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I was driving yesterday in sleet with an outside temp of about 32 degrees and the heater was not working real well (maybe because I have the cab that breathes?). The temp gauge was reading a full needle width below half which is about where it runs in the summer as well. I have not replaced or checked the thermostat since purchase, so I have no idea what temp it is or how old it is. I have replaced the heater, fan motor, and all hoses, but not the pump, and I have no problems with overheating in the summer. So my questions are; does anybody know what the approximate temps the marks on the gauge indicate, and how well does the heater work in your truck? And I have the standard recirculating fan heater type.

Thanks, Lee.
1956 with 235.

There's a quick way to check- - - -immerse the thermal bulb in a pot of boiling water for a "212 degree" reference, and/or use a cooking thermometer to check the temp in the pot before it get to the boiling point. I use a laser thermometer a lot when diagnosing overheating problems. They're cheap on Ebay, and pretty accurate. Just point the red dot at the top and bottom of the thermostat housing to get an idea what the opening temp is.
Jerry

It sounds like you have a 160* thermostat. When it gets cold, a 160 won't cut it. 180 is better. I see you're in Texas, where you can get 4 seasons in a day. If you live up in the Panhandle, I'd recommend going with a 180 or 195 for the wintertime, and switching back to 160 in the summer if the vehicle overheats. If it doesn't, just run a 180 all the time. And if you can fix the "breathing" cab, do that, too.

I live in the Central Valley and it can get chilly, definitely not as cold where I grew up, at the base of Mount Whitney, and it gets down right cold there. I discovered at a young age that a 160 doesn't do well in the cold.

When I got my first '65 C20, it was being put together by grandpa, but he passed away before he could finish it for me. He had put a loose 3/4" piece of plywood over the SM420. Being 17 when I got it, I didn't know the danger of the u-joint being right there. But it was unbearably hot in the summer, and not driveable in the winter. Eventually I got a center hump, but it lacked the flange that goes between the hump and the firewall. I fashioned some cardboard to fill the gap because I had limited supplies and no sheet metal, nor the knowledge needed for working with sheet metal. That helped, but the cab still "Breathed", and it was a miserable "B" in cold weather. I eventually found a flange and that fixed the "Breathing", if you will.

My current '65, came with a breathing cab, and I have used heavy duty aluminum duct tape to seal the holes until I can get in there and cover them with sheet metal.

Lee, I have a 59 with the standard heater,180 degree thermostat . The heat is nice a toasty, the factory gauge has never worked in the 12yrs. I've been driving on the street. Aftermarket gauge says 180, In summertime in traffic may get to 190. Factory radiator was recored 12yrs ago, engine 261.

Check the circulation in the system. With the engine warmed up & running at a fast idle, wrap your hand around the heater hoses in various places. The heat runs from the hose by the therm housing. If that is hot go to the return hose. when ever the hose is cooler on one side then the other it indicates a blockage at that point. The hoses should all be so hot you can not hold onto them for over 2-3 seconds.

George

I have no idea how much your cab "breaths" but I have 160s in both my 38 and 53 and the heaters in both will run you out. Stick your hand in 160 degree water and see how long you hold it there. My 38 also breaths well. Assuming your blower fan is working normally, if you want it warmer without overheating in the summer do what we used to do when I was a youngster and my dad before me, put a piece of cardboard in front of the radiator to block part of it off. Remove it for summer driving.

The thermostat could be stuck open but if your truck gets up to temperature you should get heat .Like George said check to see if the coolant is circulating through the heater core by holding onto the heater hose were it goes into the core on the firewall and were it comes out .Some trucks I have seen have a shut off valve ,maybe check that .If the hose is hot going in and cold coming out you could have a plugged heater core .A small difference in the two hose temperatures is okay as the coolant will be a little cooler the way back especially if the fan is blowing air through the heater core.The only other thing I can think of is a air lock or low coolant .

Thanks to all for the information and suggestions. When my trusty assistant gets back, I’ll do the boiling pot of water and stick the temp sender in so I can figure out where my temp gauge reads. Next time I drive it, I’ll see how warm the heater hoses are to the touch. The PO put in 2 shut off valves in the lines going to the core, so it will be a good test to see if there is a difference in temp on either side of the valves. Maybe there is a blockage there. I did forget to open them when I left the house yesterday (this was our first cold snap) and it took me a few minutes to figure out why I was getting no heat!
Tommy; I was in NO this past weekend on business; sorry you have to go through another hurricane.

Lee.

I had to go to town again this morning so I checked some of the items suggested. The truck warmed up quickly; in about 2 miles the indicator was halfway between the bottom mark and the middle (the outside temp was 33). I turned the fan on and the temp was ‘meh’ coming out the bottom, about what it usually is. When I got home I left it running and put my hands on the heater hoses and they were both very warm on both sides of the shut off valves I have installed. They were cool enough though that I could leave my hands on them. After I shut off the engine, I carefully removed the radiator cap and the water level was about a half inch down. Just a little higher than when cold.
So for now, I think my issue is a thermostat that is either opening early or it is maybe a 160 variant. I’ll get me a 180 and some gaskets and swap it out.
Thanks for all the ideas.
Lee.

"So for now, I think my issue is a thermostat that is either opening early or it is maybe a 160 variant. I’ll get me a 180 and some gaskets and swap it out."

A 160 degree thermostat should allow/produce a warm-up to around 180 degrees.

157-163 degrees rated thermostat - levelling out (fully open) at 183 degrees. [chevy.oldcarmanualproject.com]