Just a thought. You think it maybe leaking at the hose connections and running down off the bottom of the valve? That is if your re-using the spring clamps. I've never had good luck with those after removing them. Try replacing with the worm screw type hose clamp if you haven't already.

 

Very good point rsmarcil. It really does look like it's coming out of the valve, but this is worth a shot before I head down the long EGR/Headgasket road. I think I might also replace the hose running from the back of the heater control valve as well before I go further.

 

Yes check all of your hoses/connections before you proceed. If everything check out then I suggest installing a pressure gauge in your cooling system, atleast then you'll know what you are dealing with.
Hopefully its a faulty clamp or hose.

 

Moral of the story: Never, ever, ever trust those spring-type clamps. Installed screw-type on both in and out of heater control valve and it appears to have done the trick.

Thanks rsmarcil for pulling me back from the brink with some old-school back-to-basics. Most times it really is the simplest fix vs. the toughest.

 

No problem. Glad you got it worked out. I've had problems with those stupid clamps before and is why I never re-use them anymore.

 

I found this thread while having a similar experience with the "4 Seasons" Heater Valves; They leak through the phillips screws that hold the vacuum servo to the valve ports.

been trying for months to locate a small-but-annoying coolant leak in my 1997 4.0 liter Ford Aerostar until I finally discovered it is a drop-by-drop leak through one of the phillips screws in the Front Heater valve. (my Aerostar has front and rear heaters, the Rear heater valve is not problematic);

I replaced the front heater valve 3 times. YES, THREE TIMES, 2 of the valves were from Autozone, and one from Advance Auto Parts, all three leaked inmediately, although the one from Advance Auto Parts leaked through the vacuum servo diaphragm.

I ordered the oem Motorcraft heater halve through eBay, it has a metal-bodied servo as opposed to the all-plastic "4 Seasons" valves. Seems to me the 4 Seasons valves are not being tested under coolant pressure before being shipped from China. The fact that all these valves are leaking through one of the mounting screws tells me their design is deffective.