No heat at idle

[cthomasparr]

I survived the winter with interior heat only about 2500 rpm or over. Would blow cold air at idle. I suspected a clogged heater core but noticed that both heater core hoses by the firewall seemed to be at the same warm temperature. I was hoping to do a reverse flush but I wonder if I might have another problem. In the event where I may do the flush which hose flows into and out of the heater core?

[jolt]

Air trapped in the heater core will do this also. A bad thermostat will make the engine drop temperature when it idles or at lower engine loads. Poor water pump circulation could produce lower heat in the heater core. Both heater core hoses at the same temp could be a blocked heater core, poor or no flow to the heater core because of a blocked hose or tube from the engine to the heater core, or the engine just is not hot enough at the time.

Air trapped in the heater core can be caused by a cracked cylinder head, a blown head gasket, or the cooling system was not purged of air when a coolant change was done. In the case of the cracked head or gasket problem, the exhaust gases leak into the cooling jacket and if the heater core is the highest point in the cooling system, the gases will displace the fluid in the heater core and you will get poor heating. Check the overflow tank on the radiator to make sure it is clean and the coolant color is right as exhaust gases will generally discolor the coolant over time into a brown color.

You could try taking both hoses off and hook it up to a garden hose and see if water will flow through the core in either direction. If the water flows good then you have another problem. If it is a plugged heater core, a shop would just replace it but you could use water and compressed air to blow it out. Fill with water and then quickly blow the water through with compressed air from a air compressor. The higher pressure from the air will have a better chance of forcing the water though the heater core but there is the danger that the higher air pressure will damage the heater core and cause it to leak. If the blockage is from corrosion, a back flush will do very little unless you use chemicals that would dissolve the corrosion. Once again there is a danger that the chemicals will eat the heater core along with the corrosion. You end up with a leak and once again you have to replace the heater core. These are some of the reasons why a shop would just replace the heater core but if you have more time then money, you can try to unblock it.

[cthomasparr]

Ok.makes sense. I don't suspect the thermostat or water pump... those have been replaced not very long ago. There are no apparent engine cooling issues. I could be losing coolant. The engine has 250k miles. I haven't noticed any smoke or strange oil at an oil change.

I will attempt to flush the heater core both directions and then top of fluid and keep watch on it.

[jolt]

These two Youtube videos on draining and bleeding the air out may help. The second video is just about getting the air out.

https://www.youtube.com/watch?v=R7WAO2_Ztk4

https://www.youtube.com/watch?v=zUpXgAJ1gjU

[kumquat]

I survived the winter with interior heat only about 2500 rpm or over. Would blow cold air at idle. I suspected a clogged heater core but noticed that both heater core hoses by the firewall seemed to be at the same warm temperature. I was hoping to do a reverse flush but I wonder if I might have another problem. In the event where I may do the flush which hose flows into and out of the heater core?

Sounds like a stuck thermostat to me. Air or another obstruction in the heater core would lead the hoses to be dramatically different temperatures, and air would likely work its way out of the heater core.

How quickly does your engine get up to operating temperature (~190)?

[cthomasparr]

Engine temp on the gauge heats up after few minutes of a short drive will be the normal 1/2 way mark even when there was no heat at idle.

[Chiadog]

On my 05 Base the upper hose connected to the heater core is coolant "IN".
How many miles on your Vibe, what year, when did this start, Has the problem gotten worse over time?
I had the identical problem on my 05 Chev Uplander mini-van. After much flushing, etc. the problem turned out to be electronic control module which the climate control knobs attach to. A crude test for this is to turn ignition on, engine off, in a very quiet place and see if you can hear the temperature flapper door actuate when you quickly spin the temp control knob from cold to hot and vise-versa, pausing to listen each time. I have another test if you need it.

[YotaGT]

Hmm, I too had this problem this winter.
03' GT ~140k

It wouldn't really blow hot air unless I was on the throttle, and once I returned to a light I'd get a breeze of cold air.
I was recommended to see if I was low on coolant, while that did help, heat was so-so.

Cold weather has passed,
but I started thinking about this again not long after a clutch change (was paying attention to my Tach more often).
I've heard that engine rpm can be affected by the coolant system, and a not enough coolant hitting the thermostat, or rather splashing up against it could cyclically trip the temp sensor?
Well I'm topped off on coolant, and I've found that while driving, holding the clutch in will show the revs fall but then bob cyclically.

I'm all set to replace the thermo if it's really this easy: https://www.youtube.com/watch?v=xBeH1i9t0QU

-Sam

If this is the case, I usually go OEM for these, what do you all think?

[joatmon]

I survived the winter with interior heat only about 2500 rpm or over. Would blow cold air at idle. I suspected a clogged heater core but noticed that both heater core hoses by the firewall seemed to be at the same warm temperature. I was hoping to do a reverse flush but I wonder if I might have another problem. In the event where I may do the flush which hose flows into and out of the heater core?

I remember at some point one member here was able to purge a stubborn air pocket out of the heater core by reversing the heater core hoses, where they connect at the firewall. They are the same diameter, and the core doesn't really care which is in or out.

I had the identical problem on my 05 Chev Uplander mini-van. After much flushing, etc. the problem turned out to be electronic control module which the climate control knobs attach to. A crude test for this is to turn ignition on, engine off, in a very quiet place and see if you can hear the temperature flapper door actuate when you quickly spin the temp control knob from cold to hot and vise-versa, pausing to listen each time. I have another test if you need it.

On the Vibe, at least the 03-08's, the temperature control pushes/pulls a mechanical wire cable that actuates a flapper door. There was a common issue in the early ones where the cable wasn't adjusted properly, and setting the temp control to the cold left the door still partially open, resulting in warmer AC than desired viewtopic.php?t=1004 Unless that cable was disconnected, I don't think it is likely to cause a problem with low heat

[cthomasparr]

I was able to add a quart or more of antifreeze to my radiator and my heat returned. I thought I filled the fluids months ago, but maybe not. I will now keep my eye on the level and if it continues to drop, investigate where it is going. Thanks.

[circuitsmith]

I would replace the radiator cap.
It has two valves that are supposed to eventually purge air out of the radiator as long as coolant is kept in the reservoir.
I recently replaced the thermostat and radiator cap for the second coolant change at 10 y.o.