Which way does the water flow through the heater (no A/C on my vehicle)? There is a 5/8" hose that goes to the intake manifold and a 3/4" hose that goes to the water pump.

Ain''t that just the way it is,in the summer time when you don't want it ,it's hotter than hail in your Vette,in the winter time you freeze your bells off .you can try to flush out the core it self by taking off both heater hoses and with a garden hose flush it out in reverse.but if it is really plugged and you try a full engine flush it usually totally plugs the core and you end up replacing it anyway.I think the hose coming from the water pump is the inlet line into the core,if I'm wrong somebody jump in and correct meRon 782010-01-07 12:08:02

Yep, a good flush will probably take care of it.   If both hoses are not very hot, as hot as the radiator hoses after full warm up, the core is blocked.kstyer2010-01-07 17:53:11

You stand corected there Ron. The hose comming from the intake is the inlet or hot side. If you want to back flush you go into the water pump hose. If the core is plugged inlet hose will be hotter than the outlet. Be careful of chemicals. Some may be corrosive. I owuld just try flushing it with water. I have an advantage in my shop as I have a flush machine that pulsates water and air.

Disconect the hoses from the engine then back flush, you don't want any of that crap getting pushed into your cooling system. Good luck.

Not sure if this is same problem. But my sons 78 with air is only blower cold to Luke warm air on hear/defrost/vent. Not sure where to start. Any advice would be greatly appreciated. Thanks

I would begin with a flush of the heater core as described in the posts above. Follow that with a separate flush of the radiator and engine block. If nothing appears to be blocked, reconnect and refill. Warm to temperature, make sure your thermostat is opening and double check the temps of the hoses to and from the heater core. 

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if I remember right there is a heater control valve rs below exhaust manifold  works in vac  I replaced it because I didn't like the way it looked,i don't drive this in winter in conn but insummer this car is hotter then hell. I would not use anything to back flush  water only,chem open the road to bad things   read post   most want is written is true     hope it works ,take your time.dwa175

Not sure about the '73, but by '78 the heater system also has "doors" that regulate heat entering the car.  Our own 78 suffered a loss of heat last spring and the reason fr that turned out to be that the cable had come adrift of the control lever, positioned behind the glovebox. 

So, if the inlet and outlet hoses are both hot, check the operation of the heat control lever by taking the glovebox inner out and watching the lever move as you operate the control.

yes   my 79  cable broke   a pain in the---        but replaced, that works fine dwa175