There's two causes of backfiring and sputtering that are somewhat common and elusive. One is bad spark plug wires. The other is a plugged pickup screen inside the gas tank. The plug wires typically cause misfires and backfiring on moderate acceleration. With fuel systems with the pressure regulator on the engine, like yours, a plugged pickup screen will cause sputtering for a couple of seconds before the engine stalls or loses power when the largest volume of fuel is being pumped, which is during coasting. When cruising at highway speeds it will usually run best. You can verify that by driving with a fuel pressure gauge attached and clipped under a wiper arm. Observe the pressure when the problem occurs.
Also look at how the spark plug wires are run. They must not be bundled close to each other, especially the two rear ones on the driver's side. Current flow through one can induce voltage into an adjacent one magnetically and cause a spark to occur in the wrong cylinder.
You may want to do a cylinder leakage test to see if, in this case, an intake valve is not sealing properly. That can send ignited fuel back into the intake manifold as a backfire.
Monday, June 3rd, 2013 AT 10:01 PM