Heater Core Test Tips?

Discussion in 'Wrenchin' Secrets' started by Gary Anderson, Dec 3, 2019.

  1. Gary Anderson

    Gary Anderson Well-Known Member

    Bought this '68 Riv with the coolant/heater hoses disconnected from the heater core and plugged. PO said the A/C works, but won't for now because "...something, something, heater core bad, something else..." He had no idea what the problem was.

    It's pretty likely that the heater core is a leaker (no idea if it's original or not), but before I go to the trouble of replacing it I'd like to get some tips on making sure the core itself is the problem. So far, I've run water into the top pipe (thereby back-flushing it) and water came out the bottom pipe - so it's not clogged.

    But neither did it leak inside under the dash. I have to figure that it still might leak under hot coolant pressure, but I'm not an expert on heater cores. So, before I re-connect the coolant hoses and put manifold vacuum to the heater valve in the intake to open it, is there some last-resort test I can perform to know whether or not the core is bad? I would really rather not have hot coolant leaking onto the new(er) carpet underneath the dash. Does "not leaking" under ambient pressure (the test I already did) mean nothing at all? What are the odds in this case?

    As always, thanks so much for any insight you guys can share.
  2. BBBPat

    BBBPat Well-Known Member

    Cap off one side and put 10 lbs of air (regulated with a gage) on the other side. Disconnect the air source and it should hold the 10lbs of pressure overnight
  3. woody1640

    woody1640 Well-Known Member

    If it was unhooked/bypassed, that usually means (99% of the time) it is /was leaking.

    Who is going to waste their time bypassing it if nothing's wrong with it?

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  4. Brett Slater

    Brett Slater Super Moderator Staff Member

    Bypassed = leaky.

    If it's a Harrison (original to the car) and you have a good radiator guy, get it re-cored.

    I have a guy out here south of Boston that did mine and (very recently) another board member's - using made in US Maine cores.

    Both of us are very happy with his work.
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  5. Gary Anderson

    Gary Anderson Well-Known Member

    I may be able to do this. I have a coolant pressure tester that I might be able to rig to a hose. Good idea!

    Agreed. But because so much of the vacuum system was disconnected (including the heater valve) I'm wondering if they didn't understand why the heat wasn't working and maybe thought the core was clogged - and just gave up.
  6. StfSocal

    StfSocal Well-Known Member

    Not sure if the heater cores are as difficult to get out on the Rivis as it is on the Skylark/GS cars. If it is (which I believe it probably is), then try and get it tested the best you can. If it fails, then its a process lol. As Brett said he had his and my heater core repaired by his radiator guy and it is definitely worth it. If it isn't original then its a toss up on whether you want to recore or find a replacement. I would recommend to try and not go with aluminum if you can. Aluminium can crack and have galvanic corrosion and the hose connections sometimes are way off.
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  7. flh73

    flh73 Well-Known Member

    I rigged up a setup and used a vacuum pump. If it holds vacuum should be sealed. Unless heat causes it to leak. Good Luck.
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  8. BBBPat

    BBBPat Well-Known Member

    I had a pal that was "helping" me change heater hoses. Pretty simple job on my '73 Stage 1 with AC. I turned around to see him wrestling the anaconda and pulling with both hands. You know the rest of that story...

    Vacuum will work just as well. Put 20 inches on it and pay attention to the gage.
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  9. Gary Anderson

    Gary Anderson Well-Known Member

    Based on the 1968 Buick Chassis Manual, it looks like the heater core box is separate (joined in the middle, but) from the A/C evap box. I haven't looked too closely at it yet, but I think the heater core won't be too ridiculous to replace, if needed. And new ones are in the $50-$80 range, so I'm not sweating that too much. But I have been wrong before...
  10. StfSocal

    StfSocal Well-Known Member

    Ya the actual removal of the box fro the firewall is fairly simple. However, gaining access to the AC box in the engine bay can be quite difficult and I believe there are some nuts that need to be removed to slide the heater box out.

