I'm working on a 2.0 L engine that has a backfiring problem and sluggish acceleration. The distributor had an obvious problem in that the timing was advancing too fast...actually full advance once plugging the vacuum hose back on. I estimate the full advance was about 40 degrees, which is way too far. I installed a rebuilt distributor. Also, since I had to pull the exhaust manifold off to replace the three freeze plugs (one was rusted through) under the exhaust manifold, I noticed that the air pump pressure that is supposed to be pumped into the exhaust manifold went nowhere? It was a dead end? How can this be? Why would the factory put an air pump on to do nothing? I haven't started the engine yet because I cross threaded the connection of the EGR tube to the exhaust manifold and have to get a new one, but I'm really curious about that air pump dead end?:screwy:

Follow-up: The backfiring was caused by fouled spark plugs creating unburnt gas in the exhaust. The elbow on the exhaust manifold is no longer available at the dealership. Got a thick brass one at a hydraulic shop for $16. The timing advances normally now and the truck runs much smoother. The air pump pressure that goes nowhere...well, still scratching my head over that one?