Akom's Tech Ruminations

Keep in mind: my vehicle (RAM truck with a V10) may not be exactly like yours, but the concepts should still apply.

Typical Inadequate Heat Output Causes

• The antifreeze simply isn't hot enough, at least not where it matters. In most cases you should have noticed this already just by looking at your engine temperature gauge. You do look at that gauge right? This is a huge category and includes such pearls of common sense as:
  1. Do you even have antifreeze in there? My problem was probably triggered by a 5% drop in level - and the stop-leak stop-leaked the entire heater core as it should with exposure to air. The first thing you should do is replace lost coolant, add to expansion tank and go through a few full hot/cold cycles. If it still doesn't get better, then check other stuff (And checking for leaks wouldn't hurt)
  2. Thermostat stuck open (replace thermostat). You'd probably be seeing very low engine temperatures unless you're idling or it's just really hot out
  3. Water pump (ouch). None or barely any flow? that's an issue.
  4. Cooling fan is hyperactive. This is a non-issue for me as mine is on the crank, but if an electric fan is stuck ON, you may feel that in the winter
  5. Flow obstruction in the plumbing. Now, this you may not notice. This could be gunk/stop-leak, collapsed or kinked hoses, etc. If the hoses leading to/from the heater core are obstructed, then you'll definitely have this issue
  6. Flow obstruction in the heater core itself: same as above, except more likely - the heater core has tiny passageways and enough extraneous fluff will fill it up.
  7. These two items are checked easily: Find the two hoses heading to/from your heater core (they run from the water pump or thermostat housing towards the firewall) and compare their temperatures. I used a handy laser (infrared) thermometer - 175F on one, 100F on the other - yup, that's a problem - should be at similar temperatures
  8. Air trapped in heater core. Apparently some cars have this issue - their heater core is a high point that collects air. People claim that running the engine with the car at a significant slant helps fix this, I haven't tried - I actually started with the heater core completely empty (blown out with 60psi air), and it worked beautifully.
  9. Heater Core valve is malfunctioning. (This is a small in-line valve on the heater hoses heading to the heater core) My truck doesn't have one, but it's a possibility. In the worst case you can just take it out and connect the two ends of the hose directly with a barb - if you get hot air, that's a problem. But usually you can see it operate - there is a usually a visible lever that's controlled by your dash control
• The heater core is hot, but you still don't have hot air.
  1. If you have a blend door (a mechanical door that selects hot or cold air), you should look into the mechanism that controls it, and see if it's getting power, if the doors are sealing well, etc
  2. Thankfully I didn't have to dig into mine as pretty much anything I'd want to do with it required removing the all-in-one unit under the dashboard, which means lots of coolant everywhere and lots of Freon in the atmosphere, not to mention recharging both systems
  3. Air intake could be obstructed, restricting airflow altogether
• Is the heat/AC fan even running?

In my case, it was the heater core, as I mentioned. The heater core hoses had a 70 degree discrepancy, and while there was a hint of warmth in the air, it basically was completely plugged up. I pulled both hoses off their connections close to the water pump, and used compressed air and hot water (faucet+garden hose) blasts in both directions until water stopped coming out tinted. I had about a half inch of red sludge in my bucket afterwards.

End result? My air vents are at a toasty 170F now.