Offroad-Express

hi all.
i have a problem, my brother owns an isuzu!
i noticed a couple weeks ago it was running a little ruff. next thing i knew it had a knock it sounded to me like timing or a faulty injector. i told him to crack one injector at a time and see which one if any were playing up. he reconned none so i tore the front down and checked timing. that was all ok. i tried advancing and delaying timing on pump but had no luck with removing knock. finally i tried the injectors again. when i cracked 4 the knock went away completely so we replaced it. tried it knock came back. swapped it with 2. no 4 still knocking. thing is when its cold it purrs revs everything perfectly, once warm it idles nice revs ok but not flash and under load sounds like the piston is hitting the head.
any ideas
i would have thought a big end would have knocked more when cold same with piston slap.
could it be something in pump or a sticking valve?
thanks

EGR. If it is a manual, try disconnecting the TPS. Done 3 nissans in the last month with 2 of them having the same symtoms (one of them being mine).
If it is a auto, disconnect and block off the vacuum hose on the actuator on intake manifold.

Cheers,

hi thanks for your thoughts, i have disconnected it and left him to try it as i was limited on time however looking at the setup i cannot see why it would only effect the 4th cylinder. ill keep you posted.
thanks

so he tried it and its perfect, i havnt a clue why it affects just 1 cylinder not a very good design in my mind as that cylinder must get a hiding its entire life. anyhow. thanks heaps for the advice it was rite on the money