Hi guys, I've been trying to track down an overheat problem(temp sender def. faulty, but van does ACTUALLY overheat sometimes as well...water boiled out of overflow tank once). In part, I've noticed that my heater core doesn't blow hot...so I installed a flush T and flushed the cooling system, with flush additive...everything came out surprisingly clean(there is receipt evidence that the previous owner had a shop flush the van as well). Hoses going in and out of heater core didn't get hot, so I bypassed the core open/closed valve so that the core is always open. Still, the hoses don't get hot...so I've drained coolant, disconnected one heater core hose at the installed T, and blown into both ends(so in one case, I am blowing into the left side inlet of the engine, and I hear bubbles coming out top of engine where coolant cap is off. In the other case, I am blowing through the heater core and into the right hand side outlet of the engine, and I also hear bubbles coming out of the fill cap. In both cases, there was minimal restriction to blowing, just fluid backpressure).


So the heater core and lines aren't clogged/restricted, and the water pump is pumping some water(I replaced the lower radiator hose with a piece of clear vinyl hose temporarily, to watch fluid flow...when I rev engine, the fluid vortex's and small bubble fly through!). So why isn't fluid circulating through the heater core? Is this some kind of 3Y configuration issue, or is my water pump weaker than it should be, or is there some other explanation?

Any thoughts appreciated. Big thanks to all the contributors on this forum, this site is an incredible reference source.

Cheers,

Alex