I have a 1980 300sd (116 chassis) which I believe has a leaky heater core. The symptoms are after a drive the coolant magically disappears when the valve, which the PO installed in the heater hose before the core, is open. Close the valve and the coolant level doesn't go down. The weird thing is I haven't noticed any leaking coolant underneath the car or on the floorboard. Also, the heat works great when I have the valve open, it just eats coolant. So, from what I hear, replacing this will involve taking out the whole dash. Is this a major operation or is it doable in a weekend? Is there any special tricks I should know? Is there anything else I should replace while I have it out? Another problem with the HVAC is that all air (a/c or heat) only comes out the defrost vents. I assume some hoses need replaced to fix that, are there diaphragms in there that needs replaced as well?

 

yes the dash must come out

good chance to check for any leaking hose and vacuum element too. better check for leak also from vacuum source. you may also recover some comb and pens that has fallen off from the dash.

you may need some platic wire straps for you will cut some of them and too brittle for re-use.

 

If you are not seeing a leak or smelling coolant in the car I would do the garden hose test before taking the dash apart. Maybe a vacuum test? That coolant has to be going somewhere. Find the leak first!

 

hmmm so what is a garden hose test?

 

here are some indicators of a leaking heater core: a sweet smell in the interior with the defroster fan on, an oily film on the windshield, perhaps a leak out of the evaporator condensate drain or a slight mystery coolant loss. Always inspect the evaporator condensate drain located above the transmission bell housing.