The Stovebolt Site Forums The Shop Area Engine Shop Heater Hose Cutoff Valve

So, I installed a cutoff valve on the top hose running between the heater core and the thermostat housing. When it is open, my temperature gauge indicates that the temp rises as the engine warms up, and stops at 180° when the thermostat opens. When the valve is closed, the temp rises to a bit over 200° before the thermostat opens, and the gauge quickly drops down to 180°. So my question is, would moving the valve to cutoff the hose between the core and the water pump be any different? Also, to anyone who has installed a cutoff, do you notice this same thing and is it something you just live with during the summer when the valve is closed? My truck is a '51 3600, and my particular heater will not allow a Ranko valve.

I have never had that happen with mine. I have the same setup. If you install a thermostat which comes with the hole in it, or drill a small hole in yours, I'll bet your problem goes away. You might have a sketchy thermostat which opens all at once rather than gradually.

I put my valve on the push side. That helps keep pressure off of the core. I've never noticed any difference at all in warm up with the valve open or closed.

Good point about the pressure on the core. I put my cutoff on the top hose just because it was easier to get to that way, but I believe I will swap that to the push side and see if that makes a difference. I have a jiggle valve in my thermostat, which replaced one just like it because I wanted to be certain I didn't have a faulty one. There has to be some logical explanation for why the gauge goes to 200 or sometimes higher before that thermostat opens, when the cutoff is closed. If it is open, the temp never goes above 180. Thank you for the responses, very much appreciated.

When the valve and thermostat are closed then no coolant is moving at all. It is basically a plug of coolant in the engine behind the thermostat. You need to wait for convection to heat the water directly behind the thermostat before it opens. The gauge is measuring the temp at the back of the head and it is clearly getting hotter faster than the coolant directly behind the thermostat (not surprising).

When the value is open a small amount of coolant is circulated (since the heater hoses bypass the thermostat) and that eliminates the "plug" of water and allows the heated coolant to circulate behind the thermostat. One of the heater ports is generally right behind the thermostat for just this reason.

When eliminating the heater core I never block the ports, but rather put a small loop of hose between the two ports to allow the thermostat to open properly.

I am not for or against hose loops or cut off valves, but most (if not all) ADs did not come with a heater from the factory. They did not come with a hose loop. They plugged the holes. They worked just fine.

If you look at the photo I posted (click the thumbnail) you can see how I mounted mine. The push side is the top hose. The pull side is the bottom heater hose and the bottom radiator hose.

It will work just fine as is. I was only pointing out why the temperature appears to spike with the valve closed. I have never owned a truck with the fancy heater bypass. The only ones I have worked on in other vehicles were three way valves. This allows the coolant to flow even when the heater core was bypassed.

The reason I prefer a loop is that if the thermostat sticks closed and there is no loop the temperature will rise and you will be used to it rising. By the time you notice something isn't right there is a good chance damage has been done. On the other hand, if you are used to the temperature always stopping when the thermostat opens you will be more likely to notice if it continues to rise.

A related point- I have a short jumper hose going from the thermo housing to the water pump, as my heater is not hooked up yet. I have noticed that the engine runs a little warm with that jumper hose in place (as opposed to blocking off the heater hose outlet and inlet).

I surmise that the reason it is running warmer is because the hot water coming up to the top of the block is being directed back into the water pump- bypassing the radiator, which causes the engine to run warmer. Considering the size of the opening in the thermostat and the size of the opening for the heater hose connections, I would say that the quantity of hot water being recirculated into the block, relative to the amount of water flowing through the thermostat, is substantial... Perhaps something to consider when doing your initial start up with no heater connected?

Curious what others think about this, as I have never really considered it before.

I put a valve in the lower heater hose right at the thermostat housing and have never seen any difference in warm up or running temp, open or closed

After some careful reading of these great responses, it comes to light that I have run the hose at the thermostat to the bottom outlet of the core, and the hose from the water pump to the top inlet to the core. Please carefully read this and comment, but what I plan to do is leave the cutoff valve in place at the thermostat, and run that hose to the top inlet at the core, and run the hose from the bottom outlet of the core to the water pump. Any comments appreciated.

That's the way I'd do it...hot in the bottom and cold out the top. Location of the valve really doesn't matter, as soon as it's closed the loop is deadheaded and all flow is stopped and no heat will be transferred.

Mike B

The hot is coming out of the thermostat. The fitting by the water pump is pulling "cooled" water from the radiator and heater core (if it's connected that way). IMHO it really makes no difference which heater hose you connect where as long as the shut off is on the push side (the thermostat housing). I have mine plumbed the way you describe, upper to upper and lower to lower just for the sake of simplicity.