Honda Car Forum

My "04 Acura EL has a coolant overflow return problem which my mechanic can't solve. When the engine is hot the coolant overflows into the overflow reservoir normally but will not flow back when the engine cools, and the coolant goes down in the rad by a similar amount. This continues until the overflow reservoir is full and I have to syphon the overflow back to the rad.

I have changed the rad cap and the thermostat (Acura parts) and cleaned the overflow reservoir to make sure there is no blockage to prevent flow out of the reservoir.There are no leaks from the rad. My mechanic has pressure tested the system and he can't find any leaks. The engine does not overheat, the temp gauge always reads normal but this would change if there is insufficient coolant in the rad.
The dealership will not quote a price to diagnose, they say whatever time it takes that will be it. This could be very costly.
Can anyone shed some light on this. Any suggestions would be appreciated
Nevillea