Trouble codes

I have an 89 chevy pickup. I get a "service engine soon" light when I have been traveling long distances. It does not come on unless I have been driving atleast an hour on the highway. I have pulled code 24 and replaced the VSS. Now I am getting a code 32 (EGR). It can be the EGR valve, solenoid, or MAP sensor. Any ideas? I am confused since it does not come on when daily driving. The engine spark knocks when it gets to normal operating temp. and does so all the time.

My guess would be the EGR isn't coming on, or not coming on enough. Pull the valve off and check the passage. It should be mostly clear, but of course it will have light carbon on the walls. Don't use solvents to clean it, as you will damage the diaphragm. Use a pick or a small screwdriver to clear it out if it is plugged.