HELP Replaced heater core, now air won't blow hot.

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Tie-Dyed

Not-quite-so-new-guy
Feb 27, 2018
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Hello there everyone, I'm new to the forums because I purchased a 1986 Cutlass Supreme Brougham Coupe, the 5.0L V8 and it is starting to require some repairs. I am not great at fixing these things but am trying to teach myself. The heater core was leaking so was replaced and after this I could not get my air to blow hot except for once or twice and it has not done it since. Before I replaced the HC the defrost and heater had no problems. All the dampers are working and it blows plenty of air through all the settings it just never gets hot. I found that there was a cracked PCV valve elbow so I have that part coming in tomorrow. I have inspected all the other hoses and vacuum lines and there don't seem to be any other issues.

As far as it working after the new HC was installed, this occurred at random on two occasions when it was particularly cold. It has not worked since last year but as the weather was warming up I didn't care. It has been pretty cold the last few weeks so I have decided I should fix this problem. Driving with the windows down to keep the windshield from fogging is getting old even though my dog seems to be pretty happy with the situation.
 

axisg

Comic Book Super Hero
Jul 17, 2007
2,685
2,354
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YYZ
sounds like one of 2 things.
A: your heater control valve is shut ( operated by vacuum on AC equipped cars )

B: most likely you have air in the system. Air compresses whereas liquid does not. This makes it difficult for your water pump to pump the air out of your heater core and pump coolant in so your heater will work. aka vapour lock

Get a cold car up on jackstands in front ( get the rad cap at least 6" above the top of the inlets to the heater core ) and let it run with the cap off the rad until you reach operating temp. Once there Rev it up a few times to help burp the air out of the system ( which is trapped in the heater core ). It will splash hot coolant out of the rad and make a bit of a mess but its the simplest way to remove the air.

To make it go quicker take the heater core lines off at both the intake and water pump. Hold both coolant lines 12" above the heater core and using a funnel load it up with distilled water or 50 50 coolant mix until full. Clamp then reconnect your lines and remove the clamps then do the jackstand trick.

Once the heater core is full it will self "burp" into the radiator so keep an eye on the levels for the next few days.
 
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fleming442

Captain Tenneal
Dec 26, 2013
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I'm putting my money on the valve. It's in the coolant line between the engine and the heater core, and has a vacuum line going to it. A new one is a $10/ 10 minute fix, and is of different construction than the factory metal valve which is prone to corrosion.
 

pontiacgp

blank
Mar 31, 2006
29,270
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Kitchener, Ontario
I use this kit to bleed the system in my GP...

https://www.walmart.com/ip/Lisle-24680-Spill-Free-Funnel/29724014

6ec02b0b-6d38-405f-a943-1233276c56ad_1.083d13e0e561971810f6a718d5659537.jpeg
 

Tie-Dyed

Not-quite-so-new-guy
Feb 27, 2018
5
1
3
sounds like one of 2 things.
A: your heater control valve is shut ( operated by vacuum on AC equipped cars )

B: most likely you have air in the system. Air compresses whereas liquid does not. This makes it difficult for your water pump to pump the air out of your heater core and pump coolant in so your heater will work. aka vapour lock

Get a cold car up on jackstands in front ( get the rad cap at least 6" above the top of the inlets to the heater core ) and let it run with the cap off the rad until you reach operating temp. Once there Rev it up a few times to help burp the air out of the system ( which is trapped in the heater core ). It will splash hot coolant out of the rad and make a bit of a mess but its the simplest way to remove the air.

To make it go quicker take the heater core lines off at both the intake and water pump. Hold both coolant lines 12" above the heater core and using a funnel load it up with distilled water or 50 50 coolant mix until full. Clamp then reconnect your lines and remove the clamps then do the jackstand trick.

Once the heater core is full it will self "burp" into the radiator so keep an eye on the levels for the next few days.

Thanks for the tips. I will definitely bleed the system using your method when I change my oil either today or tomorrow.

I'm putting my money on the valve. It's in the coolant line between the engine and the heater core, and has a vacuum line going to it. A new one is a $10/ 10 minute fix, and is of different construction than the factory metal valve which is prone to corrosion.

I am thinking I will just replace the valve since the car sat from 97 to 2015, only being started sporadically and driven a very short trip around the block each time. When purchased the car had around 69,000 miles, now 92,000 and I don't think anything besides the carb had been worked on or repaired.

I appreciate the quick replies and the help.



This does look fancy. I'll probably just lose some of the pieces though.
 

pontiacgp

blank
Mar 31, 2006
29,270
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Kitchener, Ontario
This does look fancy. I'll probably just lose some of the pieces though.

It's more than just the looks that are nice, it's simple to use, works well and there is no spillage. It puts the coolant well about the coolant in the system when bleeding it
 

Tie-Dyed

Not-quite-so-new-guy
Feb 27, 2018
5
1
3
It's more than just the looks that are nice, it's simple to use, works well and there is no spillage. It puts the coolant well about the coolant in the system when bleeding it

I believe you, but I also believe in my ability to lose things, and it seems to be out of stock anyhow.

Edit: Well... I amazon primed one for 19 bucks and it will be here tomorrow.
 

pontiacgp

blank
Mar 31, 2006
29,270
20,391
113
Kitchener, Ontario
I believe you, but I also believe in my ability to lose things, and it seems to be out of stock anyhow.

there are plenty of stores that carry it, Napa has there own version. I keep mine in a plastic bag so no parts go missing.
 

Tie-Dyed

Not-quite-so-new-guy
Feb 27, 2018
5
1
3
there are plenty of stores that carry it, Napa has there own version. I keep mine in a plastic bag so no parts go missing.
Yeah I just ordered a kit off Amazon along with some brushes to clean out my engine bay. Had a gift card so that's a win.
 
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