My F150 with 4.6 has over 200,000 miles and I have kept the maintence up with no real problems to date. The check engine light has recently come on and the code is for missfire on #4. What causes this? I also have a clacking at start up which sound as if it is coming from under the valve cover. It goes away as if the oil takes time to get there. I have tried high viscositiy oil and engine flush which reduced the noise but it is still around.
Any help is appreciated.

 

It almost sounds like you're having massive issues generating oil pressure. This could be a number of different things. Everything ranging from main bearings spun and preventing oil pressure there, down to a faulty oil pump, or even a clogged oil passages. Other than the obvious, I'm not entirely sure where to help. I'd try flushing the system again and see what happens.

 

Thicker oil will make it harder to get oil to the heads on start-up.
What year is the truck? Could be a bad plug, wire or coil, depending on the year.
Inspect the exhaust manifold. Could be a broken stud or two that holds it to the head.
Could also be a loose spark plug which will rip the threads out of the head if it blows out.
How long has it been since it has been tuned up? Check the heater hoses over #4, some of the early models had clamp on hoses that leaked over time and got coolant into the #4 plug pocket and shorted out the plug wire/coil.

 

Its a 98 F150 and the noise sounds like it is under the valve cover. It stops after the engine has run for a few minutes. It's has the appearance of oil not reaching the rockers.