My 2001 Subaru Outback has a blown head gasket (surprised?!) and the quotes for repair are roughly $3500. Thermalguard says they guarantee it will fix a blown gasket. It sounds too good to be true. What’s your opinion on Thermalguard?

It’s too good to be true. A blown head gasket can only be repaired by removing the head and replacing the defective gasket. The trouble with the guarantee is that they will have your money before you find out the stuff does not work. It may also plug up your cooling system, heater core and radiator.

Depending on how your head gasket is blown, some of these “miracle-in-a-jar” sealer products will indeed work. But that’s for all definitions of “will work”.

Assuming you’re not worried about Thermalguard plugging up your header core and radiator, if it does manage to stop the head gasket leak, you don’t know how long that “fix” will last. Will it last a day, a week, a month, a year? It is only temporary and it will leak again.

Tester, a respected contributor on this forum, has seen some products (Steel Seal with sodium silicate) work for up to a year in some cases.

I recommend only trying these miracle-in-a-jar sealers only if you can live with the risk of a clogged radiator and header core, and you never intend on fixing the head gasket properly, and you’re prepared to deal with it leaking again at any time. In other words, only as a last resort to band-aid a car hoping it will last a few more miles.

If it were me, and I thought of this car a “disposable” - like it didn’t owe me anything and I was going to just drive it into the ground, then I would try one of these things.

However, if I took the car seriously as something that I constantly depend on and wanted to last then the actual fix is a way to go.

If you use the search button at the top of the page you can find plenty of discussions to read - one of the more common brands is Steel Seal and there have been quite a few posts on that one.

I’d be inclined to shop around for quotes. $3500 sounds high.

Additives of this sort are only good as a means to squeeze some extra life out of an old beater. The head gasket will need to be changed to effect a proper repair.

A coolant leak (either externally or internally) that is not a breach into the combustion chamber may possibly be stopped with a stop leak type product.
If the combustion chamber is breached then forget it.

Who gave you this 3500 dollar estimate for head gaskets? That sounds not just high, but ridiculously high.