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.