Heater blows hot when driving, blows cold when idling

[DoinkMobb]

I did a search and didn't really find anything pertaining to my particular problem.

The heater works, first of all. The air blows out hot, but when I come to a stop, the air blows cold. This isn't a huge problem really, especially since we only have 10 cold days a year in Tampa, but still annoying nonetheless.

Anybody got any ideas?

[Haitian_King]

I've had a weird thing going on like that too. Ordinarily, my heater blows hot (I'm talking "mucus membrane drying" hot), but lately, when it's on windshield, it's been blowing cooler. Nowhere near the 85 shown on the HVAC. It runs hot when it's on Face, but I don't like driving like that for long because it dries my contacts out.

[yodawill2000]

Coolant topped off ?

[Q45tech]

Thermostat partially in bypass [rubber seal ate up from lack of mainteance].

Consider that at idle the engine creating about 3-4 HP and 10-15 at a steady cruise.

Each HP is 12,000 BTU of heat and 1/3 is transfer to engine coolant.

Hard to diagnose without exact vent heat temperature which can reach 140F but is usually 40F hotter than temp setting [125F].

[Haitian_King]

I'll check my coolant levels.

The problem that DoinkMobb is having is what I've had in the summer. Sort of.

At acceleration, the A/C blew lukewarm ad at idle, it blew ice cod. Upon initial cooling start, the air coming from the vents wasn't efrigerated. If I switched the HVAC system to Windshield defroster and then back to Face, the air was bone-achingly cold.

It wasn't a major problem, but an annoyance that grew in its power.

[Don Giovannii]

Hey. I would suggest first checking coolant leve.

Next see if you have the ratio of water to coolant right

next make sure ur radiator isn't busted, mine was and was giving me a bunch of problems

next, might just be an old heater coil

[DoinkMobb]

I will check the coolant post haste...or after I move into my new house. I have a growing list of things I need to check/fix/replace on this car. After I get my house situated, I'll spend any and all free time on the car.

[AlabamaDan]

I had the same problem. I was low on coolant. Filled up on coolant and fixed everything.

[Soulpwr]

I'd be concerned as to why it was or is low if it turns out that way as well.

[AlabamaDan]

Mine was low because the filler neck cap had a bad seal.

[Jesda]

Its not unusual for some of the water to evaporate when the seasons change. I had to top off my Range Rover and determined this was the cause after I was unable to find a leak.

[RAP]

Hey. I would suggest first checking coolant leve.

Next see if you have the ratio of water to coolant right

next make sure ur radiator isn't busted, mine was and was giving me a bunch of problems

next, might just be an old heater coil

? Pray tell, how will the "ratio of water to coolant" affect anything with the cooling, or, heating system in a car?

Seems like that pure water is the "BEST" coolant. ? ? ?

Prove me wrong.

Jack

[Haitian_King]

I thought antifreeze made the best coolant because of it's ability to get quite hot without boiling off and its ability to resist freezing. Something that pure old Brita water can't do.

[Q45tech]

Don't confuse a liquids ability to move heat from one metal to another, it's thermal efficiency with its boiling point.

Almost nothing is better than pure water just that water has a nasty low boiling point and high freezing point.

Chemicals [so called antifreeze / coolant] are added to water to stretch it's operating limits at both extremes unfortunately these DECREASE it's heat transfer efficiency at the same time.

OEM system are designed for 50/50 mixtures.

Within antifreeze the PERCENTAGE and composition of anticorrosion additives should be unique for all aluminum vs iron or mixed engines.

Remember what is importantant is the temperature of engine [cylinder head, piston, rings, bearings] and not the coolant per se.............the coolant is just a reflection of that head temperature..............when coolant boils it can no longer transfer heat well because the molecules are farther apart.

[Haitian_King]

Good to know.

[DoinkMobb]

I topped off the coolant this morning, it was really really low. I turned on the heat when the car was sufficiently warmed up and...the heat was hot when driving and again cold when idling. So I dunno.

I did notice that on idle, the needle looks to be on the upper half of the 500 hashmark, does that sound low? Would that have any effect on the heater?

[maxnix]

Is the HVAC fan blowing when at idle?

[DoinkMobb]

Is the HVAC fan blowing when at idle?

Yes, it's still blowing, but just blows cold air. My fan only blows at the highest setting, but that shouldn't have anything to do with the lack of heat at idle though.

[qship96]

I dont understand how any owner allows coolant {or any other fluid} to be below specified level....especially an owner savy enough to be on this board?

Easy, quick, simple, smart, safe, and prudent to eyeball EVERY fluid in the q, except for differential , at every fuel stop, or once per week, whichever is sooner.I assumed every enthusiast automatically does this