The Mitsubishi Pajero Owners Club®

[Mark3]

Sniff tests (checking for products of combustion in the coolant) arent always accurate on the 2.8, i done many a head replacement  and half the time the sniff test came back all clear....

Typically the heads crack allowing combustion gasses into the cooling system under high pressure so you'll find your cooling system will over pressurise filling the expansion bottle to overflow and prolly not sucking it back in as it cools (like it should do) the gasses find there way around the cooling system causing your heater to blow hot and cold (cold when there is no hot coolant n it)

From cold top the radiator right up then leave the cap off and start the engine, if you see bubbles, coolant burping out and steam (which will be exhaust smoke as cold water dont make steam) your head needs to come off for further investigation.... chances are the head is cracked.

[Google]