Will used oil analysis ALWAYS detect head gasket leaks?

Joined
Nov 19, 2024
Messages
4
I have a 09 Subaru, which is one of the model years known for head gasket issues. I have been getting used oil analysis (UOA) to try to make sure I would detect a head gasket leak. However, I started loosing coolant earlier this year (May or so) and have done two 4,000 mile intervals since then. Neither report showed any sign of coolant contamination.

That said, I can't find a coolant leak external to the car. So my question is, can coolant leak into the cylinder and get pushed out the exhaust without leaving a trace in the UOA?

For information, I bought the car used at around 160,000; I don't know if a previous (I'm the 3rd) owner did the head gasket at ~110,000.
 
Sure, here is the latest report. The last two were after I started consuming serious amounts of coolant. I am adding about a gallon every other week, driving around 150 miles per week.

1732064465720.webp
 
Yeah if the coolant is getting in the combustion chamber it'll get burned and not show up in the UOA. Is the oil as clear as always, no slight haziness or anything?
 
When we owned four cylinder Outbacks, we monitored the coolant overflow. It is very easy to test for combustion byproducts. We had the head gaskets replaced preemptively at 140,000. We gave that car to a relative and it was traded in, still running, with 289,000 miles.

Keep in mind the quality of head gasket jobs is quite variable. The heads almost always need machining but some shops and many shade trees skip that and just slap in new gaskets.

Gen 3 Outbacks are great cars. We bought a 2006 3.0R new and put 255,000 miles on it, and currently own two H6’s with 156,000 and 117,000 miles respectively.
 
Last edited:
Yeah if the coolant is getting in the combustion chamber it'll get burned and not show up in the UOA. Is the oil as clear as always, no slight haziness or anything?
Javier,

I wouldn't describe it as "clear", but it is like typical used motor oil to my eye.
 
When we owned four cylinder Outbacks, we monitored the coolant overflow. It is very easy to test for combustion byproducts. We had the head gaskets replaced preemptively at 140,000. We gave that car to a relative and it was traded in, still running, with 289,000 miles.
What do you mean by monitoring the coolant overflow? The level is decreasing in there as well. I am adding directly to the radiator though, and then adding to the overflow.

When you say test for combustion by products, do you mean one of those dye kits where the air will change color if you have exhaust coming into the coolant?
 
I have a 09 Subaru, which is one of the model years known for head gasket issues. I have been getting used oil analysis (UOA) to try to make sure I would detect a head gasket leak. However, I started loosing coolant earlier this year (May or so) and have done two 4,000 mile intervals since then. Neither report showed any sign of coolant contamination.

That said, I can't find a coolant leak external to the car. So my question is, can coolant leak into the cylinder and get pushed out the exhaust without leaving a trace in the UOA?

For information, I bought the car used at around 160,000; I don't know if a previous (I'm the 3rd) owner did the head gasket at ~110,000.
I am the original owner of an '04 Forester that also had slow coolant loss but UOA found no trace of coolant in the oil. And no obvious signs of external leaks. When I replaced the timing belt, pulleys, tensioner, water pump, etc. I discovered the original water pump gasket was leaking coolant that was dripping into the lower engine cover and being absorbed. After replacing the water pump & gasket, it stopped losing coolant. I gave that car to my daughter and at about 120k miles now it still has the original head gaskets and runs great. I know its days are numbered, the head gaskets on those cars eventually fail, but it looks to have several more years and thousands of miles before that happens.
 
I'm not sure about an 09 but on our '98 OB with the EJ2.5 we had trouble with the head gaskets 2x, first was at 109,000 where it started overheating and overflowing the recovery tank, had it fixed by the dealer, then again at 186,000 miles, same thing, episodic overheating and af bubbling and getting forced out the recovery tank, also there were black gritty flecks of something in the tank, probably carbon from the exhaust. I don't think we ever had AF in the motor oil but didn't do an analysis at the time.
At that point we traded the car on a 2017 Forester, that's been good. I think they finally solved the HG problem.
 
A coolant leak into the combustion chamber(s) can be smelled in the exhaust, especially at this enormous consumption. You don’t even need to bend down and sniff the exhaust directly.

Also you can test the coolant for the products of combustion or even look for bubbles in the radiator neck.
 
Last edited:
Back
Top