Air in the system would cause a hotter operating vehicle and I suppose that could cause some sort of boil over which should be obvious.
I wouldn't jump to pull the head until I had pressure tested the cooling system. You don't necessarily need dye to do this test.
Auto Zone and other mass-marketer parts house will rent you a cooling system pressure tester for about the cost of a new one. Usually around $75.00. The test is performed on a cool engine.If you have an encapsulation pan underneath, remove it first. There are usually anywhere from 4 to 8 small bolts holding the pan on. In many cases, an 8mm socket gets them off. If you leave the pan on, you may not see where the drippage is coming from once the system is pressurized.
I pump up my system to about 17 lbs. I have a 20 lb. cap. Automotive theory manuals indicate that if you pump up too much, the water pump seal may be damaged, hence I back off a bit. Once the pressure is on, you start looking for leaks. You can bet that if it's external, you'll soon see drippage on the ground underneath the car. You then know where to start looking.
Benzmac once commented that this test is a good way to know just how tight to get the hoses snugged down. I use this approach whenever I change hoses to help avoid breaking the plastic upper neck on the radiator.
If you see no leakage after 5-10 mins of being under pressure, then it's possible you have a leaking head gasket and the test for that is a whole 'nuther story.
1991 300-SEL - Model 126
M103 - SOHC
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