I have a 2006 Pontiac Vibe. Same engine as the Toyota Matrix. The heat blows fine but isn't warm enough.

I checked the antifreeze and the level is fine and there are no leaks.

The hose into the heater core is hot. The out hose is fairly warm.

The heat is warmer when moving than when idling.

High is counter productive. The heat is only semi helpful when the blower is on low or just above.

I'm wondering what to do next?

Change the thermostat? The engine never overheats, seems to get up to temperature, but it took a long time to do it the other night when it was 8 degrees F.

Flush the heater core?
Backflush the heater core to clean it of debris and make sure that it has decent flow.

Something else?

What are the next steps in troubleshooting this problem? What have I missed?


2 Answers 2


If the engine is warm, the air flow through the heater core is off, and one side of the heater core isn't burning hot like the other, then there's a good chance your heater core has debris in it causing it to block.

If your thermostat was stuck open, there's a good chance the engine wouldn't be able to come up to temperature especially on a cold day. If it's stuck closed then it would overheat. Since that's not happening, there's a good chance the core is blocked.

If you decide to try to flush/backflush it with a garden hose or something, make sure you don't use too much pressure, or you'll blow out the heater core, especially if it's blocked. I might use a gravity feed - a hose from the bottom of a full bucket, like an IV drip but bigger :)

Flushing it might be the best diagnostic - you should be able to tell if it's blocked or not if your gravity feed doesn't get much flow in one direction.

After you're done, make sure you flush out any air bubbles that might be in the coolant loop. Tip the front of the engine up (park uphill or such), pop the radiator cap off (careful if its hot!) and run the engine, keeping a close eye on the engine temp. Add fluid if/when your reservoir starts to get low as it fills in any bubbles.

If you want some tips on what to do, and what not to do, this jackass makes a pretty good show of clearing a blocked heater core:


  • Thanks for the video. Informative and entertaining. Both of these answers have very good info so I'll pick a winner after I figure out what the problem is.
    – hortstu
    Jan 11, 2014 at 19:12

If the heater hose is hot going into the core, this means there is flow through it. If it wasn't getting flow through it, it wouldn't get very hot ... warm, maybe, but not hot. My suggestion here is it may be the blend door which is not functioning correctly. If you are unsure what the blend door is, it's the piece inside next to the HVAC unit which directs air through the heater core or by the AC coils (hot or cold blending to get your desired temp). Some blend doors are connected directly to the dash switch, while others are actuated by a motor. Not sure which way it is on yours, but you should check this as your problem.

  • My blend door is controlled by a dial. I can feel resistance when I turn it and the temperature of the output changes if I turn it. I would imagine this allows me to rule out this as a potential cause?
    – hortstu
    Jan 11, 2014 at 19:13
  • Since you can feel a difference in the temp, I'd suggest the solutions spelled out by antiduh probably can help you in the end. Jan 11, 2014 at 21:10

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