Hi All, well as of this past week, it is cool enough here in Washington to need the heater for the first time in my truck (1954 GMC 102-24 w/248). Unfortunately, I found I get a fogging on the windshield from the defrosters when the heat is turned on (no dripping on the floor) indicating a slight leak from the heater core. So I shut the heat off until I can fix the situation. Well...in doing research, I found a couple of postings stating that running higher-than-standard pressure in the cooling system can cause havoc with the heater cores/valves in our AD trucks. Lo and behold: my truck came with a 13# radiator cap on it from the previous owner. I looked on-line (Stant catalog), and I should have a 4# cap. So...my question for all of you is: any reason other than just getting it wrong that you can think of that the previous owner was running a 13# cap? Any reason not to revert to a 4# cap? Thanks - Roland.
Probably pure ignorance, and I don't mean that in a negative way. If it screws on, it must work, ya know? ETA if it does have some other problem that becomes apparent, it's something that needs attention, not a band-aid of a higher pound cap.
Update - happily now running with 4# cap! I think Redneck Packrat is correct! I got myself a 4# cap today and almost convinced myself I had bigger problems! When I took off the old 13# cap (cold), I noticed the radiator wasn't full - so I topped it off, filling it to the top of the fill neck. Then I put on the 4# cap. Idled for a bit, drove up the rode for about 1/2 mile and took a look. The overflow tube was spewing my new coolant out so I put the 13# cap back and verified it was no longer pumping coolant out the overflow. I drove about 5 miles and came home to review what could cause the pressure to be so high in the cooling system (and dreading a compression leak into a water jacket ). Went on-line and found the fifth line in this post by VWNate1 quite helpful!: "...I fill to maybe 1" above the tubes when hot , it's WAY below the filler neck as any excess gets puked out quickly ." Well, by now the system had warmed up and expelled the excess coolant during it's initial stint with the 4# cap and my swapping it out for the 13#-er. So I put the 4# cap back on, brought it back up to temperature and it worked!!! Just ran it at speed (50-55mph) for about 5 miles and let it idle a bit - no coolant coming out the overflow with the 4# cap. Truck running at normal temperature the whole time; top of radiator nice and warm (meaning it isn't empty!!). I suspect the previous owner filled the radiator to the top, and kept upping the cap pressure until it would hold the pressure in (which turns out to be something around 13#) without realizing he was risking the heater core/valve. Icing on the cake: my heater no longer fogs up the windows, in fact it is dry and warm!!!!
I'm so glad there was a quick, easy, and relatively painless (cost of a 4# cap) fix for the problem! Out of curiosity, I just checked the '50 I had trailered home last week and have not yet had a chance to do anything to. It has a 13 on it! Also has the heater hoses looped. Wonder if previous owner did the same thing: filled radiator to top, blew coolant, upped the pressure til it stopped barfing, then had to pinch off his heater core! Talk about manufacturing a problem THAT is a lot of the reason I'm reading all over this board before I even take up a wrench and start playing with the ol' gal.
Check out the warning against the heater core sold by this forum's sponsor: "Not for use with modern upgrades over 4# radiator pressure caps."
Hey Roland, and Bill. My GMC radiator has an overflow tube that terminates inside the radiator tank, therefore, no matter what radiator cap you use, it won't hold pressure unless you plug the overflow line... I put an overflow tank on mine, just so I can keep a closer watch on the level. Mounted it on the firewall just about level with the radiator top tank. Water flows in when warm, flows back to radiator when cool. Translucent tank lets me check level anytime the hood is open. (You know, when I'm at Home Depot and people come up and ask questions about my truck!)
