Having problems with your 1984 Mercury Grand Marquis ?
My 1984 grand marquis does not have interior heat.
\015 I replaced thermostat. Both heater core hoses are hot. Top radiator hose is hot,bottom hose is cool. Car is full of coolant,which I squeezed top hose to purge air as I added coolant while car was running. Had warm air for 5 minutes,never to return since first thing this cold morning.\015
Sounds like a faulty blend door under the dash. The blend doors direct air thru the heater core and the vents. Your model should have vac controls so you could have a vac leak or a stuck door.
Re check the coolant level, it most likely dropped when your engine reached temp for thermostat to open and took what you had just poured into radiator and put it in the block.
Repair Help & Product Troubleshooting for 1984 Mercury Grand Marquis
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