'89 closed system, no temp gauge yet, overheating?

89 4.0 closed system.

Engine recently bought back to life somewhat after sitting for 2.5 yrs.

It's a closed system and aftermarket temp gauge is suspect (gets up to and pegs at 240 very quickly). Bottle and cap appear OK.

Once I start it cold and idle/drive around, it runs OK for 5-10 minutes. Once it gets to the 240+ mark and sticks, it starts to run very poorly, sputters, wants to die.

Would high engine temps cause such a poor running condition so quickly?

How to I tell it's really over heating in a closed system with no boiling?

Also, I keep reading about fan clutches but the fan appears to rotate in realation to engine speed as if it's a direct drive.

What happens when you turn the fan with your hand, you will not turn a direct drive fan with out slipping the belt.

Overheating causes an engine to run poorly.

Have you changed:
thermostat
Water pump
coolant


the coolant temp sensor for the factory temp guage is at the rear drivers side of the head, check the connections there and replace if it looks nasty or you change all the stuff above and still have the same symptoms.

when refilling with coolant you should remove the coolant temp sensor in the rear of the head and fill the system until liquid flows out the hole for the sensor to avoid air pockets in the block.

Pulling the radiator hoses and flushing fresh water through the radiator to make sure its not clogged or gunked up.

Hopefully you haven't blown a head gasket by overheating it so much.

good luck