Engine Peformance Poor, steady miss, terrible fuel mileage. What's wrong?
A steady miss indicates one of two things: a cylinder that isn't firing because of an ignition problem, or a cylinder that has lost compression.
The first step in diagnosing this kind of problem is to identify the dead cylinder. A professional mechanic can do this quickly by hooking the engine up to an ignition oscilloscope and displaying an ignition raster pattern. The dead cylinder will show a firing voltage that is significantly higher or lower than its companions depending on the nature of the problem. He might also do a "power balance" test and/or a compression test to find the dead cylinder.
One way you can find a weak or dead cylinder is to momentarily disconnect each of your engine's spark plug wires one at a time while the engine is running. When the plug wire is removed from the spark plug, there should be a big drop in idle speed and idle smoothness. When you pull a wire and there's little or no change in idle speed or quality, you've found the bad cylinder.
It makes no difference whether you remove each plug wire from the spark plug or the distributor (or coil pack on distributorless ignition systems). The idea is to simply disconnect each cylinder for a moment to see if it makes any difference in the way the engine runs. The one that makes no difference is the problem cylinder.
CAUTION: Disconnecting spark plug wires while the engine is dangerous because you risk getting shocked. You can minimize this danger one of several ways. One is to wear rubber gloves and use insulated spark plug wire pliers to momentarily disconnect each plug wire. Another is to make sure no part of your body is touching or leaning against any metal surface on the vehicle (the fender, hood, grille, etc.). Or, you could turn the engine off, remove a plug wire, restart the engine, note any change in idle, then repeat for each of the remaining spark plugs.
If you disconnect the plug wire from the spark plug and hold the end of the wire close to the plug terminal or other metal surface, you should see a spark and/or hear a crisp snapping noise if voltage is getting through the wire. No spark would tell you the plug wire is bad, voltage is arcing inside the distributor cap (remove and inspect the cap for cracks and carbon tracks -- replace if any are found) or a dead coil on a distributorless ignition system (Note: on most distributorless ignition systems, each coil fires two cylinders. So if both cylinders are dead, you know for sure the coil is not working. If only one cylinder is dead, however, it's not the coil).
If all of the plug wires seem to be sparking okay, the next step would be to remove the spark plug in the problem cylinder. Fouling is a common cause of ignition misfire. Examine the end of the plug. If the electrode is covered with deposits, clean or replace the spark plug. Also, note the type of deposits on the plug. Thick, black, wet or oily-looking deposits would tell you the cylinder is burning oil (probably due to worn valve guides, rings and/or cylinder wall). If the deposits are a powdery black, the cylinder is running too rich (probably due to a leaky injector on a multipoint fuel injected engine). If the deposits are brown or gray, it indicates a normal buildup. However, the plug may be fouled because it hasn't been changed for a long time, because it is the wrong "heat range" for your engine application (you need a hotter plug), or because of frequent short trip stop-and-go driving.
In any event, if the plug is fouled you should probably remove, inspect and clean or replace all of the spark plugs.
If the dead cylinder is getting spark and fuel, the only thing that's left is a compression problem. The most likely causes here would be a leaky valve (probably an exhaust valve since they run much hotter than intake valves and usually fail or "burn" first), a blown head gasket (this usually involves two adjacent cylinders, however), or a rounded or badly worn cam lobe.
A compression check will verify if the cylinder is developing its normal compression. Little or no compression would verify any of the above problems. A leakage test could also be used to further diagnose and identify the nature of the problem (valves, head gasket or cam). Air leakage through the exhaust port would indicate a bad exhaust valve. Air leakage back through the intake manifold would indicate a bad intake valve. Air leaking into an adjacent cylinder would indicate a blown head gasket. Minimal leakage would indicate a rounded cam lobe.
Leaky valves would require removing the cylinder head and having a valve job performed. A leaky head gasket would require removing the head and replacing the gasket (and probably resurfacing the head to restore flatness). A cam problem would require removing and replacing the camshaft and lifters (old lifters should never be reused with a new cam).