why did my truck start surging

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. It could be as simple as low coolant level (would result in coolant temperature switch giving erroneous readings to the computer), it could be a bad throttle position switch or I have seen intake manifold gaskets fail on the D21 causing a vacuum leak and give this problem. Even if the warning light bulb is burned out the scan tool will still interrogate the computer in your car.