White smoke from the exhaust indicates engine oil remaining on the bore that the rings haven't scraped off -loss of oil control. Black smoke from the exhaust indicates dirty fuel injectors on a diesel engine. Dirty fuel injectors aren't a serious problem. The rings gets progressively worn down. A slight escape of wind from the dipstick is the first stage of ring failure. It will get worse until gasses and oil start escaping the dipstick holder. Reduced compression or blow-by on the rings has nothing to do with the condition of a diesel pump.
Reduced engine compression/white smoke are the result of the piston rings not sealing tight with the bore. 'Blow-by' on the rings results in less power transferred to the crankshaft. The compression gasses that escape past the rings are forced out via the sump. This results in the sump leaking oil.
The dry/wet readings must be very close to each other. To do a dry test, unplug the sparkplugs for petrol and remove the glow plugs on a diesel engine. Place compression tester inside glowplug and crank engine 10 times till the needle indicates maximum compression. Note this reading and write it down. Repeat test for remaining three cylinders. All four readings must be within 10% of the vehicle specs and very close to each other. Should a reading be out, insert 10ml engine oil into the glow plug and repeat compression test. The oil seals any faulty rings. Should the reading differ from the dry reading, it indicates a failing ring. Above sea level the compression readings on all four pistons must be within 80% of the specs.
A mechanic must perform a compression test on any used vehicle before purchase. And don't use the mechanic next door to the retail outlet - ensure you obtain an unbiased opinion.
If there's no air,gasses or oil escaping the dipstick holder, but the compression readings are not correct then the cylinder head is cracked or there is a blown head gasket. Look for a white emulsification on the dipstick. It would indicate a cracked cylinder head, leaking engine coolant on the piston.Check the dipstick end for gear oil or engine oil. Unscrupulous sellers add SAE90 gear oil to mask faulty ring symptoms.