Heater won't stay hot

[shane74]

Ok, I must be doing something wrong because my heater won't stay hot unless I'm on the gas. It doesn't have to be heavy throttle, but definitely a few hundred rpm's over idle. I have replaced the t-stat and heater cores (front and rear heat in my burb). It still won't stay hot. Also, the water pump is not leaking, so I don't think I'm low on coolant. I'll check it tonight when I get home, but does anyone have any ideas? I really need to defrost my windows so I can see out of them to drive

 

[Can Can]

I would suggest that you might have some air stuck in the cooling system. Pop the rad cap off when the truck is cold and run it until the thermostat opens. Keep the engine running for a while and watch to see if any big air bubbles start purging out of the rad neck........

That's where I would start. Other than a thermostat or a plugged heater core I can't think of anything else off the top of my head.

 

[bnc04]

check this thread out as well.

http://coloradok5.com/forums/showthread.php?t=125158&highlight=heater

 

[POWERMAD]

Is overheating at an idle?
What's the idle set at?
Belts tight and in good condition?
How are the heater hoses routed, in good condition?
Are the radiator tubes in good condition and not half plugged with crud?
When was the last time the cooling system was flushed out?
I would start with that.
Then you can then check the heater hose fittings in the block for restrictions and check flow when flushing the block.

 

[bigbadchev84]

i would try putting a piece of cardboard in front of the radiator, maybe it is just to cold outside for your radiator to get very warm, around here we have to block off most of the airflow to the radiator so that my heater will warm up

 

[shane74]

Canmore -
I plan on trying to purge the air when I get home.

Ron -
No overheating at all. It heats up to operating temp and stays there.
Not sure on the idle. I don't have a tach, but I haven't noticed a change.
The belts are new and tight.
The radiator has good flow through it.
The hoses are routed in the factory locations and seem to be "clean". I am going to couple the hoses together and flush the system tonight. I don't want to flush old crap into new cores.
I am also going to replace the water pump. I can get a new one for $25 and it's less than an hour to change it out. Then I'll have all new components in the cooling system. If that doesn't work, then I might have to block off part of the radiator during the cold snap we're having.

 

[dyeager535]

Shane, my heater wasn't struggling a bit in November up in the mountains when it was into the teens.

I'd really steer clear of the water pump. I've never been a fan of replacing parts that may not be at fault, regardless of cost, and with a typical 100,000 mile life span...

Anyways, if the engine isn't overheating, then that should tell us your pump is working. It has one impeller inside. Either it slips on its shaft, shaft pulls out, or it works. (or the blades rust off apparently heh)

Other cooling system issues would be plugged lines or cores, if there is a valve inline with the heater core coolant lines that is not working right,(never seen these on trucks, but who knows?) or for some reason your heater blend doors aren't working right. Are they vacuum? I'm not up on how the AC blend system works, (besides on vacuum) but depending on how it is setup, if it had a vacuum leak, I would think the blend door movement would be affected, reverting back to whatever is "normal" position. I know on my '83 it leaked because of the vacuum setup on the controls. Nothing like what you are experiencing though.

How do the cooling lines feel when it is idling? Does one get cold as it idles?

If one line is cold and the other is hot, that should tell you the coolant isn't circulating through the core(s) at least.

 

[diesel4me]

It might be--

We had a T-bird at my friends shop recently that had a factory short block installed by ford when the head gaskets went 6 months ago(factory warranty,the 3.8 V6)--anyway,the owner complained of little or no heat at slow speeds,and the engine would climb into the red zone on the temp gauge on the highway--
After checking all the usual stuff,like radiator and heater hoses with collapsed inner liners,bad thermostat,clogged radiator,heater valve,clogged heater core,we finally gave up and blamed the water pump--it seemed highly unlikely that it would be bad with 58 thousand miles on it,and we assumed the dealer must have put a new or rebuilt one on a new short block---WRONG!!--we took it off,and the impeller had only 2 rusted blades on it!!--the rest were floating in the coolant in the form of rusty flakes!--granted,I havent seen this very often on a chevy small block pump--but anything is possible I guess..

Its possible that if its REAL cold out, the motor wont get up to temparature at idle--as another poster stated,sometimes you have to block off the radiator some with cardboard---a fan clutch stuck in the locked position might overcool the engine at low temparatures too--but if the thermostat is working its not too likely...I have the same problem with my diesel--it seems to run cold at idle too,but so did my VW diesel--just the nature of the beast I guess...

 

[dyeager535]

If the engine itself isn't overheating at idle or under load, still don't see how the water pump is the culprit here.

As you said, anything is possible though, and if the coolant system wasn't taken care of previously, it could happen I guess. Just don't see the pump moving enough water to cool the engine though if the impeller is shot, and his IS cooling ok.

 

[dontoe]

I've had the same problem a few times over many years of driving Chevys. Couple of times the water pump was the culprit, couple of times it was only low on water.

 

[shane74]

Well, I got it fixed. It turns out I was a gallon low on coolant. I thought I had it topped off, but I guess I didn't let the eng. run long enough to open the t-stat. I topped it off and everything is good to go now. I also couldn't understand how the water pump or t-stat could be bad if it never over heated or ran cold all the time. Well, I guess I learned a valuable lesson..ALWAYS check the easy stuff first!

Thanks for all the ideas though guys. I really appreciate it.