Okay, so the in-line heater control valve was stuck and clogged. Right now I just have a temporary hose connector in place so water can flow freely. I pulled the heater core and it flows fine. I disconnected the return hose (I believe top hose driver side) to the engine. I ran the motor and no water is flowing at all. The truck runs fine and the water pump functions for the engine cooling. Why would there be no water flowing down the heater core hoses? Anyone have a diagram of in/out hot/cold layout of the water pump/radiator/heater core system? My manual does not show what I need. This thing is simple. It should not be that hard.

 

The heater water valve (18495 in pic) is spliced into the inlet to heater core hose.

If no water is flowing, the heater core (18476) is plugged up with rust & scale or you have the hoses reversed.

 

So I checked all the hoses to make sure they were clear. Also, I checked the tube the water goes through under the carb. Flushed out the heater core. Water will flow either way. Hooked everything back up. Still no heat until I took it for a drive. The thermostat must have to open before the hot water starts moving around?

Does anyone know why the water flows through the pipe under the carb?

 

The pipe under the carb heats up the gas/air mixture to make it run better when the engine is cold. Modern thinking says you want a cold gas/air mixture to get maximum efficiency.

Many folks do not run the coolant under the carb.

 

Thanks Emil,

I did some digging myself. It's actually to prevent icing in the carb. The water would not be flowing unless you have the heater control valve open. On a cold start, the water would still be cold for a few minutes and still not flowing until the t-stat opens anyway. If outside temperature is cold enough to build ice under the carb, then you would most likely be running the heater water/fan at the same time thus preventing the ice build-up. In warmer climates, it can be deleted for simpler hose routing.

 

Carb icing can happen in 40 degree weather. The air is pulled in so fast that the moisture in the air will freeze over idle bleed holes and such.

 

Exhaust manifold butts up under the intake to help preheat on a cold engine. most (?) have a a restriction valve in the exhaust to shorten heat up time. (cold intake will condense fuel along the runners and cause poor drive ability.)
the core flows water but will it flow air. look through the core and be sure it isn't packed with crude. the old systems weren't as well sealed and may be pulling air from leaks in the ducts and plenum box. also check the thermostat isn't stuck part way. while monitoring the engine temp you can cut and lay cardboard in front of the radiator. temp on both the gauge an your duct thermometer (hand?) should also rise. other than hard driving once the thermostat cycles the engine temp should read fairly steady. If its swinging say something like 1/3 to 3/4 I would remove and replace he thermostat. (or test it first, but at that point just replace it. Don't get the cheapest one!)
Good luck