Would a head gasket sealer work to stop exhaust pressure from going through the cooling system and overflowing the reservoir
It might work for a little while but ultimately you need to replace the head gasket, and repair any collateral damage it may have caused. (If coolant gets into the oil it can cause significant engine damage.)
If you are experiencing increased exhaust pressure you need to find and fix the restriction.
You seem to be describing a scenario where combustion gases are entering the water jacket through a damaged head gasket.
So, 1st, make sure that's happening.
If you start your engine cold with the coolant cap off does the coolant immediately start to overflow or bubble?
Watch Scotty's video on testing for combustion gases in your coolant.
If there's no doubt that you have a blown head gasket then in your shoes my thinking would be
"won't hurt to try something that might delay the inevitable."
They've come a long way with these products, however, don't expect any miracles.
If you have a tiny pinhole leak it may buy you some time.
I'm not endorsing any of the products but K&W makes one called FiberLock® and they used to offer a complete refund if it doesn't work
https://www.youtube.com/watch?v=v0U8Iyf0kk4
@jack62
No it doesnt bubble at all the level just climbs slowly for no reason. No overheat and heat works perfectly
@jack62
Here is a pic of it
https://s.amsu.ng/WNOi9vDFvbmN
@jack62
Level starts of normal when cold
@jack62
To be certain you might want to use a head gasket leak detection kit like the ones Scotty talks about to check for combustion products getting into the coolant.
https://www.amazon.com/Lisle-75500-Combustion-Leak-Detector/dp/B0007ZDRUI
@jack62
But if it's not that what else could it be
Coolant overflowing into the reservoir could be due to a bad radiator cap,
Heck, stop at the parts store a buy a new radiator cap.
That would be a cheap and easy fix.
Be sure that you're only filling the coolant reservoir to the "Cold Fill" line.
Then, if it isn't the radiator cap, do the combustion gas test to rule that out, .. (or in)
@jack62
I would say cross that bridge if and when when you come to it. First test to see if you have the head gasket leak. If it tests positive, which seems likely, that will confirm it. If it tests negative then it is time to start looking for other causes.
@jack62
Would a screw on cap also do that because mine doesn't have a radiator cap only the reservoir
A bad cap there will have the same result. The system needs to be pressurized when hot.
Think of starting any car with the radiator cap off.
Things will be fine until it gets hot and then the coolant will begin overflowing out.
Having a faulty cap on the expansion tank won't allow the system to properly pressurize and it will overflow when hot.
I have noticed that the seal is kinda bad and is leaving black residue on the reservoir threads