Actually diagnosis is pretty easy. I've lost count of the number of cars I've had come in over the years with a large number of new parts and the same problem still there.

In essence all they need to run is a spark in the right place, fuel and compression.

Time and time again people get asked the same basic questions, what is the fuel pressure, is it timed up right, what is the compression reading? The fuel pump works isn't useful information. I've measure the fuel pressure at the fuel rail and it is within spec is useful. Just as the timing belt is not broken isn't useful. I've looked at the timing marks and they line up AND I'm sure the bottom pulley hasn't turned on its rubber damper is useful.

And you know what happens when you ask people those questions; 9.8 times out of 10 the OP goes off and buys new spark plugs, cap, leads, oxygen sensor, followed by an AMM or fuel pump. The scatter gun approach just doesn't work and it is expensive. Frankly it is laughable.

I don't know anyone who does this competently and professionally who looks at a car and discerns it need an AMM by sight alone. We do the basic tests, figure out what the missing element is and address the issue.

Just be systematic, but I guess that is just too boring.

And I even know how to spell know, but I'm no know it all;-)