Consequences of adding Blue Devil headgasket sealer to a car with no obvious problem?

Adding "Blue Devil" sealer or any other non-approved chemicals to a motor vehicle is not necessary, a waste of money, can be dangerous (destructive) to the engine and may void the new car warranty. I've been doing auto repair since 1974. :-)

A modern head gasket in a cast iron block/aluminum headed engine does not actually seal in the way that say a jar seal does or a cap on a pop bottle. Instead you have a set of TTY bolts that are in essense giant springs that put a constant force of many tons on the gasket.

Stop leak gasket sealer is based on the notion that a gasket is like a paper gasket it is a fixed thing and it degrades over time and gets "loose" The stop leak works its way into the gasket filling up the voids. Old head gaskets might have worked that way but not modern ones since the TTY force insures that a constant pressure is on the gasket even if the gasket gets squashed down.

It very much depends on how the sealer works. Some more modern sealers are liquid until they come into contact with the kinds of conditions found within the cylinders during combustion at which point they "go off". I am lead to believe that "Steel Seal", a brand available in the UK works in this way. This therefore makes it effective for sealing a gasket that's blown between a water jacket and a combustion chamber. This type of sealant wouldn't do anything to a car which didn't have a head gasket leak.

Other sealers work by adding a material or hardening in some way. They may, for example, add some sort of "silt" to the coolant system which is deposited around the system. Adding too much of this is enough to cause blockages in heater matrixs and radiators.

Before doing anything to repair a non-existent head gasket leak, have either a compression test or leak down test carried out as this will generally be cheaper than one of these liquid gasket repair fluids.

hey all I can tell you about the product first hand is... my o4 jeep cherokee has 219k miles I use it only to pull my boat because im not pulling it to salt water with my 2020 tundra, it blew a head gasket my mechanic recommended blue devil or the junk it, put it in and now runs as good as the day I bought it! you decide

It depends.

The engineering staff at Blue Devil no doubt did at least some engineering work to make sure their product was compatible with the majority of engines.

However, the engineering staff at all of the automotive companies designed their engines and cooling systems to work with plain coolant.

I think you would expect that in most cases, Blue Devil or similar products will not cause major problems, but there are almost certainly engine configurations that Blue Devil did not anticipate or design for. Also, what if your particular engine isn't in the nominal condition (i.e. a partially plugged heater core or maybe runs slightly hot because of a marginal radiator)? Then you are getting into scenarios where the outcome is unpredictable (risky) that neither Blue Devil nor the auto companies have designed for. So in the case where you have a blown head gasket on a beater vehicle, the risk/reward of using this kind of product is certainly worth it. But to add it for no reason? Some risk with no benefit.