I just recently found out that my old car has an “unrepairable” leaking head gasket. (It is unrepairable because of the cost of repair vs. cost of better car.)
Well, both a friend and also my mechanic said to try the various ‘stop leak’ formulations, as this certainly won’t make things worse.
Standard instructions are to add the solution and then allow the car to idle or run for 15-30 minutes. Then top of radiator if needed.
I did this and the darn thing leaked like the a sieve (usual since the head gasket gave way) during the warm up period but after about 8 minutes sealed up like a drum. How does this stuff work?
Most of the block seal stuff consists of soft metal shavings (lead? tin? aluminum?) in some sort of gunk. Basically, the theory, as it were, is that if you put enough junk into a cooling system, it will clog anything. Stop leak may or may not seal the gasket, but it also may or may not clog the radiator as well, and it usually makes the coolant look like… well, like it has block sealer in it.
FWIW, I’ve heard good things about K&W sealant (I could be wrong about the name, but it’s something like that), but I’ve never heard of anyone using it to fix this bad of a gasket leak. To use it, you have to completely flush the antifreeze from the cooling system, run the engine with the stuff in it, then flush again and let it dry before refilling. It’s a lot of work, but it’s the only stuff I’ve heard of that permanently fixed a coolant leak (gaping holes in head gaskets not included.) The guy that recommended it to me said that his machine shop used to use it regularly to prep flathead Ford blocks (which apparently are usually slightly cracked), and never had a comeback.
I don’t know about head gasket leaks, but up until just a few years ago, my mom had a 1967 Mercury Comet with a cooling system that constantly developed new leaks. In the early 80’s, I showed it to a venerable old mechanic, thinking that he was going to laugh and tell us to scrap the car, or at least send us to a junkyard to look for a new radiator.
Nope - he had a solution, one that worked for over 10 years: Black pepper. Get a can of ground black pepper, and dump the entire thing into the radiator core. Run the car for a half hour, and add coolant as necessary - the pepper will lodge in the leaks and clog them. When it starts to leak again, add another can of pepper.
That car consumed two or three cans of pepper a year for almost 10 more years, and ran until 1993, when my sister put one too many tankfuls of unleaded into it without additive, and it cracked a cylinder when dieseling after she shut off the ignition.
The other advice that mechanic gave us was to never change the oil. Just add oil when necessary - the dirt and metal residues served to clog up the oil leaks. Same principle as the pepper.