1988 Chevy Van overheating

You might could have a cracked water jacket or plugged up water passages on the block itself. If you were to have a crack jacket you would more then likely be losing water/coolant some where. Some times it will appear in the oil on the dipstick, but we are not always that lucky or unlucky. If you do have flow in the radiator itself more then likely there is something up with the cooling system on the block. You can use a infrared thermometer and look for variations on the outside of the block. It could even be an issue with the intake or head gaskets if they were improperly installed. Not very likely but has happen`d before. You wouldn't notice very good flow tho with that one.

Hope that can give you some ideas. Best of luck with it.