2002 Ford Focus 2.0 SOHC, manual Trans has 150000 miles. Shut engine off. Would not restart. Backfired in engine compartment, kicked back on cranking, then started, but would not continue to run. Timing belt OK (no cracks or wear), timing positions on cam and crank match. Cylinder 4 spark plug oil fouled, but engine did not miss or run rough before engine wouldn’t start. ECM did not throw any trouble codes. What ever happened did so in between shutting the engine off and ten minutes later when I tried to start it back up. Good battery, good fuel pump.
Not finding the question in there but I will assume you would like me to try and make some sense of what may have just happened.
Oil Fouled Spark Plug
Having found an oil fouled spark plug in the number four cylinder is a huge indication of internal failure. Now is the time to go one step further and perform a compression test on all 4 cylinders.
Low Compression
Having low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. However, low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.
If compression is low in one or more cylinders
Engine Backfired
An engine will backfire if a valve is open when spark is applied. Since you noticed no indication of the valve timing being off then the valve must have been stuck/bent when the engine was shut off. Then when you cranked the engine over you had an issue because of it.
What do I do Now?
Once you have confirmed you have low compression you will need to disassemble the engine in order to determine and access the actual damage.