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Introduction
A misfire in a 3.0 Ford engine occurs for all the same reasons as any other engine that has a misfire. A misfire should be diagnosed from the simplest and most likely to the more complex. A worn or dirty spark plug is the first thing to inspect. If the spark plugs are OK, then the entire ignition system from the plug wires to the coil and the control module would be suspect.

Finding the Misfire
Look for any cracked or loose vacuum hoses and any other vacuum leaks. A fuel injector can cause an identical miss to a spark plug if it is operating erratically. Look for a leak in the injectors around the body of the injector. With the engine running, disconnect and reconnect the injectors one at a time, paying particular attention to the amount of RPM drop on each. Look for a cylinder that has very drop in RPM, in comparison to the other cylinders. Listen to see if the miss also dissipates in that same cylinder. Listen to each injector for erratic clicking or operation. Use a rubber hose. One end goes to your ear, the other is placed on top of an injector. If an injector is suspect, have it cleaned with a cleaner directed to the injectors only.

Causes of a Misfire
Check to see if the "Check Engine" light is on and, if so, what the codes are. A faulty crankshaft sensor or cam sensor can cause a misfire. A mass air-flow sensor can cause a miss by transmitting a false signal to the computer, causing the computer to create a lean or rich mixture. An oxygen sensor can do the same thing. The computer itself can cause a misfire, although not as common, through a malfunction or a bad main ground to the computer.

If everything checks out above, then the mechanical aspect of the engine should be checked. Start by removing all the spark plugs and disabling the ignition by unplugging the ignition module. Take a compression test on all cylinders and keep track of all the readings on a piece of paper. Hold the throttle open and rotate the engine about seven times, making sure the same number of engine turns remain constant on all cylinders for an accurate reading. The cylinder balance should be within 10 percent or less on all cylinders.

If one of the cylinders is lower than the others, one is leaking somewhere. This would cause a definite leak if there is an unbalance. To determine if it is a valve or piston ring one of two methods can be used. The best method is using the compression gauge hose and attaching an air source to it. Listen for the escaping air. If it can be heard at the exhaust, it is a bad exhaust valve. If it can be heard through the intake manifold the intake valve is bad. If it can just be heard with the oil filler cap removed, it is the rings and the air is passing the rings and into the oil pan or crankcase.

Leakdown Testing
A regular leakdown tester would be the best tool to check for cylinder leaks, since it would indicate to what extent the damage is by the amount of air bypassing the valves or piston rings. If you do not have a leakdown tester, the simplest method is not bad to use. If a problem is found, it doesn't matter where it is. The engine must come apart to fix it. It is no simple fix. Just pour 1 oz. of oil into the cylinder that had the lowest reading and take the compression test again. It the pressure goes up, the rings are bad. If the pressure stays the same, the valves are bad. By putting oil into the cylinder, it seals the rings for a few turns, thus restoring compression if the rings are bad.

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