antifreeze. A pin hole or tiny crack can put oil into the coolant.

Thanks very much. Every knowledgeable person I've talked to has agreed that the oil cooler is the likely culprit. Added wrinkle is I had to move boat about an hour from its location to my home slip. Boat ran great, turned up to WOT and no issue with Temps. But when I got back I had about a half quart or so of coolant in bildge under engine and sugnes it came out up top at cap..
There's a good chance it was the expansion tank hose which was in place but lose and so coolant just leaked out. Expansion tank had little coolant. But it also makes me worry about head gasket since burping coolant or boilover is a symptom of head issues.

I am going to move forward with replacing the oil cooler as I still think thats most likely cause. Challenge right now is finding one. Coming up empty with various parts suppliers who are now waiting to hear from Volvo Europe on supply.

from a marine engine expert friend....

'Why not just bypass the cooler for an hour or two and see what happens to coolant levels and oil content. It's easier to connect the inlet and outlet water hoses than all the other ideas tossed out."

psneeld said:

from a marine engine expert friend....

'Why not just bypass the cooler for an hour or two and see what happens to coolant levels and oil content. It's easier to connect the inlet and outlet water hoses than all the other ideas tossed out."

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Thanks, will consider. Not sure how easy it would be to bypass.