the mechanic "cracked" the injection lines, which stopped the flow of fuel into those injectors. You CANNOT crack the injectors loose until the injection lines are removed. Well, you could, but you would then twist the injection lines all to heck, destroying them.
As such, compression was NOT effected, only fuel flow. You have to LOOSEN or remove the injectors themselves from the cylinder head to release compression. Loosening the fuel lines has nothing to do with releasing compression, only stopping fuel flow through the injector. No magic or voodoo involved. If your mechanic is telling you otherwise, find someone else, he is clueless.
Did it run away on its own oil, and would cutting fuel supply then stop it? Possible. If it was not getting enough oil on its own and needed the additional fuel, cutting the fuel would then slow it down, which would effect to slow the oil vapors, or blow by of liquid oil, and it would die.
Diesels are actually quite simple, and a Perkins 4-108 is among one of the simplest engines. A simple compression test will indicate the engines viability, yet it seems almost impossible to get men who call themselves mechanics to perform that test. Far too often, hamfisted and/or criminal/larcenous and or incompetents will condemn an engine that minor or basic repairs could restore to prime condition.
I had a 4-108 in a skid loader. Bought it with a rod knock she banged so loud you could hear her a block away. Was blowing oil like a locomotive, but never ran away. I tore it down, had the crank ground and serviced, bought one rod (big end turned blue), reused the old rings, threw in a full gasket set and seals, and threw it back together. While I was at it I took the pump apart and put in a new shaft seal and just gave it a visual for wear. I dont think I had more than $500 parts into it including doing the crank, and it ran like a brand new engine.
Most failures are caused by neglect. In the case of my 4-108, an oil sender popped and she blew out all her oil. Boats by their nature are often sadly neglected, or the work people pay for is much too often a rip off. Find someone competent, or better yet buy a book on the engine and do it yourself. Its really not a rocket engine.
If compression is good, if the injector tips are dry and all equal in appearance, look elsewhere. Smell the oil and feel it between your fingers. Is it thick and oily, or thin and watery, and does it smell very strong of diesel. Is the oil level overfilled? These would indicate a fuel leak into the engine from either the lift pump diaphragm or the injection pump driveshaft seal. If the level gets high enough and the crank starts whipping it, oil and fuel vapors can get into the breather and feed the engine enough to runaway. Rarely will engine wear cause a diesel to runaway and I consider it doubtful. If the rings and cylinders were so worn as to permit such excessive blowby the engine would be an oil burning smoking beast and hard to start. Look elsewhere before condemning the engine.
And we can stop with the "prince of darkness". That Lucas pump is every bit as good as a Stanadyne. Neither are as good as an oil lubed pump, but the lucas CAV on that perkins regularly exceeds 5000 hours without any trouble. However, that may be where your problem lies. If the governor lets go the pump will go into full fuel flow and you will see a lot of black smoke. But again, all troubleshooting should begin with injector removal and a compression test.
