Heat Momentarily to No Heat

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Joined
Mar 5, 2025
Threads
9
Messages
32
Location
Texas
Okay everyone, I’m lost. Been trying to figure out why I’m not getting any heat in the cruiser. Here’s what’s happening: if I have the AC on and turn it to full blast heat, I get perfectly hot air that comes through the vents for about 5 seconds and then it gets warm, and then it’s just cool air after 15-20 seconds or so. I can repeat this multiple times too, going from cold to heat and it’s consistently hot for a bit and then goes cool again.

After reading some threads on here I thought that my heater core may have been backed up but when I did a reverse flush it came out spotless. Roughly a cup or so of coolant came out and then nothing but perfectly clear water was flowing through. So unless I did it completely wrong (didn’t have water pressure up much at all because I didn’t want to cause any leaks in the heater core), that isn’t the issue. Blend door still activates when I turn the heat on, I can hear it hum and it goes from cold to hot momentarily. Heater valve is new and engages when turning heat on so that’s not the issue. Thermostat is also new as all that was replaced when I replaced all the heater hoses and did a rear heater delete. Brand new radiator, block and everything drained and flushed multiple times. Any ideas what could be the issue?
 
Not sure what your setup is, but it certainly sounds like you have an air pocket in the system some place. Let us know what year you have so we can focus on something more specific.
 
Can't help if you don't post vehicle details and recent repair history.
1996 1HDFT HDJ81. Heat was always this way, even before the cooling overhaul. Things done during cooling overhaul: brand new radiator, thermostat, radiator cap, new radiator hoses and heater hoses, rear heater delete, and ACSD delete.
 
Not sure what your setup is, but it certainly sounds like you have an air pocket in the system some place. Let us know what year you have so we can focus on something more specific.
1996 1HDFT HDJ81. I thought it could be an air pocket but the cooling overhaul was done months ago, figured any air pocket would be resolved by now with all the driving and heat being on full blast. Also, inlet hose to heater core is hot and then after a little bit, outlet heater hose gets hot too so it seems that hot coolant is getting to the heater core.
 
Have to ask, have you checked the coolant level and has it changed ie: drop, add, drop, add??
Coolant level has been stable the last 2 months. Did all the drop and adding when we originally flushed everything and overhauled the cooling system with new radiator and hoses etc.
 
1996 1HDFT HDJ81. I thought it could be an air pocket but the cooling overhaul was done months ago, figured any air pocket would be resolved by now with all the driving and heat being on full blast. Also, inlet hose to heater core is hot and then after a little bit, outlet heater hose gets hot too so it seems that hot coolant is getting to the heater core.
this sounds like an obstruction in the fins of your heater core.
 
Can't be the fins; may be the tubes but the fins only serve to dissapate heat in the air. There's no way that can be intermittent...unless it has legs. Which is a possibility.
 
sounds like the heater valve / cable is installed backwards. un hook the cable and manually open and close it, then see how the heat works
 
That would make sense, if the problem wasn't intermittent.
 
sounds like the heater valve / cable is installed backwards. un hook the cable and manually open and close it, then see how the heat works
Unfortunately this isn’t the issue, when the valve is fully open the cabin gets amazing heat for like 15-20 seconds before it starts to turn to cool air. Happens everytime. Even as soon as it goes cool, I can turn AC on and flip it to cold and then immediately go right back to heat with AC off and I’ll have the heat for 15-20 seconds before it starts getting cool again.
 
That would make sense, if the problem wasn't intermittent.
That’s why it’s confusing me. It’s not like I barely have heat or have heat here and there. I have amazing heat immediately when I turn heater on but then it slowly turns cool.
 
If the heater system lines are hooked up correctly, the core is not obstructed internally or externally (fins), the heater valve allows you flow coolant into the core to get it hot, but it wont stay hot.....sounds like there isn't enough coolant flow in the circuit "somewhere". Maybe the heater core IS plugged more than you think??

The A/C doesn't come on at the same time does it? I've had that happen on other vehicles. You said the blend door is working so maybe not. I'm not familiar with 80's.

When I mentioned the lines being correct, it's because I've seen heater hoses hooked to the same side of the system. Both hoses essentially on the water pump outlet side so the system filled but not enough pressure differential to get flow.
Obviously not on an 80 series Landcruiser. Just throwing stuff out there since it sounds like all the possible causes have been looked at.

Good luck with it!
 
Also want to add: with every back flush of a heater core I’ve seen, they’ve looked horrible. Red/brown tinged water. Mine only was coolant tinged for 5 seconds and then immediately was clear, not even the slightest red tinge to it.
If the heater system lines are hooked up correctly, the core is not obstructed internally or externally (fins), the heater valve allows you flow coolant into the core to get it hot, but it wont stay hot.....sounds like there isn't enough coolant flow in the circuit "somewhere". Maybe the heater core IS plugged more than you think??

The A/C doesn't come on at the same time does it? I've had that happen on other vehicles. You said the blend door is working so maybe not. I'm not familiar with 80's.

When I mentioned the lines being correct, it's because I've seen heater hoses hooked to the same side of the system. Both hoses essentially on the water pump outlet side so the system filled but not enough pressure differential to get flow.
Obviously not on an 80 series Landcruiser. Just throwing stuff out there since it sounds like all the possible causes have been looked at.

Good luck with it!
With you saying it could be plugged: when I did the back flush, coolant came out and then it was instant crystal clear water. I didn’t have any red/brown tinged water and gunk that everyone else has when they back flush their heater core. Mine was crystal clear water that came out the exact same as it went in. That’s what’s baffling me. The inlet and outlet hoses are connected in their appropriate locations.
 
Honestly, it sounds like the blend door isn't staying where it should be all the time.
 
I maintain my cooling systems, changing the coolant as required and use distilled water. I had a cooling problem with our 4Runner many years ago. I was a mechanic at the time and everything looked good. Radiator looked brand new looking in the filler neck. When I changed the T-stat the inside cooling passages looked spotless. No visible corrosion. The water pump was pumping, the belts were good, fan clutch, you name it, it looked good.

There were no IR guns or anything back then, but one day I bumped up to the radiator fins when it was running hot and it was not hot. It was cool in the center and smoking hot along the edges. I took it to the radiator shop our company used, they got it apart and all the tubes looked clean on top. Flipped it over and the majority of the tubes were visibly plugged looking at the core from the bottom. I forgot what the percentage was.

I asked how this could be as everything looks spotless and I change coolant like I should. I was told the iron block and aluminum castings still "shed" particles over time and any other debris that gets in builds up over time in the bottom of the block and clogs the radiator tubes.
My 52 Ford has a huge 4 row radiator and I've had it rodded out 3x since I've had it. Looks clean other wise. I've seen corrosion from improperly maintained systems and hard water deposits. This was not the case.

So my LONG WINDED story (sorry) to say, Don't assume the core is ok. It's a lot of work to pull it so verify the other possibilities first. Like the blend door since you can hear it, but can't see it.

Good luck with it. I'm looking forward to what you find.
 
Well if the wire attached to my heater valve was slipping it could do the same thing
 
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.
 
There is an excerpt from the service manual for just this problem, in the Resources forum, 80 series section.
 
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