My 1988 Mercury Grand Marquis still over heating

Hopefully it's something simple like maybe your timing is off a bit. When you check your engine oil , does the oil look normal ? *( no bubbles or chocolate milk looking stuff on dipstick ) . This would be a sign of a blown head gasket that also would cause overheating.

Try bleeding air from system like this. Remove cap from overflow tank fill with antifreeze to full mark leave cap off and start vehicle and let idle also you will notice level in tank start to rise this is normal once thermostat starts to open level in tank will drop add more antifreeze to full mark then put cap back on.