I guess the engines section is where the heater circuit questions go.

455 in a 72 H/O, dripping coolant to a measurable loss in system volume from engine side of firewall, inboard from where AC condensate would drip. Is coolant, happens without AC system on. I read in another thread that firewall side leak is heater valve diaphragm and on the carpet on the inside is heater core. Other behavior is fogging on windshield at shutoff, but I never noticed whether it was inside or out.

May need to get Valve, heater water temp control, 410572, if diaphragm is indeed toast. That is 71-73 all w/ AC. Looks like I have the newer one already on there.

 

Most of the time your heater core has a leak somewhere. But leaks start as seeping, perhaps only under heat & pressure. Until the seep turns into a coolant leak without warning.

I just went through a mystery leak on an A/C equipped ‘66 Starfire. My symptoms were an observable coolant loss from the reservoir, small (2”) puddle below the base of the firewall, but no dripping inside the cabin. 90% of the time, if there’s a coolant leak, one of the brass pipes cracks at the heat exchanger and you get coolant dripping into the cabin. (Aka the glycol foot bath).

It was only leaking down the firewall, not into the cabin. Weird. In my case it turned out to be a loose hose clamp on the hose from the back of the block to the lower hose at the firewall. I just tightened the clamp up and it was fixed. Of course I was 2+ hours into it before I found the problem.

In your case, the easy (ish) fixes would be to replace the heater valve at the back of the block, any hoses you suspect and put fresh clamps on. A previous owner (or you) may have reused hose clamps. Look around for a coolant drip trail on the block and firewall, the car may leave you clues as to the failed part.

If you get into replacing parts, consider cutting off the hoses so you don’t upset the soft brass heat exchanger. Hoses are quick to replace, the heater core will cost you a day. And they’re getting harder to come by. Way easier to gently xacto knife or razor blade the hose off the brass pipe than to yank it off and get into swapping the heater core.

If it were me, I’d bite the bullet and replace the core + most all needed parts like hoses, clamps and whatnot just to be sure everything is fresh in one go. If you prefer troubleshooting, the quick/cheap fixes are hoses and the heater valve.

Fogging in the cabin suggests a heater core leak, more than an engine side problem, but you probably know that. I don’t know much about post-66’s, but is it possible that coolant is pooling in the heater box in the cabin and going up through the defroster vents to escape? If so, that sounds like a time for a heater core replacement to me.

Coolant troubles are annoying, you never know when a small problem will become a big one without any warning.

Hope I’m not wasting your time, there are many here more knowledgeable than me, but perhaps these comments will help you.
Chris

 

I forgot to check which side the fogging was on, inside or out. Good advice.

I think common wisdom is getting your core fixed as opposed to getting a new one.

 

If you cant determine the exact leak location and it is only in the engine compartment, it could also be the passenger side rear freeze plug. Hope not though. I would almost rather replace the heater core than that plug.

 

If your car has A/C, the leak is almost certainly from the heater control valve at the rear driver side corner of the intake. Be aware that your car needs a heater valve that is normally closed and opens with vacuum. Most vendors incorrectly show the later valve that is normally open and closes with vacuum.

 

Koda,
Below is an old post from a discussion in 2020 about the different heater control valves:

https://classicoldsmobile.com/forums...-valve-144421/

 

https://classicoldsmobile.com/forums...-valve-145499/