I reviewed all the many postings re engine coolant problems and couldn't find one related to my situation, so, here goes. The coolant overflow reservoir on my 2GM20F fills up as I run my engine and does not bleed back into the heat exchanger. The reservoir will overflow if I don't transfer coolant back to the h.e. This problem appeared after 12 years of heavy use. I replaced the radiator cap, checked and tightened the bleed hoses to and in the reservoir and found nothing. The seat for the cap appears to be ok (no scratches, junk, etc.) Help!