Ok, slightly off topic since the car I'm going to be asking about is a 2001 Focus with the 4 cyl Zetec engine a friend owns, but I'm asking it in a general info method. His car basically runs like crap when cold at any rpm, and has a bad stumble just past idle. After harrassing him for several months to check out the fuel system, he found the in tank fuel pump pickup clogged with a nasty sludge that resembled a yellow paste blocking up all kinds of stuff, but mostly the pickup screen. There's supposed to be some kind of bypass on the pump pickup when the car goes to full throttle, so I'm guessing the yellow gunk has now found its way up in to the fuel rail and injectors, and that a bottle of STP injector cleaner isnt up to the task of flushing things out.

What I'm wondering is, we pulled the exhaust manifold off the car, and I can see that the exhaust valve stems are coated with a snow white residue. I've never seen valves stems burned/carboned up that color before from running so lean. What are the chances he has really boogered the top end of the engine running the car so lean for so long? Would you think a compression test should be done to verify if he has toasted the valves?