Subaru Outback

My 1997 Subaru has 160,000 Miles. Mechanic A replaced my old engine with one with 50K on it. After 4 weeks the car heated up. It was out of coolant. I took it to mechnic B for checking, he said that it was the headgasket. He checked it for hydrocarbon, and it was positive. I took the car to Mechnic A for explanation. After two days I picked up the car and it has been fine for the past 7 weeks. I am puzzeled.

Offhand, sounds like Mechanic A did not top off the coolant.
Maybe Mechanic B’s test for hydrocarbons picked up residue from the original engine.

Any reason why you’re inserting Mechanic B into the middle of Mechanic A and yourself?
This muddies the water when it comes to a problem.

When you took the car back to Mechanic A “for explanation,” what did he say? What did he do in the two days he had the car?

Why are you puzzled?
You gave the car back to Mechanic A, told him that Mechanic B diagnosed it with a bad head gasket, and Mechanic A replaced it.

What’s so hard for you to understand about that?

BC.

Unless the headgaskets were replaced I would be surprised if this problem doesn’t reoccur. The shop may have used some type of sealer in the coolant to put a temporary fix on this. I recommend you keep a very close inspection of the coolant level to make sure it doesn’t go down.

The coolant was flushed clean by Mechanic A. I got the feeling that Mechanic A was not trustworthy. I used him as a default.