Heat only blows hot after being parked

Joined
Aug 12, 2015
Messages
785
Location
Central NY
My 91 Vette. Just trying to figure out the fundamentals of heating at this point because it makes no physical sense.

new t-stat, new water pump, new coolant(drain n fill 2x with distilled water then 50/50 mix)

no matter how hot the coolant gets it refuses to blow hot air. Both lines on the heater core are hot. The only time I get hot air is when the car is driven, parked, and started back up again within a few hours. Only blows hot for a minute then gets ambient again.

my only assumption is the blend door being jammed up, but that still wouldn’t explain why it only blows ambient temp air at all times. Even if half and half I would assume it would always be a constant temp, but it’s not. Why would it only blow hot for a minute if there’s coolant running through the core?

clogged up heater core? Reversed hoses on the heater core? Faulty new t-stat?
 
my only assumption is the blend door being jammed up, but that still wouldn’t explain why it only blows ambient temp air at all times.

Yes, yes it does.

You are smart to assume blend doors; most people have no idea what that is. They will blend air and you can't feel a change. Toyota and GM are the biggest problems with this, it's not massively common IME with other makes.

I don't know YOUR car, but check this out more. On Toyotas/Lexus, you can just open up the blend door motor and clean off the accumulated copper wear particles in the dielectric grease, put in clean dielectric grease, and motor on. The GM units are not so easy to split open, but usually only $20 to replace. The metallic wear particles get into the greases and confuse the sensor, that's the base problem.

I have repaired a few 90s and later GM vehicles with new motors (not repairable like toyota); look more into that. There should be specific Corvette forums that have specialized knowledge on this; ask there.
 
My bet would be that you have a vacuum leak. Those HVAC systems are vacuum controlled...and if you don't have enough vacuum, you lose control of the blend doors. Do you have a lumpy cam, or high performance engine...those can lead to vacuum issues.
 
My bet would be that you have a vacuum leak. Those HVAC systems are vacuum controlled...and if you don't have enough vacuum, you lose control of the blend doors. Do you have a lumpy cam, or high performance engine...those can lead to vacuum issues.

All stock motor, but I would not be surprised if it’s a vacuum leak. A lot of the rubber hoses are falling apart. Haven’t had a chance to sort through all of them yet.

edit: looking at some blend doors and looks like the vac line running to the HVAC vaccum tank is rotted out. Going to replace it first before I start pulling the dash apart. Thanks! Would’ve never thought of that.
 
All stock motor, but I would not be surprised if it’s a vacuum leak. A lot of the rubber hoses are falling apart. Haven’t had a chance to sort through all of them yet.

edit: looking at some blend doors and looks like the vac line running to the HVAC vaccum tank is rotted out. Going to replace it first before I start pulling the dash apart. Thanks! Would’ve never thought of that.
Happy to help, I hope it solves your problem! Post some pics of your ride if you get a chance.
 
Made a noob mistake.

replace vacuum lines - no help. Surprisingly the rotted line still held pressure.

those heater hoses that I thought we’re hooked up, actually were looped under the expansion tank. Someone on the Vette forums mentioned checking the evap core which requires pulling the coolant expansion tank quickly to check. I remove the tank and see the heater lines jerry rigged and the heater core connections in the open.

most likely it’s leaking. Since I was planning on pulling the dash to install the new blend door anyways, I’m gonna replace the heater core while I’m at it. Luckily I saw this before I did the blend door. I’d be mad if I had to do the job twice
 
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