1991 5.0 mustang gt
coolant backing up into reservoir not returning into radiator overflowing
leaving radiator empty

 

Sounds like a bad radiator cap.

 

thanks 67mustang302 i bought new cap. will try on way home from work
and let you know.

 

tried new cap. still doing same thing

 

check to make sure that the hose that runs from the radiator to the resevoir is secured properly at both ends, if its loose it can suck air instead of coolant when the motor cools down. if that checks ok check for pressure while running, its possible to have a bad head gasket push all the coolant out.

 

Sounds like a head gasket to me, a block test needs to be run.

 

ill do a block test. i was thinking head gasket also but there is no other
signs of blown head. still runs strong, no water in the oil and no water coming out of exhaust. what about a pluged radiator? when i try to burp the coolant system thermostat opens and coolant is pushed out of radiator?

 

Does it run fine when warmed up, but after you shut it off the coolant doesn't go back into the radiator? Or is it filling the overflow and spilling out when the car is running?

 

it overflows when its running. when you shut it off and let it cool it sucks
back some coolant but not all. leaving radiator low and overflow bottle too
full.

 

found coolant leak behind water pump. could this cause the overflow problem?