Leaking injector causing cylinder to be washed

[Djw085]

Is it possible for a rebuilt motor to loose compression in one cylinder after around 40 mins total running time since build due to a leaking injector washing the cylinder?

[Duncan]

Absolutely. The rings won't bed if the bore is fuel washed.

How much compression has gone missing (how does the leaky cylinder compare to the others?)

and, did it have a missfire for those 40 mins?

finally, once the injector is replaced you may still be able to get the rings to bed properly by running it in hard on a dyno to save having to remove, disassembly and re-hone.

[Djw085]

It didn’t misfire when I first started it it started misfiring after I ran it for 20 mins then changed oil and filter (as directed) ran it again and the miss started shortly after that. It had next to no compression in the 3rd cylinder.

[KiwiRS4T]

13 hours ago, Djw085 said:

It didn’t misfire when I first started it it started misfiring after I ran it for 20 mins then changed oil and filter (as directed) ran it again and the miss started shortly after that. It had next to no compression in the 3rd cylinder.

Did you refill with mineral oil? Have you repaired/replaced the faulty injector? As per Duncan's suggestion try running in procedure,

Stick it in third, accelerate briskly (not with full boost if turbo) to about 6000 and then lift off completely til revs drop to about 2000 and repeat for about half an hour. Do not be alarmed if you get a puff of blue smoke when you floor it as oil will be sucked past the non sealing rings on the overrun (so  check your oil level regularly). Keep an eye on your temps and (if you have a gauge) oil pressure. Then repeat the compression test.

Will not work with synthetic oil!!

[tridentt150v]

I'd do a wet comp test first...to be sure its the rings and not a valve issue