wino39

Gunk(oil) showed up in the radiator overflow tank. Have done a coolant flush and replaced thermostat to no avail. Vehicle now has sporadic (but happening more often) episodes of overheating. You can stop the vehicle, place in neutral, rev engine to 2500-3000 RPMs and temp returns to normal(most of the time). Have to top off coolant frequently and is nearly impossible to get all trapped air purged.Virtually no oil consumption (130,000 miles on vehicle) and great power. Any definitive testing method to determine if this is a head gasket?? If this is the problem does any one have opinion on engine sealants? Can this stuff harm my engine and make matters worse? Or is it worth a try? Anyone ever heard of "Liquid Glass" (a sealant)? Anyone care to share a range of cost for this repair ??? Thanks.