I finally got the OBD2 scanner. 3 error codes. 2 might be relevant P1131 - O2 sensor (phone's flat, can't read the message right now but pretty sure it said something about running lean) and P0171 - running lean bank 1. The other error code was engine oil temp sensor but that doesn't seem connected.

Some sort of vacuum leak is still on the cards (I cleaned and checked the PCV valve when I cleaned the throttle body).

It's pretty cramped around the intake with the gas injection. I tried some Start Ya B@stard but need to get the air intake out of the way to check vacuum lines and spray around the harder to reach parts of the inlet manifold. I guess degreaser will do the same job as brake cleaner?

If I can't find anything I'll take it to a mechanic and get them to test the intake for leaks and, if that fails, a compression test. (Yeah I could have done that first but that's like admitting defeat. I never take the easy path)

The other thing I'm thinking is a burnt valve/seat is likely to be on the exhaust and more likely to be rich exhaust gases and not cause the sensors to pick up a lean reading? I pulled the O2 sensor out and cleaned it anyway. No signs of carbon but I didn't think you'd see that if it's running on gas most of the time. I've googled "would a misfire caused by a burnt valve cause the engine to run lean?" and reading the results on that just in case.

It's probably a stupid 2 cent O ring on one of the injectors. Hopefully this will help someone else. The only thing I'd do differently so far is check what happens to the idle when I disconnect the coils. Still, it was probably due for plugs and coils (the plugs weren't too flash after 35,000K anyway - but no obvious signs of one bad plug/cylinder either).

If you had a burnt valve it would be easy to track which cyl it was, dos it miss under acceleration and cruise or will it surge on cruise, or dos it just run rough at idle. You did look inside the pvc valve and see the spring loaded flat plate. If you cant find the problem look for a mechanic with a scan tool with ford software, with mine I can see all the live data from the pcm and will give you a good insight of what is happening under the cam cover.

The miss is only noticeable at idle. It's like sitting in a car with a lumpy cam. It just bobs away (and putting at the exhaust). That's why I called it a "lumpy idle". Feels perfect above 1000rpm. Even the fuel economy around town at 17L/100km doesn't seem like the miss is a constant drag on the engine although if it's supposed to be 12 or 14 that's almost a dropped cylinder's worth?. It could be a small vacuum leak that doesn't haven't much impact at higher rpm? I checked the pcv valve by blowing through it after it was clean. No back flow. Normally I just plug away at these things until I am completely defeated and demoralised and then I seek help. But it's like having to ask for directions in front of your missus.

Yep, a little jump in revs when I sprayed around the inlet manifold with degreaser (needed to be cleaned before I work on it anyway).