Intermittent is how mine was for a while so I had the under bonnet lash up and then the jumper until I sorted it. Some other relay behind the main group would click when testing mine [1990 19j] No glow plug relay on pre Defender model diesels just a heavy duty ignition switch unit. Can be...

Re my post above, when trying to sort my starting issue I made up a little jumper cable and plugged one end into the brown live feed [ at relay position] and touched the other end on the white/red to solenoid. Started every time. Connected small white from ignition switch through a test bulb to...

Send it back.

And the boost fail could be from something as simple as a pipe that has split or come off. Whooshing noise points to this, had it happen on a car once and power was gone.

Well, nice pics, could be anything but you have forgotten to tell us what vehicle year / type you have and what if any non standard extras fitted that may need the extra cable.

If air is getting in it has to be at the pump, any thing after would show as a leak.

To check for air coming through the system could you disconnect fuel pipe in the engine bay and run the pump with pipe in a clear jar to check.

As above, a problem selecting gears is a clutch, main box issue. Transfer box, put the diff lock in and see how much movement you have at the flanges [both props need to be off to do this properly] If there is a lot of slop when one flange is held and other turned it is likely a center diff issue.

The original ones still there? If so mark their position, cut off and weld replacements in the marked place.

As above @kermit_rr re thermostat. I often say for older vehicles you can not beat having an aftermarket capillary type temperature gauge, keep original for looks and fit it somewhere else. They read in numbers/degrees and no electrics to bother with other than light. Hope you pass on what you...

All series gauge's are getting well old. The one in mine just about lifts off the bottom when up to temp. Check what temp you actually have with a IR temp reader, aim at thermostat, top hose, rad bottom hose. I had a job to find a new thermostat for my Ninety last year, first replacement did...

Sorry to say but that is not a crack. That is the result of corrosion working out from the inside of the chassis box section. What is worse is that someone in the past has done a rubbish job of patching the area probably hiding worse underneath. What is the rest of chassis like? You cant beat...

No, not that I can see.

When I was trialing a lot with my series I had those flanges that had a screw fitting in them and replaced with grease nipples. After a dirty day fresh grease pumped in. If there was water in there don't know where it went but never had to replace a bearing.

Do you drive through over axle deep water a lot? Although a 1990 truck mine is a late made 19j with the old axle lube system. Has 200tdi engine these days.