Plugged heater core?

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.

Hi Stealthee,

Any chance the thermostat might be sticking at the open position?

Coolant is always flowing trough the heater core so I don't think it may be clogged...

Oscar.

If thermostat was stuck open then the operating temperature would be affected.

Operating temp is 100% normal.

Well I flushed the heater core, and that was not the issue. The only way I get heat is if the climate control is set to 90. Anything below that and its cooler air.

After engine is sufficiently warmed up how do the heater core inlet and outlet hoses feel? Paticularly the outlet hose?

Also, is the thermostat stock/OE? Rated at 195 degrees.

I'll have to wait til I head somewhere to check the hoses.

As far as I know thermostat is OE. Had perfect heat all through the end of winter.

Also, when the cooling system was serviced did the shop use Dexcool or some aftermarket extended life type antifreeze. With Dexcool the coolant will be orange. Others green/yellow.

The reason why I ask this is some switch over to the EL green but the system has to be properly and thoroughly flushed of all Dexcool or sludging may develop later on.

It has Dexcool in it. I actually want to switch it over, but figure I'll wait a bit to do a total flush and refill.