Dear fellow tsikoteers - I need your thoughts.

I drive from manila/pampanga to baguio atleast twice a month for the past three years and didn't experience any cooling related problems not until recently. Minsan longer drives pa during holidays, oftenly loaded with 4 to 7 adults including myself.

During my last trip, I noticed my temp exeeding half up to the 3/4 mark. I experience this until now regardless of either uphill or patag after driving for 3+ hours.

Visually checking, laging napupuno yung overflow container at meron traces of water being spilled out from it. Should be normal kasi yun nga ang purpose ng overflow container when internal pressure exceeds its threshold thru the pressure relief valve ng radiator cap. Correct me if I'm wrong.

The problem is that water is not being sucked back to the radiator as water cools down (as water contracts) when slowing down or even when leaving the car parked overnight.

Most recent action I did was to replace the radiator cap with a new one. Federal Mogul yung brand and pressure rating is according to spec (0.9) but problem persisted.

Ride is a 3.1L 4JG2 TDIC Trooper that is religiously maintained. Been using plain distilled water with no coolant eversince and I drain and replace this once to twice a year or soon as I see traces of kalawang.

Any thoughts what could be the problem? What could I have missed maintenance-wise?