Heater Core Replacement

Old May 26th, 2019, 07:42 AM
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Heater Core Replacement

Shortly after starting the car (which was hesitant to start, had not been run since last Sunday) this morning my wife gasped at a tiny stream of coolant on the all-weather passenger floor mat trickling by her new white sneakers. Once wiped-up, the tiny trickle stopped or otherwise diverted somewhere else like the top of the carpet.

It would seem to me that this is probably a dreaded heater core problem (seemingly everything else in the cooling system has been replaced this spring) and I have a few questions for the board regarding my 1972 Cutlass Supreme Convertible 2bbl 5.7L V8
  1. This seems like a heater core issue but, what would a heater valve issue be? I can tell you that even with the fan all the way down and system off, it seems like heat comes out the vents
  2. Other than having my original heater core rebuilt or hunting NOS, are there many differences between replacement heater cores, recommended model?
  3. I noticed just one heater core, the OSC w/AC model on Rock Auto, mentions bolt-in replacement. Should I be fooled by that? For sake of time, would purchasing a Spectra brand at AutoZone or NAPA brand be less advisable? Open to suggestions.
  4. I have heard that the dash of the Oldsmobile is actually configured a little better for replacing heater cores than its GM brethren, how long in labor hours should I expect to replace this part?

Thank you in advance for your help. And, if there's an existing thread somewhere for reference, my apologies.

Last edited by SanTan Devil; May 26th, 2019 at 07:44 AM. Reason: more info
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Old May 26th, 2019, 03:17 PM
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Changing it is a PITB, but doable, just takes time, and effort, and two guys helps. Also most the new ones are Alum. I would have my old one fixed, that's what I did.
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Old May 26th, 2019, 05:12 PM
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Originally Posted by SanTan Devil View Post
This seems like a heater core issue but, what would a heater valve issue be?
Heater valve failure almost always results in a large coolant leak at the valve diaphragm. I suppose there could be a fault mode where the valve either does not open or does not close (such as loss of vacuum), but you wouldn't really know it had failed that way unless it was stuck closed and you got no heat in the winter.
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Old May 26th, 2019, 05:21 PM
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With the valve the leak is usually on the outside of the car. Heater core is on the inside.
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Old May 26th, 2019, 05:21 PM
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I replaced the one in my Wife's 84 Riviera about 5-6 years ago with one of the new aluminum ones. It lasted about 4 years and started leaking again early this year. I actually managed to find a NOS GM heater core on e-bay. It was expensive but as much work as it was to change it there was no way I was using another new style one.

If you dont have a factory service manual see if you can find one. The Buick manual has step by step procedures and makes it as easy as it can be. Still was a complete weekend job. Your Cutlass is probably a little easier.

Not sure if you will find someone to recore yours. In my area there really are no radiator shops left in existance.
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Old May 26th, 2019, 05:45 PM
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If the car is non AC, it would be a lot easier. IIRC you can remove the glove box and work through there. On an AC car, yes it's a major PITA.
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Old May 26th, 2019, 06:23 PM
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Both my cars had AC and I have replaced the heater core multiple times in the past 48 years. There are 6 nuts on the engine side of the firewall that need to be removed in order to drop the HVAC box into the passengers footwell. One is behind the inner fender liner so you need to unbolt and move the bottom of the fender liner or cut a flap in the appropriate area (I recall the CSM gives specifics on the location). I accessed that nut the first time I replaced the heater core by reaching up from the bottom of the fender liner and opted to leave it out upon re-installation, which made subsequent replacements so much easier.

If you aren't a skinny high school kid, it may be best to remove the passenger seat (if you have buckets, that is) to give more room to lie down and access the under dash components. I have got to the point that a heater core removal/replacement is a few hour job, but for the first time it may take a bit longer.
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Old June 8th, 2019, 09:12 AM
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I'm on heater core number 3 currently. The first one burst with the original water pump and the second one burst within two weeks of install with a new pump and leaked all over carpet I had just installed. The issue I found was there was no restriction on flow into the heater with the new aluminum intake and hoses and they kept over pressuring and failing as a result. I installed about an 11mm socket or so in the hose going to the heater and have not had an issue since. Heater works great just as before but doesn't burst.
I also would spend the extra and buy a nice copper core instead of the cheap aluminum ones if you replace your old one. Also take care not break the vent cables. And if you don't have to replace the insulation on the heater blend door, don't. It keeps it from going "Clunk!" when you adjust the heat. But if you do, just take note of how quiet it is or isn't before reinstalling. 👍
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Old June 8th, 2019, 09:49 AM
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Anybody ever just do a bypass and wait on it? Is there a disadvantage to that other than not having heat?

I currently have an aluminum unit in-hand to replace what I assume is the original but, know where to get a brass one. Not sure how much that will cost though.
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Old June 8th, 2019, 09:57 AM
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Bypass is exactly what I did for a long time between replacement. I have a flush kit TEE installed in the hose and I just plugged the hoses together to make a circuit and cut out the heater.
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Old June 9th, 2019, 08:38 AM
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Yes, you can bypass the heater core. Just be aware that there are two different diameter hoses (5/8 and 3/4). I used a hose nipple on the intake in place of the heater valve, then just connected the hose from the water pump to the nipple.


Originally Posted by kiloman_11 View Post
The issue I found was there was no restriction on flow into the heater with the new aluminum intake and hoses and they kept over pressuring and failing as a result. I installed about an 11mm socket or so in the hose going to the heater and have not had an issue since.
Yep, the vacuum controlled heater valve on AC cars and the hose nipple on non-AC cars have a 1/4" hole to act as a flow restriction (the 5/8 inlet hose and 3/4" outlet hose also assist with this). Too much flow through the heater core will cause the tubes to expand and rupture.
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Old June 9th, 2019, 09:33 AM
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I wish that flow restriction was more talked about. Seems like it's only something you hear of AFTER you've ruptured coolant all over your car.
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Old June 9th, 2019, 12:07 PM
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The NON A/C restrictor nipples are sold through several reproduction houses. (HHF714 Fusick) The A/C heater valves are available through "4 seasons". Make sure you get a normally closed unit for 72 and down (A-bodies).
Check the heater blend door for adjustment and proper seal.

Last edited by droldsmorland; June 9th, 2019 at 02:51 PM.
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Old June 9th, 2019, 12:30 PM
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Originally Posted by kiloman_11 View Post
I wish that flow restriction was more talked about. Seems like it's only something you hear of AFTER you've ruptured coolant all over your car.
Yes, I agree. That is one reason why I replied to your post. I blew out 4 or 5 heater cores from the 1980s to the 2000s by using a plain unrestricted nipple. One core blew out after the very first 5000 RPM run. That plain sucked. Then somewhere around 10 years ago I learned about the restriction in the heater valve and hose nipple and now the heater core has survived many 5000+ RPM pulls.
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