Please tell me if you think this is a terrible idea:
What if I temporarily plugged the timing cover bolt holes and the water holes on the front of the block (could use bolts and rags)? I could plug the intake manifold's secondary (small) hose as well. Then I could squirt water into the return hose with no thermostat and see if it leaks out through the oil pan drain. It seems to me that if nothing comes out, then the engine/heads/intake manifold are not the problem and I could rest assured that I botched the timing cover/water pump part.
