#8 Cyl "weak"

You can not detect a lean condition on a EFI car by spark plug color unless it is right after a wot run because they run lean anyways. If the spark plugs are melted, yes, but not by color.

The one plug not firing is because the carbon track allows a easier path to ground, therefore no spark. You need to find out WHY it is doing this. Most likley oil is getting into the combustion chamber. Now how it is doing this is the part that determines all else. Is it the valve seals, valve guides, piston rings, intake manilfold gasket (leak from valley to intake port) even a cracked head or block as well as the head gasket could be the cause, most likley though the valve seals or piston rings.

Clean or replace the spark plug. Examine the cap and rotor for carbon tracks or damage. Use an ohmmmeter to measure the resistance of the #8 spark plug wire. You should see about 2000 ohms per foot of length. A 3 foot long wire would measure 6000 ohms or less. Then do a cylinder balance test.

Cylinder balance test:
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 99 for everything OK, or the number of the failing cylinder such as 22 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at Walmart.

Do a compression test on any cylinder that fails the cylinder balance test...

Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.