1) Both are a possibility, but damaged valves are much more likely. As Dan said, he never saw sticking valves on a V6 and he has done a number of them...So, a quick summary:
- Last month, having received the car (GTV 3.0, not run for several years), I pulled the cam covers, visually checked the cams and applied some oil to the cams & cam followers; checked the condition of the cam belt, and ran a compression test on all cylinders. I had zero compression on cylinders 1 and 6, and healthy compression on the other four
- I then attempted to start the engine; it did start, but ran roughly and wouldn't idle. Aside from misfiring, it didn't make any strange noises - I did not hear any clattering or metal-to-metal contact. I shut down after a couple of minutes max. (We now know the valve timing was out; and with decent compression on only four cylinders it's no surprise it ran like a dog.)
- Today I ran another compression test. I now only have decent compression on cylinder 3, with zero on the other five cylinders!
- I borrowed an endoscopic camera and looked at the piston crowns - there's no obvious sign of damage to the pistons; I could see the depressions in the piston crowns for the valves, and some letters stamped into the centre of each piston crown. However - it was a cheapo camera and the screen resolution was not great, so I might have missed signs of contact between pistons and valves
- Whilst running the compression tests, I attempted to rotate the cam followers on any cam lobes that were not under load. They all rotated freely. However I noticed at least one (inlet on cylinder 6) where the two neighbouring buckets on the same cylinder were sitting at different levels, presumably meaning one of the two valves is not closing properly. Given cylinder 6 has never had any compression, this would make sense.
- Although I don't have the kit to perform a leakdown test, it seems clear that I have several valves not closing correctly. My questions are:
1) Why - are they more likely to be bent due to contact with pistons; or are they somehow just sticking due to lack of use over several years?
2) Why are more cylinders now showing no compression - and does that push us more in the direction of bent or sticking valves?
Either way - it seems further dismantling is going to be needed to get to the bottom of this...
(Sorry - that turned into a bit of an essay. If you've read this far, thanks for doing so - and if you can offer further advice on those two questions, even better!)
2) Probably the result of turning over the engine with incorrect timing. Yes.
As I said, get a set of second hand heads as changing all the valves and redoing the heads costs probably more than 2 second hand engines. Get the spare heads checked over and bench tested, skim them and install with a complete new engine sealing kit (head gaskets, main seals, water seals etc.).