Misfiring, backfires, and loss of power tell me there's a timing issue. Check the timing. It should be between 8-10 degrees BTDC. It's a 98 with over 200K miles, so when was the last time the timing chain was changed? While checking the timing, watch for timing stability, as well as proper setting. If the timing bounces around, change the timing set. For best results over the long run, specify a double roller chain and gear set.
Also, perform vacuum leak checks using a can of carburetor cleaner. Spray all vacuum lines and fittings, as well as the intake manifold mating areas. Also spray the air intake after any air measuring sensors, to see if maybe un-metered air is being introduced into the combustion system.
Find a retailer that loans tools and check the fuel pressure at the regulator and after the regulator if you can. Decreased fuel pump output can give a good idle, but starve the engine of fuel at speeds and under load.
Loss of power could also be related to a plugged catalytic. If you can, disconnect it temporarily and see if that makes any difference in the power output.
Wednesday, June 15th, 2011 AT 4:21 AM