I Have A 90 Marquis My Heater Core Was Behind The Fire Wall And Under The Dash Somewhere Behind The Glovebox. I Was Told The Whole Dashboard Would Have To Come Off To Replace It. For $400 I Couldnt Do It So I Jus Bought A Nice Sized Coat To Ride Out The Cold Mornings With. Hope I Helped!
The above answer is pretty good but if you'd like some more detail read on!
There is a box of sorts which is located behind the passengers' side of the dash. Inside this box there are two heat exchangers. One which has hot engine coolant running through it for heating the air, the other is used by the A/C to cool the air. From your dash controls you select if you want heat or cooling and where to direct the air (windshield, vents, floor) and inside the box 'trap door' like flaps open and close to accomplish what you requested.
This box is bolted to the firewall (firewall is simply the sheet metal between you and the engine). If you open your hood and examine the passengers' side of the firewall behind the engine you should be able to find where 4 hoses are attached. Two of these hoses carry the hot coolant to and from the heater core, the other two carry the 'Freon' like gas to and from the A/C evaporator to cool the air. And if you look closely you may spot the bolts or studs that attach the heater box to the firewall.
Now if you were able to find everything, it starts to explain why changing the heater core which is inside that heater box is soo expensive! To get to the heater box parts of, if not all of, the dash have to be removed. Then there are wires and cables attached to the box which have to be carefully marked and removed without damaging them. From the engine bay the four hoses (BE CAREFUL!!! ENGINE COOLANT IS VERY HOT FOR HOURS AFTER THE ENGINE IS SHUT OFF AND CAN CAUSE SEVERE THIRD DEGREE BURNS, USE CAUTION!!!) must be removed. The A/C lines should be purged by a certified licensed mechanic to avoid release of gasses to the atmosphere (which is environmentally BAD and ILLEGAL too). Then the box must be unbolted from the firewall and removed from the vehicle. Once the box is out then you must disassemble the box to the point where you can remove the old heater core and install the new one also being careful to not damage any sealing gaskets around the heater core or box covers removed to gain access.
You just replaced your heater core! Congratulations! But wait you still have to mount the box to the firewall, attach all hoses, cables, wires, replace lost coolant by filling the radiator, and have your A/C recharged!
On my 1988 Marquis, in September 2007, I was quoted $600 for this job by a local gas station, with a good reputation, on Long Island, NY.
Oh, and to explain as to why heater cores need to be replaced there are several reasons. Poor quality part to begin with, corrosion from not replacing engine coolant as per service schedule (few people realize that engine coolant becomes corrosive with age) or just a lot of years of service. A 1988 vehicle is now almost 20 years old! Two signs of a leaking heater core are a chokingly sick sweet vapor coming out your vents or the tell tale puddle on the carpet under the passengers' side dash.
One final note of caution: MOST ENGINE COOLANT IS TOXIC IF INGESTED!
ADULTS, CHILDREN, and PETS WILL DIE IF SWALLOWED!!! Do not dump anti-freeze anywhere. Take it to your local gas station or a town recycling center for proper disposal.
OK, how many of you that have read this are getting the yellow pages out to make an appointment to have it done professionally?!
Or if your adventurous... get at it!
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