'69 Cutlass S, 350, 350TH, 2bbl, factory air.

It appears that I have a bad heater core. I posted info in another thread, but haven't gotten any feedback, so I'm guessing nobody is reading that thread anymore. I thought I'd post here and see if anybody has any ideas or experience with this. The last page of that other thread with all of the background info is here:
https://classicoldsmobile.com/forums...tml#post948761

Problem: With the A/C running, I have a substantial coolant leak. The coolant is running from the A/C evaporator drain. This does not seem to be happening unless the A/C is running, because I have run the engine without the A/C and did not see any leaking.

There is no evidence of leaking in the passenger compartment. The passenger side floor and the area around the heater box is dry.

The car has a new radiator, new high-flow water pump, and all new coolant hoses and clamps. There are no coolant leaks under the hood at all. Cooling system has the original dealer-installed coolant recovery tank, which is all complete and operational. I'm currently running a 13 psi radiator cap and a 195 degree thermostat. I never noticed any leaking before I started all of this underhood work, but I can imagine how a new, higher flow water pump and flushing the cooling system could cause a new leak.

According to the chassis service manual, the 2bbl models did not use a heater control valve in the heater core to manifold line. I haven't yet checked that line to verify this, but my recollection is that there is just a hose nipple at the engine end.

Although it seems pretty unusual, I can imagine how I could have a heater core leak that would only manifest by leaking out of the evaporator condensate drain. What I can't figure out is why this leak would appear only when the A/C is running... but it pretty well makes the newly refurbished A/C system useless until I figure out what's happening and get it fixed. After working under the hood for the last 10 months, I was anxious to start driving the car again, and I'm quickly running out of pleasant driving weather.

Any ideas or suggestions?

 

Are you sure it's coolant and not just condensation?

If your heater core is leaking turn on your defroster and see if you get a film of coolant on the inside of the window.

 

Also check your coolant level as it should be dropping in the radiator.

 

Yes, I'm absolutely certain it's coolant, not just condensate. I collected a lot of it in a drain pan. After things cooled down a little, it took almost 2 quarts of fresh coolant to bring the level back up in the radiator.

I couldn't get my head under the car while this was happening to verify where the coolant was coming from, but it must be coming out of the condensate drain. Nothing else is wet, and there were two steady streams running about like a faucet just slightly turned on.

The engine was also running pretty hot while this was happening. I don't have a temp gauge on it, but I'd say it was close to overheating. The engine actually dieseled when I shut it off, which it's never done before, ever.

I'll try the defroster and see what happens.

 

The only way that the A/C affects the heater core is via the heater control valve. If you have the correct valve for a 69 Cutlass, it is normally closed and opens when vacuum is applied. Temporarily pull the vac hose off the heater control valve and plug the hose. Run the engine until warm. Does it now leak coolant, even with the control in the HEAT position?

 

You guys never cease to amaze. Joe, I did as you suggested and removed and plugged the vacuum line at the heater control switch. I didn't see any leaking with the controls set to heater or defrost, or with the temperature on warm or cool. The heater was also blowing cold regardless of the temp setting.
I reconnected the vacuum line and restarted the engine. With the heater set to heat/warm, the heater was now blowing warm air, but still no leaking. I moved the control to defrost, and didn't see any windshield fogging. But it was now leaking. I'm sure this was a coincidence and had nothing to do with the defrost setting.
Before I started, I had jammed a paper towel under the heater hose connections at the heater core. It was dry until now, but suddenly this was wet. Once the engine was really warm, I could see a tiny amount of coolant bubbling out of the new inlet hose (the one I replaced). Apparently where I can't see underneath the hose, it's leaking like crazy.
I'll fix that and test some more and report back.

Joe, it sounds like the heater control switch is working normally, doesn't it?

Thanks guys very much for the insight and ideas.

 

Well, that seems to have done the trick and solved the leak. On the 3/4" heater hose at the firewall, I replaced the double-wire clamp with a worm gear clamp. As much as I dislike worm gear clamps, it sealed much better.

Apparently the heater control vacuum switch is working correctly, and the leak had nothing to do with the A/C being on aside from the extra load causing the engine to produce more heat faster. Only after the engine got hot and the cooling system had fully pressurized did that connection to the heater core start leaking.

The funniest thing is that even while it was leaking like crazy, when looking from above the leak was hardly discernible. If I hadn't looked really closely at the end of that heater hose, I'd never have suspected that connection. Everything I could see from above was completely dry. I never risked boiling my head by looking for the source of the leak while the coolant was really running fast, but afterwards when it slowed down to a drip and I looked around that area, the only thing I saw that was wet was right around the evaporator drain. Very deceptive.

Thanks very much, guys, for getting me on the right track. I'm very, very glad to have this little mystery solved. Now I can get on with other things and actually have a chance to drive this car again before winter sets in.

By the way, when I was thinking I was going to need a new heater core, I shopped around a little and found copper/brass replacements to be very hard to find. I think I only found 3 places that had them. Summit actually had some in stock, and for considerably less cash than the others ($50 vs $80). Aluminum cores were everywhere, and were much less expensive than the copper/brass cores.

Thanks again, guys!