Mga masters, I need your expert opinion.

My brod-in-law bouht a 1994 Delica Chamonix the other day from Pampanga. While I was test driving it last night, I notice na kpag dehins umaandar yung van, mahina yung aircon (halos wala na nga) pero, once I drive on, lumalamig especially kapag pumalo na ng mga 80+Kph. I told him about my observation pero sabi daw nung pinagkuhanan nya at nung mechanic, normal daw yun for diesel rides, especially kung 2nd hand. Is this true? Yung Delica ko kasi dehins naman ganito. I'm asking kasi 'wawa naman ang utol ko kung palpak yung van na nabili nila.

One other thing: unlike dun sa Delica ko na electronic na yung thermostat control, dun sa Chamonix nya, eh manual yung pinipihit. Tried adjusting it pero no dice.

Thanks po mga masters!