Also at 3,000 miles you need to check and adjust the valves. The 1978 and up Buses had hydraulic valves that did not need adjusting. Lining up the valves for adjustment vary from engine to engine, but a general rule of thumb is to have the cam lobe 180° from the rocker. So on the valve you're going to adjust bump the engine until the valve opens, then two more bumps will bring the cam lobe 180° opposite.

To Adjust the valves, put your screwdriver in the adjuster slot and loosen the lock nut. Measure the clearance between the adjuster end and the top of the valve. Don't put any pressure on the adjuster or you won't get an accurate reading. When you have 0.006" clearance, tighten the lock nut and recheck it. You want a light drag when you slide your feeler gauge. It's all feel and once you do it a few times, you'll get the feel for it. We had a saying about these engines; "A tappy engine is a happy engine" it's better for them to be a little loose than a little tight.