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1974 L18 620 Heater Core just warm - no hot hoses anywhere

Cardinal Grammeter

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Sounds like stuck open thermostat EXCEPT bottom radiator hose is COLD.  (ambient temp is upper 20's F)


180* stat, temp gauge runs at about 25% of "operating range band."


NOTE: FYI, I don't think this is a factor BUT:  System has a jar of original Bars Leaks (brown pellets in brown liquid) in it (which saved my bacon, 100 miles from home, heater core hose pops pinhole, hot water blasts big jet of water that splatters onto the right kick panel.  Massive leak.  But the BL stopped it in about 2-3 seconds.  Released pressure cap, continued trip, didn't replace hose until 2 days later.   Why was there BL in the system?   Extremely slow loss of coolant, can see seepage from rear of head gasket - not enough to drip on floor - that area of engine has a region that just "stays wet."


It sounds like engine simply isn't making enough heat.  (If stat was stuck open, bottom hose would be warm like all the other hoses - but it is cold.)


I have another car that has 70 HP and it has a toasty warm heater and hot hoses in the same weather.


. :confused:

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Take the heater hoses off and flush that bar's leak out in both directions. I will never use that shit for this very reason. Flush in both directions until only clear water comes out.


Now replace the thermostat to be sure.




It's a truck so it has an over size cooling system for full throttle maximum effort load hauling is over 100F heat. In cold weather it would be normal to drive to work and have the rad water cold, I've done this before and the bottom hose was cold!. Hell the heater (a rad itself) being on was all it needed to remain cold.

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Yes, good point about being truck.   


Just thinking.... heater core is so small, maybe bypass control valve and leave full on 24/7.


I was thinking too, that control valve could be partially restricting.


We'll see how eager I am to play with coolant in the winter (!)



There was a non-620 thread where they said, when open, manually tug on valve to make more open - it will open another 1/16" of so - makes a difference.  I also think they said they had to do this each time they wanted to use the heater.  Anyhow, tried it (very easy to slip cable off and hand cycle) and it may have helped - definitely with truck just parked and idling - I opened the door and heater blew a warm plume of air out. 


But then stopped engine, let sit, started and drove 20 min and not that toasty warm air blowing from he heater box.  Perhaps have to cycle the valve literally each time like I think they said.

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The valve isn't a by-pass. it's is open for heat or shut for off.


Heater core is likely plugged with bar's leak. I did the reverse flush and the water was coffee/milk colored with what looked like dozens of dead flies floating in it. That was the stop leak pellets. Nasty shit. Flushed both directions 3 or 4 times with a garden hoes and the heater set to full hot. (valve open) Fixed the problem.

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Could possible be core and or valve, but I did go through one winter w/o stop leak and both were checked/cleaned when I assembled the heater box.  Very little heat output, but I didn't focus on it because the Vent door on the box had bad seal foam and was leaking enough cold air to neutralize what heat was coming out of the box.  This year I redid the foam seal and was expecting more than I got now.


The stop leak went in maybe 3 months ago.


But still, there are no hot hoses on engine in 25-30* weather.  I mean nothing even close to 130* which is point where it's too hot.  Perhaps in sub 30* weather, I need to block off some of the radiator?


For next week or so, I'm going to aggressively manually cycle the valve and see if there is a difference.

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Look at the valve while you work the heater control from cold towards hot and you can see the lever moving. Make sure it's fully open. The cable may be off or too much slack and not moving the valve enough.


If still no heat, reverse flush it several times so you know it's open and clear. If water won't pass through the core the valve isn't open or the core is obstructed. It's the only way to know for sure.


If still no heat replace the thermostat. If you have a 185f thermostat it will regulate the temp to near that even if you are at below 30f outside. That's it's job. The rad may not warm up but the heater hose comes directly from the side of the head and it should be hot. 

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  • 2 weeks later...

In the immortal words of DatzenMike, "I will never use that shit" regarding old school Bar's Leak.


Finally got into sealing core and flushed it.  Wow, what crud and what was interesting is that the first direction I flushed, water was clear.  But the second direction, major brown with rat shit sized lumps.


Kept flushing until ran hot water tank dry.  Would be clear, and then another tube would free up and I'd get a slug of more brown shit.


NOTE:  But the stuff does work, it literally plugged a pinhole blowout in the 90* bend core OEM hose to valve when I was almost 100 miles from home.  So that's the end for the Bars Leak.  But if I ever have to use something, it will be the aluminum dust shit.


I'm going to pull the valve and flush, drain block (and radiator) and fill with fresh antifreeze.  The stat is working.  Hopefully there won't be too much Bars in the radiator.


NOTE:  I was flushing core with 150* hot water and the core gets too hot to touch (obviously).  It was nowhere near that before this, so I am optimistic that I will have heat.  Especially with all core sealed around the perimeter.  (Actually, the front panel has a good foam seal.  But there is nothing on the sides or front.  There should be a lot of leakage around the front and there is a lot coming from the RHS slot for the top core tube.




Running 150* hot water each way and all clear.  All done?  Nope!


Discovered if you completely empty the core, you get more crud out of it.  Then, when you flush it again, you get another slug of brown crud.  You have to empty the core between each flush to get that additional brown slug-worth.  SlugWorth....???  Naaaa..... couldn't be....




When you rattle the core, you get more crud.  I'm guessing this is now the 40 years of sediment that was in the core.  I've ultrasonically cleaned it, an LA Awesome caustic flush and it still keeps producing more and more sediment.


Guess I'll just keep at it.


EDIT3:  HaHa It Get's Hot Now!


When I started flushing, I was using 150* water.  So I'm flushing the core - back and forth - ran out the hot water tank.  Now, after ultrasonic cleaning, and rattling the sediment out, if I run 150* water into it, in 30 seconds, it is impossible to handle the core because it is so hot!  I've definitely made a drastic improvement.  I'm going to ultrasonic clean with white vinegar for maybe 1 hour - or enough to see a little brass shine on the inside.

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RE:  getting sludge out of block:  I'd knock the freeze plugs out and hit it with a 13HP 4000 psi pressure washer.  You would be surprised how clean that will make things.




I had it out today in 25* weather and although I put some thick pile house carpet "mats" on my bare floor boards - it actually got too hot in he cab!  Have some weather in the teens coming and I was working to get it done in time.  Also modded a new windshield washer pump in too, so the washers are 110%.


There is no doubt, that it's a scale problem.  I am lucky that I kept working on the core after it was flushing clear.


On way to know if the core is OK is as soon as you fill it with hot water, it will be too hot to handle.  If you can handle it, it's got scale.


Also note that the core is split into two circuits:  one pipe flows through half into he tank opposite the pipes, then from that tank, back through half the core to the other pipe.  If there is crud in tank w/o pipes, that will be a tough one.


Oh, and also seal up the gaps around the core with adhesive backed low density foam tape from M-D (they have the softest.)  You will eliminate a cold air leak along the top pipe slot in the heater box and also air leaking around the core and not being forced through it.


My feet get warmed up real good if you can imagine that!  And this is with a 180* stat.


It could also be true that ultrasonic cleaning the core was the magic bullet.  I was able to get just over half he core in my dental class cleaner.  This produced cloudy water when I poured it out.

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This heater core is Killer. 


Was driving around this evening in rain where conditions were requiring big time defrosting to keep windows from fogging.  Heater kept entire cab windows fogless AND that was with heat control throttled back.


I just can't get over how effective it now is.

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