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Problem with my heater

I’m having a problem with my truck’s heater. It puts out hot air when the temp is above (roughly) 20 degrees outside. When it gets very cold (right now, it’s -5), the heater blows only slightly warm air. This happened last winter, however, it was only really cold for about a week. When the temp hung around 20-25 at the coldest, I didn’t have any problems and forgot to fix it. Any help is appreciated.

What year, how many miles? When was the last time the cooling system was checked? When was the last time the coolant was flushed and filled? With a new thermostat and radiator cap?

The symptoms you describe are typical for a thermostat that is failed open. The coolant on cold days gets cooled too much from the radiator, and keeps the engine too cool as well. A new thermostat will fix this. Couple that with a coolant flush-n-fill and a new radiator cap should keep it fixed.

1999 - 84K. I’ve had it since 2006. I’m not sure when the last time the cooling system was checked / last time the coolant was flushed. If was filled last winter. I think the thermostat is the original.

I’ll replace the thermostat and flush/fill the coolant and see how it goes - thanks for the help.

Also, my “check engine” light came on the other day. I took it to Autozone and they hooked it up to their handheld. It said that plugs 1,2 and 3 were misfiring (all 3 on the passenger side). I haven’t noticed any power issues / backfires / chugging, etc. Do you think this could be a serious issue? The plugs were replaced in June 2009.

Thanks

[i] my "check engine" light came on the other day. I took it to Autozone and they hooked it up to their handheld. It said that plugs 1,2 and 3 were misfiring (all 3 on the passenger side).[/i]

I would consider new wires for the ignition. A set of OEM wires (not the fat colorful fancy ones for the hot rods) would be a good idea. Wires have a limited live and 100,000 miles range is certainly not a bad time to replace them.

I don’t know the B4000 but some cars have some common parts for each side, that might be worth checking. For example if they each have a coil shared on that side, then that coil would be suspect.

The super cold may have an effect on the aging vacuum operated water valve you’ll find in the heater hoses near the firewall.