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Heater Hose Routing

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by jimmie, Nov 13, 2001.

  1. jimmie

    jimmie Newbie

    Oct 28, 2001
    Likes Received:
    gold country california
    Allright you guys.
    Need alittle help with this in order to end the debate.
    Ive got a non-AC '71 GMC w/350 motor, pretty close to stock.
    Ive got a heater hose fitting in the side of the waterpump and one on the radiator close to the cap.
    Which of the heater core hoses goes to which fitting? Top to ? Bottom to ?
    Also, one hose fitting is smaller than the other. Is this the input or the output?
    Appreciate the help.
  2. Steve_Chin

    Steve_Chin 1/2 ton status

    Mar 6, 2001
    Likes Received:
    Foster City, CA, USA
    I'm not sure how the '71 Jimmys are routed, but my '72 Blazer routes the hoses from the crossover on the intake manifold into the heater core, then back out of the heater core into the water pump. The smaller diameter (5/8" hose) is the hot water supply, while the larger diameter hose (3/4") is the return to the water pump.

    That said, the side of the radiator with the cap on it is the *cold* side of the radiator that feeds the water pump. You won't get much heater action if you use the fitting on that side of the radiator to supply water to the heater core. Likewise, the fitting on the side of the water pump feeds the inlet of the impeller (also cold coolant). The way that your hoses should be routed is hot water supply either from a cylinder head or the intake manifold coolant crossover to the heater core. Then, from the heater core to either the cold side of the radiator or to the side of the water pump. If you live in a cold climate, I'd suggest plugging the fitting on the radiator and using the nipple on the water pump because you'll get more flow that way (less dilution of signal to the heater core).

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