This morning i removed air filter, glow plug wiring, went for a drive, got it nice and hot.
Removed glow plugs, disconnected cut off solenoid.
Compression test on all cylinders.
cyl 1 435 psi (406 a few weeks ago)
cyl 2 424 psi (391)
cyl 3 420 psi (413)
cyl 4 431 psi (413)
all pretty good, no great differences.
I'm guessing increase over previous results due either to doing it hot, or as its a new head the valve seats are perfect.
Any, more inconclusive results really. No further forward to finding the problem. Will change bottom, top, and bypass hoses next weekend, possibly test the stat for the 3rd(?) time. Then I have nothing left to try.
Anyone want an 87 plate 90, with brand new galv chassis, new galv bulkhead, 5 new tyres, shocks, springs, all bushes, ball joints, handbrake shoes, rear shoes, diff bearings and seals, xfer box seals, filters, new head, water pump, radiator etc etc... makes me weep.