1990 Other Chevrolet Models Occasional overheating

due to you saying you lose a gallon or more of coolant you still have a serious leak. Have a pro pressure check your system to see if there are any visible leaks. If not then you may have a head gasket or intake gasket blown. It could possibly be a heater core as well or even a water pump. But a pressure check should give you a very good indication

no visible water leaks even after vehicle sits, nothing on the ground except below muffler on humid days. heater blows heat, am suspecting head gasket. will have to change intake gasket also to work the head. will a compression test identify which head gasket is faulty? or in for a penny in for a pound and have to do both, plus intake and valve cover gaskets?

first let's get teh muffler thing. The water you see there is probably from the a/c drain that is in the heater area. It is normal for this to happen. It doesit whenever you have the a/c on. humid days you would see it more. As far as the compression test, if you pull all the spark plugs out and the gasket is blown between the cylinders then it would show up on a compression test. But a pressure check would show a drop in pressure indicating a leak. Which if there is nothing external it would mean internal. If you drain the oil and get water from the oil, then that would be either something cracked or an intake leak. If not it would be a head gasket or soemthing cracked. By the way if you continue to drive and it continues to overheat then it may have something cracked as well. Also if you do one head gasket you may as well do both just as a precaution. One other thing, check the lifter valley on both sides for a crack. Some of these were cast thin in those areas and have cracked there. It would be about an inch down from where the head mates to the block.