overheating honda

ok, i need to pick everyones brains.
buddy's wife is driving a 99 civic. car has been notoriously overheating. buddy's engine mechanic with yrs of experience.
anyways, here's the scoop:
- with radiator cap open, good fluid flow through radiator
- fan kicks in
- thermostate replaced last year and this saturday
- no unusual noises
- 170K miles
- no unusual smokes
- runs short on coolant in few days
- no bubbles in the coolant. we ran out of ideas, and i thought it has head gasket leaking exhaust into coolant, but that you can see in the radiator, looks like foam. negative for this car.
- no accessive fluid drippage out of tail pipe
- replaced radiator cap
- expension reservoir is filled with fluid to proper level
- per his wife, it overheats more if driven faster
- i don't know how good or bad drive belt is. all this was collected from him over the phone. i know him, he knows his stuff.
we are out of ideas. bottom line is - it's not that complecated of a system, there's only few areas of concern - pump, temp sensor, radiator, belt. pump obviously works, as when revved, it'l blow coolant out of radiator. fan works. both upper and lower radiator hose are the same temp, so no blockage in the engine.
- no leaks underneath, it's parked in garage
will appreciate y'all's help

 

How about a good coolant flush? Could clogged heater core, bad engine temp sensor for starters.

A visual check is not the most accurate. I would do a engine compression test and a coolant system pressure test.

 

thank you. fan starts, so temp sensor is ok. i told him to flush. he will not do anything that involves $$, can't efford for one, then if the worse comes to worse, we'll just get a new engine with maybe 40K miles on it for $400 and swap.
but they don't have a spare car, so we need quick fix. she has to drive in traffic for at least 45 min twice a day.
maybe we'll simply pull thrmostate out and let it go like this for some time. i know, it'll throw ECM out of kilter, no big deal.

 

ok, i think we got it.
so far, it appears that when they replaced the thermostate saturday, they did not have heater core valve opened to circulate fluid through.
i am guessing, but as it was pretty cold here through the spring, she turned the heat on while driving to work. that sucked some coolant into the core and created a void in the system. air locked the circulation.
car did not overheat when she drove back home, so air probably bled itself.
buddy had that car refilled today at 5 30 am, on cold engine, with heater core valve open - left it running for over 30 min, everything's fine. temp went up to normal and just stayed there.
FESTINA LENTE - rush slowly.
it is always darkest under the light pole.