1971 350/270 HP overheat problem - NCRS Discussion Boards

1971 350/270 HP overheat problem

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  • Rick Stecchi

    1971 350/270 HP overheat problem

    HELP... I cannot stop my car from spilling over. Everything is new!!! You name it it's new..... bored .020 with a very mild cam.... Everything is new cap, pump, alum rad etc....and has less than 200 mi on it.

    Gauge goes to 180 (accurate) Spills over until the 50/50 antifreeze gets low.

    What else can I check....

    Thanks
  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15678

    #2
    Re: 1971 350/270 HP overheat problem

    If the temp doesn't get over 180 and coolant escapes from the overflow, then either the pressure cap is bad or you are overfilling it. There must be some expansion volume, and your owners manual will tell you the correct level hot or cold.

    If you install a coolant recovery system as was used on later models, you can fill to the very top.

    Duke

    Comment

    • Patrick H.
      Beyond Control Poster
      • December 1, 1989
      • 11643

      #3
      Re: 1971 350/270 HP overheat problem

      It would also help to know if he has a small block with the coolant recovery tank, or a radiator without the recovery tank.

      If he does NOT have the coolant recovery tank, I'm betting a 95% chance he's overfilling it. It is VERY common. It even happened to me, and I thought I left plenty of room.

      Just keep driving it until it stops puking coolant every time. It will seek its own correct level. Realize too that if you have a replacement radiator (copper/brass replacing copper brass) then it does not have a mark on the side indicating the correct level, so you'll have to "guess" until you get it right. Or, let the car find its own level and mark it for future reference.

      The reproduction radiator cap I used at first didn't help either. A new RC-15 from GM solved part of it.

      The only time my 72 with the coolant recovery tank boiled over was 13 months ago at Windsor when the rubber gasket on the original cap broke.

      Patrick
      Vice-Chairman (West), Michigan Chapter NCRS
      71 "deer modified" coupe
      72 5-Star Bowtie / Duntov coupe. https://www.flickr.com/photos/124695...57649252735124
      2008 coupe
      Available stickers: Engine suffix code, exhaust tips & mufflers, shocks, AIR diverter valve broadcast code.

      Comment

      • Duke W.
        Beyond Control Poster
        • January 1, 1993
        • 15678

        #4
        Re: 1971 350/270 HP overheat problem

        Patrick - I don't think that year had a coolant recovery tank, but you know more about '71s than I do. The owner's manual, CSM, and AIM should have information on the proper level. Can you quote it?

        Also, it's important to distinguish between "expansion tank" and "recovery tank". An expansion tank is a pressurized vessel and is either remote in the case of alumininum radiators or integral with brass radiators, and if the system is not equipped with a recovery tank a suitable expansion volume must be maintained.

        A recovery tank is an unpressurized container, usually plastic, that the "overflow" hose routes to - instead of the overflow hose routing to the bottom of the car when a recovery tank is not part of the system.

        Systems with recovery tanks can be filled to the very top as a pressurized expansion volume is not required. As the hot coolant expands, it opens the pressure relieve valve and the excess coolant flows to the recovery tank. When the coolant cools, it contracts and draws coolant back through the cap's vacuum relief valve.

        Models that were not originally equipped with recovery tanks (They became standard on most GM models by the mid-seventies.) can be easily retrofited with one, and if a recovery tank is installed coolant can (and usually should in the case or OE reconvery tanks) be filled to the cap seat and to the "full cold" level on the recovery tank. The system will self purge air over (usually) several hot-cold cycles and the level in the recovery tank may initially go down, then it should stabilize once all the air is purged.

        Normal coolant level check is just inspecting the level in the recovery tank, and most are marked "full hot" and "full cold". This is why cars originally equipped with recovery tanks have small "ears" on the radiator cap, since removing the cap is not the normal way to check coolant level.

        As long as the level in the coolant recovery tank is consistent, the system is full, and they should usually be checked cold. Obviously the level will be higher when hot due to coolant expansion, and the hotter the coolant, the more will be pushed into the recovery tank, which is why cold checks are best to ensure consistency. If the level in the coolant recovery tank drops between cold checks, then the system is loosing coolant somewhere. Normally, coolant should not have to be added between coolant changes, and there should be no need to remove the radiator cap unless until the next coolant change.

        Duke

        Comment

        • Patrick H.
          Beyond Control Poster
          • December 1, 1989
          • 11643

          #5
          Re: 1971 350/270 HP overheat problem

          Duke,

          Yes, I should have used the term expansion tank. I think (?) that 1973 was the first year for a recovery tank as you describe, and a 71 or 72 will have an expansion tank for a small block w/o AC.

          Patrick
          Vice-Chairman (West), Michigan Chapter NCRS
          71 "deer modified" coupe
          72 5-Star Bowtie / Duntov coupe. https://www.flickr.com/photos/124695...57649252735124
          2008 coupe
          Available stickers: Engine suffix code, exhaust tips & mufflers, shocks, AIR diverter valve broadcast code.

          Comment

          • Rick Stecchi

            #6
            Re: 1971 350/270 HP overheat problem

            There is an expansion tank on it but it flows right out the tank overflow tube until the radiator gets low. Tried new 15lb & 16 lb rad caps as well. I am afraid it will overflow until there is to little antifreeze left causing major overheating.

            Engine has 200 miles on it..

            Thanks for the reply thus far......

            Rick

            Comment

            • Duke W.
              Beyond Control Poster
              • January 1, 1993
              • 15678

              #7
              Re: 1971 350/270 HP overheat problem

              So it has a Harrison "316" aluminum radiator with a remote aluminum expansion tank? Please confirm.

