Detroit Diesel 12L series 60 825 hp

[Roger bing]

I just did injectors on a 825hp series 60 for a friend of mine. Barred engine over and set injector heights accordingly (81mm) after installing unit injectors. Note motor was guaranteed to be in correct position at this time as valves were exactly where i set them during tune up in april. In addition, each injector had to come down. I attributed this to new springs. Boat had run over 100hrs since tune up flawlessly. Upon testing boat today the motor hesitated to come up to speed and lagged the starboard motor by 400-500 rpm. it eventually caught up and ran fine while matching load and producing top rpm (2350). Is it common to get a faulty reliabuilt product? Can a unit injector be incorrectly “spraying” at mid range rpm -1200-1800 and correct itself after?

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[Roger bing]

sorry for the late reply. in our travels since my last post, i actually found a cracked exhaust manifold. which was welded and replaced. apparently the #1 hole on the manifold was a problem on the marinized 60’s . detroit even added an additional fresh water feed to the front of the manifold in an update as they were not circulating well in the front end of the motor.

yes tony…. the 825hp is a 12.7L. I agree with you and told the owner to first swap the boost sensors from engine to engine and see if the lag transfers motors. they are easy to get at.

as for the injectors i changed at the 3000hr mark, the “lagging” motor now throws clean exhaust water at 2350rpm and 99% load while the starboard is pushing black water at same rpm and load. owner is set on doing starboard injectors in spring. they are due lol.

[Roger bing]

I got the same impression as I had the boost issue with my 300 cummins in my boat where the acceleration lagged throttle position. I got the code reader back and am going to run an injector test again to assure we didnt get a bad injector from detroit diesel. After that ill look into how the motor balances boost and fuel during acceleration. I was moving around the back of the motor by the turbo to bleed the engine.. perhaps I knocked somthing off.

[Tony Athens]

Let me ask a few things 1st..

1) Are you sure these are 12L Series 60’s & not 14L units? Unless I fell asleep for a few yrs during this engines development, the 12L series 60 is maybe 20+ yrs old or more, and I was not aware they ever had ratings that high for these early series 12L 60’s…. But, you can bring me up to date,. We’ve installed about a dozen of these engines and to me, they are top drawer anyway you look at them. At their peek, Cummins could not compete as a 14L Marine engine..

2) The way you describe your issue, it sounds more to me as your fuel delivery system has a boost sensing feature as part of it , and it is slow to react based on available boost–Just like a “lazy aneroid” on a Bosch P7100 fuel injection pump………………….

Got a shop manual for the engine? That’s where I would start, or, you need to hook up with someone that really understands this engines fuel system.. I am not your guy………..That engine ruled the roost in the truck market for years, so finding good truck guy from that era should be easy.. You need a forum that deal with “line haul” truck drivers–You know, the “breaker breaker” guys when CB’s were part of life…

Tony

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