IamNobody

I picked up my son from school a few evenings ago and he got in the car and said he was cold. It has been pretty cold the last week or two (which doesn't happen to often here in So Cal) so I turned on the heater. Whenever I turn on the A/C or heater I always put my hand by a vent to make sure the air is coming out at the proper temperature. When I did this the air was nice and hot like it should be. Then I came to a red light and while idling at the light I put my hand by the vent again but this time the air coming out was now cool. I thought WTH, it was just blowing hot air just a minute ago why is the air now cool. Now I'm concerned so I keep my hand by the vent. The light turns green and I take off and as I'm accelerating I feel the air get hot again. I stop at the next red light and notice that the air was again cooling down. Again the light turns green and I take off and once again feel the air get hot again. So I'm thinking is this related to engine RPMs and why would the RPMs effect the heater. We get home and I park and then say to my son to hold a minute I want to test something. I check the air temp and again it is cool. So I bring the RPMs up (trans is in park) and again the air heats up.

The only thing I can think of is that the heater core or one of the lines is partially clogged and when the RPMs come up the pressure in the cooling system also increases which means higher water pressure which in turn forces more water through the clog and then more water through the heater core. When the RPMs go down at idle the water pressure decreases and so the clog is able to restrict water flow which in turn causes not enough water to the heater core causing the air to cool down.

Does this sound possible? Has anyone else ever have this problem? In all the cars I've owned in my 35 years of driving I've never had this happen...either the heater worked or it didn't.

thuxel

Check you radiator fluid level. I had a problem when at a light my AC would get warm and the heater wouldn't heat. I checked my radiator fluid and it was low. I added some antifreeze and it fixed the issue.

IamNobody

Yup that was the problem. It's now working like it should. Thanks.

Nuke

Another satisfied customer/member!

JimC

Coolant doesn't just evaporate or disappear. It is leaking somewhere so you need to figure that out now.

My first place to look is the thermostat housing. Take a look at the back side particularly because that is where it usually splits. Look for any wet areas or evidence of leaking - it can look like a bad water pump but 99% of the time it is the housing. Look under the intake manifold and you will probably see coolant puddled in the valley.

IamNobody

I thought the same thing but I cannot find any leaks. No puddles under car, engine and engine bay nice and clean and dry and same for thermostat housing. It wasn't bone dry. The plastic reserve tank was filled about a third of the way to the cold level line. I just bought the car last year and never checked it (I know..stupid me ) so maybe it has always been low. Haven't needed to use the heater until recently and the engine temp has always been good so just never thought about it till now. I will keep an eye on it and if I notice it go low again I will definitely look deeper into it.

Also coolant does not evaporate but water does which normally you have a 50/50 mix. So being here in So Cal where it gets up to 108 degrees in the summer...well......

IvanRad

exactly how can people say coolant doesnt evaporate in an engine that gets almost 200-250F on a hot day.

IamNobody

I guess people who live in colder parts of the country are more use to water freezing rather then evaporating.

kput

I'm having the same issue. I added coolant but it's still doing it. So next step is to make sure there is no air in the system.

2005Redfire6

My heat hasn't worked in months.... Coolant level is OK, just need to check for air I guess.