              Are you saying that coolant is expelled until the tank is empty?

              The coolant "full level" is when the expansion tank is half full when cold.

              Duke

              Comment

              • Jack H.
                Extremely Frequent Poster
                • April 1, 1990
                • 9906

                #8
                Clarification...

                In '71 the BASE engine car used a small Harrison aluminum radiator with companion expansion tank and metal fan shroud. When you added heat dissipating options to the base car (automatic trans and/or A/C), the radiator changed to a small Harrison brass radiator, no external expansion tank, and fiberglass fan shroud. This is a point of confusion among some unfamiliar with '71 SB specifics....

                On the problem the poster relates, radiator is puking coolant via the overflow hose although the coolant temp appears normal (180F), putting science to work vs. knee jerk guesstimates is the route to go. The symptoms described can result from several causes:

                (1) The cooling system has been overfilled and it's naturally relieving itself of excess coolant during thermal expansion.

                (2) The rad cap is faulty and failing to hold system pressure.

                (3) There are secondary leak(s) in the system causing the system not to hold pressure.

                (4) When the system was initially filled, air pockets were trapped (engine block, Etc.) and these are working their way out resulting in discrete 'burps'...

                Stant makes a Jim Dandy diagnostic pressure meter that most competent auto shops have in their tool crib. It's basically a hand pump with pressure gauge. There are adaptors for the pump allowing the mechanic to mount a suspect rad cap, pump up pressure and observe/verify the rad cap's release point vs. its specification. BTW, 'curd' particles sitting on the radiator's sealing lip can stick between the rubber gasket of the cap and generate extraneous air passageways....

                If the rad cap's check out, the mechanic removes the rad cap adaptor from the test pump and installs it directly on the radiator in lieu of the rad cap. Now, the engine can be started and the pressure profile of the cooling system observed/monitored as the engine comes up to temp.

                Trapped air pockets will trigger the pressure meter to 'bounce' as they work their way through the system. Secondary leaks (rad hose attachment, faulty clamps, same issues with the heater core, OR the dreaded pinched/blown head gasket allowing exhaust gas to penetrate the cooling system) will show the pressure meter steadily climbing to and beyond the system's rated critical point (15 psi).

                If a small secondary leak (hoses/clamps) is suspected, the mechanic can use the hand pump and artificially raise system pressure to force the leak inlet point to become an outlet and visually identify the leak source(s). If the worst is suspected (exhaust gas entering the cooling system), then it's time to move to another analysis system and inspect the gas content of a sample of coolant for the presense of CO2, Etc.

                Bottom line, there's no need to guess about what's happening and a decent, trained, mechanic should be able to troubleshoot and repair the minor problem sources within an hour's worth of shop time....

                Comment

                • Terry M.
                  Beyond Control Poster
                  • September 30, 1980
                  • 15601

                  #9
                  Re: Clarification...

                  Don't rule out the possibility of a compression leak into the cooling system considering the recent rebuild. There are test strips available which will test the coolant for combustion byproducts. I have also seen exhaust gas analyzers used to test the vapor from the cooling system for exhaust by-products.
                  As Jack suggest, a competent shop should have these kinds of materials at their disposal, and should know how to use them.
                  Terry

                  Comment

                  • Rick Stecchi

                    #10
                    Re: 1971 350/270 HP overheat problem

                    It ha a replacement Dewitt aluminum radiator and a new replacement expansion tank. The car can be driven for 40 minutes ore more and does not overheat. Once the car is turned off, within 5 to 7 minutes up to 3/4 of a gallon of antifreeze is spilled from the overflow tube on the expansion tank and the tank is empty. The coolant is no hotter than 180 degrees as it is spilling as I measured it with a thermometer.

                    Comment

                    • george romano

                      #11
                      Re: Clarification...

                      I just went through this very same problem with my 72 SB at in the weeks preceeding Carlisle. Every time I stopped the car, shortly afterwords it would "puke" coolant out the expansion tank overflow hose. I thought like you; let it seek its own level, and it will stop. It didn't. I drove it 300 miles to Carlisle without a problem. Just before I pulled in the gate I noticed the temp guage was reading about 210. I parked the car and let it cool. When I was leaving the event Friday night, the car reached 210 again before I got off the property. I pulled it over and removed the rad cap. It took almost 2 gallons of the emergency water I was carrying. I filled it with the car running.
                      Only thing I can think of was the coolant level was low to begin with, and was airating while the engine was running. The "puke" was a foamy green stuff, not a liquid. Since I filled the system up that night, it hasn't lost a drop of coolant, and that includes a 300 mile drive home. In the rain I might add.

                      The tank says to fill 1/2 when cold, but I would fill it while its hot and running. If you overfill it, let it "seek its own level" once again.
                      Like I said, my car hasn't lost a drop since.

                      George

                      Comment

                      • Duke W.
                        Beyond Control Poster
                        • January 1, 1993
                        • 15678

                        #12
                        Re: 1971 350/270 HP overheat problem

                        Take your radiator cap to a parts store and have it checked that it will hold 15 psi. Most parts stores have a pressure check tool.

                        Duke

                        Comment

                        • Rick Stecchi

                          #13
                          Re: 1971 350/270 HP overheat problem

                          I want to thank eveyone for their help.

                          I think we resolved the problem by drilling a 1/8 hole in the thermostat. Drove it twice yesterday for about 30 minutes each time after drilling the hole. The temp never went over 170 and it didn't puke after the ignition was turned off.

                          Thanks again for all the suggestions and help.

                          Comment

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