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Get a copy of your engines Repair and Maintenance Manual (aka - "Shop Manual') and verify your injection pump timing.

Grey 'smoke' coming out the exhaust is usually a sign of either 'over-fueling' / unburnt fuel and mostly is caused by incorrect engine timing; but, can be caused by low compression.

If the timing is found to be correct, then look for broken / sticking piston rings or an exhaust valve that is cracked or not making a complete seal with its valve seat ... or is simply 'hanging up' (not making its full 'stroke' ... such can be caused by a pin hole water leak inside the exhaust manifold, etc. 'back draining' into a combustion chamber when the engine is shut down), etc. etc.
Then I would suggest that you do an 'air pressure leak down test' of the combustion chambers (instead of or in addition to a compression test). With the injectors removed, High pressure air is put into each combustion chamber with each piston in turn at bottom dead center, and you look for 'where' the air is leaking out of: - crankcase breather (broken/sticking piston rings), exhaust system (ex. valve problem), air intake ( intake valve problem), into the cooling water (blown head gasket), out of an adjacent combustion chamber (blown head gasket), etc. etc.
Verify your injection timing FIRST, as on most small diesel engines the injection timing should be checked about every 500-750 engine hours as a routine 'maintenance item'.

hope this helps.
 

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Got the manual, and know how to adjust the valves, but the timing marks are IMPOSSIBLE to check while the engine is in situ, unless I use an inspection camera... and I don't have one.:(

This also would not fit with the fact that it seems to have, at least temporarily, fixed itself...:confused:
Open/remove the injectors, rotate the crankshaft until piston #1, etc. is exactly at 'top dead center' and put a 'mark' on the crankshaft pulley. Look in your engine manual to see exactly how many degrees 'before top dead center' the injection pump 'begins' to deliver oil and put that BTDC mark on your crank pulley. ... open the high pressure tubing thats on the outlet of injection pump and hand crank the crankshaft - oil should appear out of the injector pump 'exactly' when the crankshaft is rotated to the 'before top dead center mark' - BTDC.

If the combustion temperature changes due a 'throttle' position change, then the timing of ignition will also change accordingly .... those OEM marks on the crankshaft pulley are an 'average' of where the ignition via compression begin and the 'speed' of the piston will cause variations in combustion chamber pressure when the ignition starts.
With combustion anomalies, always start by verifying 'injector pump' timing, first.
 
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