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1974 620 pick up all stock is over heating and the coolant disappears. no smoke outta the tail and it only happens in warmer weather. ive notice in the am i can drive it for an hour and no problems but in the day time when its warmer within 20 minutes im stopping to top coolant off. once there was coolant on the passengers floor idk from where. ive checked all the connections and they look good. another thing actually the heater doesnt blow hot either. wth?


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Sounds like your heater core or a related component is dying. Have you noticed any coolant on the ground around the truck or just on the floor? 

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 Find the rad over flow tube under the rad cap and run it into a plastic water bottle. See how much comes out, if any. Constant over flowing is probably compression entering the cooling system from a bad head gasket. The pressure exceeds the rad cap rating and it burps out. First try a new cap. You might even get the cap from a newer 620 that has a coolant recovery system. This rad cap allows fluid out into a container, but when the engine  down, it siphons back in keeping the rad topped up. This is not a fix for a bad head gasket but notmally you only need to add coolant top up once a year.


Over heats because the rad is low, or runs too hot all the time? Running too hot all the time could be the thermostat. If any doubt, it isn't worth the bother to test them. It has to come out anyway so just replace it and the gasket. Spend at least $10 on a good one not the WallMart $2.99 one. Stock is 180F


No heat from the heater just means the heater valve is closed. Maybe the cable is loose or off the halve. Check that the heater hose from the head goes into the cab to the heater. If it has been looped down to the lower rad hose inlet to the water pump to by-pass a leaking heater, pinch this hose closed or cut and put large bolts in the hose ends and gear clamp in place.



Assuming it's not the above listed problems... over heating is either poor water flow or poor air flow through the rad.


Poor water flow...

Rev engine while watching the lower rad hose. Rad hoses rot from the inside and soften. If too soft the water pump suction will collapse it and choke flow.

Take rad cap off and look at the cooling tubes. Are they crusty with deposits? This is hard water scale from dissolved minerals. Take rad out and lay face down. Fill with CLR over night to dissolve deposits. Always use distilled water.

Fan belt tight? Does it squeak? Tighten or replace

Look on underside of water pump just behind the pulley. There is a weeping hole... is it wet? Seal is gone replace pump.


Poor air flow...

Shroud missing around fan. Shrouds increase the fans efficiency to move air.

Rad fins are bent or blocked with bugs, leaves or debris.

Grill blocked by fog lights or 'bush bar'.

Rad support has openings allowing air through around the rad. Seal these up so air has to go through rad.



Have cooling system pressure checked. System has to be able to pressurize. If not, there is a leak and it may now be obvious.

Try snugging up all hose clamps. Follow all hoses and metal lines, there is even one coming from the intake below the carb and the heater hoses inside the cab.

Always run anti freeze coolant mix. (after the problem is fixed)

Paint the rad flat black. Flat black radiates heat better than gloss paint.

Tune the engine up. Retarded ignition or lean mixtures will run hotter.

 Remove all spark plugs. If one is much cleaner than the others this cylinder may be getting coolant into it.

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Seems like heater core or hoses. Have u looked under the dash,specifically passenger side.

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