I have 229M miles on my 2002S. I recently changed the heater core but now find a thick foamy substance in the overflow. The spark plugs are dry. No foam on the dip stick but it needs refilling constantly. Is it a head gasket issue or some other gasket? Thanks

 

You need to do a leak down test to find out why you are getting thick foamy buildup in the tank.

Chances are either a head gasket, cracked head, or failed oil cooler.

And what needs refilled constantly, coolant or oil?

 

I add 2 oz oil per 100 miles. I also notice that I am getting a moisture under the drivers mat. But the rug area to the right of the gas pedal is dry. Ugh
My heater is working but I fear winter coming on

 

That's a significant amount of oil to need to add in such brief time.

Does the car have a sun roof? Are the front drains affixed and not leaking?

 

Thank you for the drain lead. got it opened and done.
Head gasket / cracked head repair are beyond my skill set and would cost much more than the car is worth.
I'm not sure that a gasket failure in the oil coolant-oil filter housing is the problem. there are no leaks in that area.
I add oil before traveling and find it gone after 30 miles.
Thinking I should put it up on ebay for parts