Volvo Repair: coolant leak / overflow, volvo 740 gle, radiator hoses

Question
Got an '86 volvo 740 gle, non turbo. 175,000 miles. decent shape.  having a coolant overflow issue.  water pump ok - 1 year old.  hoses ok. Also, no coolant in oil. Temperature gauge is in middle and steady. Lately, when I check the overflow tank it tends to be low on just local driving. I fill it up to proper level. I drive for a while and it tends to overflow. I have not seen any coolant leak from engine body / radiator / hoses /pump, etc. Could it be the thermostat?  Can I remove the thermostat if I live in Southern California and drive in moderate weather? Could it be the overflow tank pressure cap? or overflow hose?    thanks, Don

Answer
Easiest and first thing to do is just replace thermostat. It should be replaced every 30 to 60 thousand miles anyway. Unless you have tester for pressure cap, cheaper just to also replace it as preventative maintenance. Not recommeded to run without thermostat because if engine is too cool and not warming up fast enough , can have effect on fuel injection running rich all the time. Also can lead to buildup of engine sludge over time especially with around town driving.