Heat Momentarily to No Heat

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TheBussman said:
heavy contaminants are difficult or impossible to flush out if they are present. I have seen casting sand restrict a core and have a little less output then it should. even after flushing i cut it in half and found the sand inside about half of the core was restricted. All of the flushing water on this car looked crystal clear the whole time. This particular case was not on a cruiser but its still possible.
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I’m thinking maybe I should do a CLR back flush on the core to see?
 
Do you have a thermal couple you can put in the heater core to see if the core is actually changing temperature and not your heater controls going bad and moving blend doors when it shouldn't?

If you have something like a section of hose that is in the core that could get pushed against the outlet and block passage until you shut off the flow and jt can fall out of the way...until you turn it on again to have it happen again. Might be easier to test for that by starting with the heater valve mostly closed and see at what point the flow is enough to pick up the blockage. If you find your heat is consistent at half open...that might support the theory.
 
My symptoms were different. I had constant heat except when the slider was all the way left.
Just to the left of the temperature set slider is a small vent, a couple of slots. The cabin temperature sensor is behind this. If the sensor flow is blocked it may "think" the cabin is already warm and switch to outside air. You can first try the easy thing, blow it out gently with compressed air. This did not work for me but I saw this somewhere on MUD. I had to take the control panel off and clean the slider contacts with contact cleaner. This action fixed mine.
I have not found the previous post that helped me. It was more than 5 years ago.
 
clx16 said:
Do you have a thermal couple you can put in the heater core to see if the core is actually changing temperature and not your heater controls going bad and moving blend doors when it shouldn't?

If you have something like a section of hose that is in the core that could get pushed against the outlet and block passage until you shut off the flow and jt can fall out of the way...until you turn it on again to have it happen again. Might be easier to test for that by starting with the heater valve mostly closed and see at what point the flow is enough to pick up the blockage. If you find your heat is consistent at half open...that might support the theory.
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IMG_7841.webp

Some updates for everyone: If I have the temp slider all the way to the left on cold and slowly move it to where it is in the picture, I get barely warm air but it maintains it. As soon as I hit the middle 25, it goes back to the same temp as if I had it over all the way to the left. But if I have it all the way to the left and rapidly slide it all the way to max heat, I get about 5-15 seconds of perfect heat then it goes to cold. Maybe I need to try to back flush the core with CLR like I’ve seen some do on here.
 
doc_random said:
My symptoms were different. I had constant heat except when the slider was all the way left.
Just to the left of the temperature set slider is a small vent, a couple of slots. The cabin temperature sensor is behind this. If the sensor flow is blocked it may "think" the cabin is already warm and switch to outside air. You can first try the easy thing, blow it out gently with compressed air. This did not work for me but I saw this somewhere on MUD. I had to take the control panel off and clean the slider contacts with contact cleaner. This action fixed mine.
I have not found the previous post that helped me. It was more than 5 years ago.
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wouldn’t this just have an effect if you have the auto activated? Rather than manually changing the controls?
 
Maybe check your control assembly as per the service manual excerpt in the Resources forum. It sounds more like an electrical problem to me.

If you really think flushing theheater core will solve the problem, I'd recommend using Thermocure instead of CLR. I've never seen CLR clean anything that wasn't already mostly clean.
 
verseddreams said:
View attachment 4045555
Some updates for everyone: If I have the temp slider all the way to the left on cold and slowly move it to where it is in the picture, I get barely warm air but it maintains it. As soon as I hit the middle 25, it goes back to the same temp as if I had it over all the way to the left. But if I have it all the way to the left and rapidly slide it all the way to max heat, I get about 5-15 seconds of perfect heat then it goes to cold. Maybe I need to try to back flush the core with CLR like I’ve seen some do on here.
Click to expand...
That sounds like a control issue. For argument's sake, what if you removed the cable from the valve and physically set the valve all the way open?
 
I wish the 80 dash peeled away that easily
 
something to mention... you might pull the controls out and flip the switch on the back to disable he Auto function completely and try that. Also when you pull it out you will find that there is a little white tube that attaches to that grated area on the bottom left... that i believe is a thermostat and you should have another one that is external up front behind the stock bumper. always a chance you have something messing with those.

also, at least on the US LHD versions there is a a/c controller box that is under the dash that if it is not all the way plugged it, it will mess with things.. Mine had the previous owner shove an air freshener next to it and the plug was partly off... took me a while to figure out that issue.

anyway i think from all you've told me that you are likely dealing with a control issue not actually an issue with the coolant.

I am happy to be wrong but if you can't put a thermal couple in the core and manually set your heater to on by disconnecting the cable. then it is probably time to rip in to the dash and check out the controls
 




Just noticed your RHD maybe you have a LHD valve installed? they are different.
 
My 80 was doing the same thing. Coolant level was staying steady, cooling system was bled yet the heat output was intermittent. However i kept getting air in the system even after repeated bleeding. Then one day the rig developed a misfire and coolant level had dropped. Turns out the head gasket was leaking the whole time. You might want to look there. How many miles are on your rig? Mine head gasket went at 283,000 miles.
 
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