    These instructions are for the A-body cars, but it gives you an idea of removal: http://www.buickperformanceclub.com/heatercore.htm
    Last edited: Dec 5, 2019
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  11. 1969RIVI

    1969RIVI Well-Known Member

    The Riv's are alot easier to get the heater cores out than the A-body cars. Mine is bypassed also due to it leaking. It never leaked inside the car (I dont think it actually can?) But it used to drip onto my header and it looked quite embarrassing at a stop light:rolleyes:. You can get a replacement at rockauto for fairly cheap. I will be tackling that issue in the spring amongst a bunch of others on my spring to do list.
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  12. Gary Anderson

    Gary Anderson Well-Known Member

    I have a replacement heater core on the way ($85, I think). I also called a local radiator shop about repairing the current one (if necessary) but he said they could only re-solder the tubes if that was where the leak was. So I just said screw it and bit the bullet. I really didn't want to risk going through the motions of rigging it up to test it and then making a mess. I mean, the OE core could still be in there, 50+ years old. I already installed a new heater valve in the intake manifold and have some new heater hoses at the ready. Along with a slew of new vacuum hoses, I'll have to source a good vacuum port to open the valve properly once the new core is installed. I think I'm gonna have to replace the temp gauge sending unit because I can't get a solid ohm reading from it. All of this stuff is typical (and expected) when buying an old car without any history of its care and repairs. Un-stuck the LF brake drum this weekend - huge driving improvement; the dash instrument cluster will also be coming out soon so I can check the vacuum lines and lube up the heater controls. And if/when the heat starts working properly, the A/C is supposed to do the same. We'll see how it goes...

    And while having heat is a good thing with winter coming (even in LA), my priority is prepping the body for paint. So this week will be making final decisions on buying replacement door and trunk or fix what I've got.
    1969RIVI likes this.
  13. Brett Slater

    Brett Slater Super Moderator Staff Member

    Sounds like your radiator guy is lazy.

    For a couple hundred bucks and a result of perfect OEM fit, he could have ordered a new core.

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  14. Gary Anderson

    Gary Anderson Well-Known Member

    Replaced the heater core today. Got a new one via AutoZone that fit perfectly ~$85 delivered. My setup is heat & A/C, so they're different than non-A/C cars. Heater box has 6 bolts to remove, plus disconnecting the cable for the inside trap door, and unplugging the resistor.

    My core had obviously suffered a leak in the past. No big deal to change it out, but doing so also gave me the opportunity to clean out the box. The car had been sitting for a couple of years at the seller's house, and some time before that at the previous owner's house. Mice had completely infiltrated the box, inside the dash, and anywhere else they could nest. The heater box and the area in front of the A/C evaporator was loaded with mouse crap, nut shells and sticks - there was even a worn out rag inside there! Nasty, nasty stuff... Happily, I could tell there was no space for the mice to get behind/past the A/C evaporator (which would have necessitated removing that box and the blower as well > fender removal time).

    So I got it all brushed out, vacuumed out, washed and rinsed, new core into the box; everything went back together perfectly. I had also installed a new heater valve into the Edelbrock intake manifold, and ran a direct vacuum line from the carb to it. Before making the hoses permanent, I used a radiator "flush-and-fill" kit to back flush the entire system. Slightly rusty water/coolant came out; ran water from a hose through the system until everything came out clean. Changed to new coolant hoses and turned on the engine to finally get some heat. Got the blower (as usual) but no heat. I discovered that the heat valve needs substantially more vacuum pressure than what's coming out of a carb port. So I installed a second vacuum port into the intake (there was already one there dedicated for the brake booster) and ran that hose directly the heat valve.

    And it opened. And I now have heat inside the car. Still needs some vacuum lines replaced behind the dash (gauges are coming out soon) and I bought a color-coded vacuum hose kit online with much more than I'll need to do the job, as well as adjustment/lubrication of the cables for the blend door and the heater/defroster door.

    It is so satisfying bringing old cars back to life!
  15. StfSocal

    StfSocal Well-Known Member

    Sounds like it went pretty smooth for you, glad you got it up and running!
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  16. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Vacuum for the heater control valve is supposed to be controlled by a line coming from the heater controls at the dash. With the AC running, it will shut the valve, keeping coolant from entering the core. If the AC is not switched on, it will allow coolant to pass through the core, warming the air.
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  17. Gary Anderson

    Gary Anderson Well-Known Member

    I was forcing the water valve open with direct vacuum to check for flow through the system. The thing the Chassis Manual tells is that whenever the temp wheel is even slightly rolled up, that will send vacuum to the heater valve, allowing hot coolant flow to the heater core. Beyond that, heat temp is controlled by cable to the temp blend door.
    Smartin likes this.
  18. BBBPat

    BBBPat Well-Known Member

    You can run the AC and or defrosters and use warm air. the temp control on the dash THROTTLES the heater valve. Goes from cold, to warm, to heat.
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