I am having an issue with an '89 Shamrock with a cummins 6bt that is baffling me. At idle up to cruising rpm the engine sounds and runs just fine, however, if you try to go above 2200 rpm through to open throttle the engine sounds like it is derating until it comes back to normal operating rpm. Any adjustment to throttle increase has the same effect. Coming from a tractor trailer background of diesel repair i can only describe it as this...if you try to go wide open throttle the engine will accel normally until just out of the rpm range you are trying to achieve. Just before it hits, the engine sounds like an engine brake is being applied for about 2 seconds, then comes back to normal rpm's until a throttle change is made. It only does this at high rpm's. I do not know if it is a fuel or boost issue. Where do i begin trying to figure this out. I am used to Mack and Mercedes engines, this cummins has me scratching my head.

thanks in advance for any advice

 

It sounds like a fueling issue. Easiest thing to do is check fuel pressure going in to the injection pump and seeing if it's tapering off at RPM due to a clogged fuel filter.

If fuel pressure is good, it sounds like some sort of wonkiness in the injection pump. Is it a rotary injection pump? The marine applications from 1989-1993 didn't all use the same injection pump as the Dodge application from 1989-1993.

 

I'm with Begle, what pump is on the engine?

 

I'm with the other posters, check for fuel supply issues and check fuel pressure.

Sounds like "fuel surge", which is what happens with a plugged filter, line obstruction, or bad supply pump.
I've had it on my carhauler, with dual tanks, and the switching valve sometimes sticks between tanks, starving the lift pump out.

It's similar to a 4 barrel carb with an almost adequate fuel pump, runs like crazy, falls, runs like crazy, falls.
Let off it and all is "normal".

Mark.

 

Lack of fuel or timing slightly off. The governor springs may be weak and not allowing it to fuel at high RPM's.

 

true on the fuel supply, and timing. and i'm unsure of the RPM range in a marine application but if it is anything like an on road truck anything after 2100-2300 and you are going nowhere fast due to the governor setup. it may not be designed to turn higher than 2200 rpm.

 

Thats kinda my thought. Those older engines in an industrial application would only turn 2200 rpm's. I'm not sure if its the same for marine applications but i bet its not far off.

 

does it have a "speed limiter" on the pump? you might be running into that, I know they came on some VE pumped commercial apps.

 

Most industrial and marine 5.9's I've worked on are rated at 2800rpm's. But these have been a-pumps and p-pumps. They are all spec'd differently for sure. These guys gave you a good place to start. You might also check fuel lines and make sure they aren't being sucked shut, rubber or plastic. I have seen this happen more than once.

 

Thanks everyone fore the great replys. I have checked the fuel lines for kinks, etc. The fuel filters were changed prior to my original post. Something i noticed was that at the dock, in neutral, i can rev all the way past my problem rpm's with no boggin whatever. Dont know exactly what this means to anyone but figured i'd share. My next plan is to check preassureand out of the lift pump but i also think the tank selectors look restrictive. And lastly i was wondering if i should instal check valves at the tanks so the lines don't drain back so easily.

Thanks again