Okay, I Google this question but can't find the
answer but me and some Co workers were taking
What would happen if you drive a car/truck without
a cap on your overflow?
It all depends on the year, make and model of vehicle and what type of cooling system it has, some cars the recovery bottle is just that, it stores the coolant that is pushed out during full temp operation only to be reintroduced back into the system as it cools down, on these systems there is no pressure on the recovery tank so a lid only keeps out foreign material, the other systems are pressure recovery tanks, these hold pressure as the vehicle warms up to operational condition and has enough room in it too also hold the increase as the coolant heats up and cools down, on these systems the pressure is important for them to function properly so any air in the cooling system can have dire consequences to,the engine, so you can't run with the cap off on one of those systems. The way you can tell is just by looking at the cap the pressure system will actually twist on, the others will just push and snap on. HTH
Good anwser shoe man, right on the money
Ditto on what joemom posted. Great post tennisshoes, 10 points for you.