Plugged heater core?

This is on my 2007 Grand Prix GT (Supercharged)

This is a copy and paste of the thread I made on GPF, I just thought I might be able to pick the brains of you intelligent gentlemen. :D

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So after searching I haven't found a definite answer.

Starting yesterday being a cooler day, I noticed my heat was intermittent. I had it set on around 75 and had heat, then it seemed like it was blowing cooler, then it got hot again, so I thought I was imagining things.

This morning it did almost the same thing. Then this afternoon, setting it on 75 felt like I had it set on 65.

My first thought was the coolant was low and bypassing the heater core. I stopped at Advance Auto and picked up some coolant and topped off the overflow, as it was almost empty. The car was still hot so I couldn't check to see if the radiator needed any added.

The whole way home it was the same with almost no heat at 75, but would get hotter air if I jacked it up to 90.

The operating temperature is in the normal range so I am just looking for confirmation, or other possible solutions.

By the way, I should add the coolant system was flushed around 103k miles according to a sticker under the hood. It stands just shy of 127k now so I don't even know if a flush is an answer.

Thanks in advance.

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I would say plugged or low coolant as well.

Short or bad controller board?

Only three things I can think of.

Either an issue with the coolant or a control issue. I drove around for a winter in my Supra with no heat cause I was car-tarded and never checked it out. It ended up being a solenoid that was bad... Simple things :p

I believe all GM products use hot bottles, meaning the "overflow" tank is pressurized with the coolant system. Our cars use regular overflow tanks that are AFTER the radiator cap, meaning they are not pressurized with the coolant system.

If this is true and your "hot bottle" was low, then your system really was low on coolant.


Edit: Has your heat worked properly since your coolant flush?

The coolant was flushed before I bought the car. It worked fine all winter. I just had it on 75 yesterday because it was a little cool and I was only wearing a tshirt. Yesterday was the first time I noticed it do this.

Also I checked the radiator once I got home and the radiator was full and the overflow was still full.

There should be a control valve on the heater core. Unhook it set it to open and see if it stays hot.

That should tell you if its plugged or the control board.

Accessing the heater core is a major pain in the ass. If I go that far I may as well replace the whole damn thing.

I don't believe there is any coolant valve on the heater core plumbing of any car. Coolant always flows through it, but a flap in your HVAC unit opens and closes to allow air to blow across the core.

It makes sense that your heat gets hotter at higher RPMs as more coolant will flow through it. Let me know if you figure out what the issue is, my old 3000GT had this same problem and I never did figure it out.

A restricted heater core can very well cause those kinds of symptoms, a lot of the newer GM's have a passive heater system that can bypass and flow the coolant around the heater core if it's restricted. We've seen a lot of those types get restricted simply because they have limited flow, and there is opportunity for debris to take up residence... It should be pretty easy to tell if there's good flow, both hoses will be hot.

If it is restricted, blowing through it with a garden hose both directions repeatedly will often do the trick.

That is a pretty complex HVAC system that monitors all duct temps and door positions. It requires a scan tool that can interface with the HVAC system to properly diagnose anything...

Have you ever changed the cabin air filter?

I have not changed the cabin filter. I have only owned the car since March 1. The only things I have changed on it since I bought it is the oil and filter, and the front tires.

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Originally Posted by Alex3000gt

I believe all GM products use hot bottles, meaning the "overflow" tank is pressurized with the coolant system. Our cars use regular overflow tanks that are AFTER the radiator cap, meaning they are not pressurized with the coolant system.

If this is true and your "hot bottle" was low, then your system really was low on coolant.

Edit: Has your heat worked properly since your coolant flush?

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Some GM's use the "hot-bottle" system, some dont. The 07 GP's dont, and the overflow tank is only suppose to have a very small amount of coolant in them so they get low easil but usualy dont cause problems when low.
Sounds like RealMcCoy is on the right track....
Make sure your radiator is full.
Get the car nice and hot and turn the heat all the way up. Grab both heater hoses at the same time and if your heater core is working correctly one will be alot cooler than the other.
If its plugged they will both be hot. So then take both hoses off and flush it back and forth. Youll be amazed with the crap that will come out. Especially with the GM's DEXCOOL...that shit gets SO nasty.

This is where I would start since its relatively easy and free and regardless if its the problem or not its still a good thing to do for maintanence.

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Originally Posted by Bluejediz

Some GM's use the "hot-bottle" system, some dont. The 07 GP's dont, and the overflow tank is only suppose to have a very small amount of coolant in them so they get low easil but usualy dont cause problems when low.
Sounds like RealMcCoy is on the right track....
Make sure your radiator is full.
Get the car nice and hot and turn the heat all the way up. Grab both heater hoses at the same time and if your heater core is working correctly one will be alot cooler than the other.
If its plugged they will both be hot. So then take both hoses off and flush it back and forth. Youll be amazed with the crap that will come out. Especially with the GM's DEXCOOL...that shit gets SO nasty.

This is where I would start since its relatively easy and free and regardless if its the problem or not its still a good thing to do for maintanence.

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Ah, good to know.

On my 3000GT I filled up the heater core with CLR for five minutes then flushed it out both ways with a hose. Still had no heat. That car drove me nuts, I'm so glad I parted it out!

Might be a wonky heater control valve.