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Old 25-06-2010, 20:01   #1
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Cummins 6bt Fuel / Boost Issue ?

Hope this is the right place to post, first post!

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
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Old 26-06-2010, 04:46   #2
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What fuel system is on this 6BT? there were several. Post a pic of the pump if needed.

are you making excess smoke?
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Old 26-06-2010, 05:21   #3
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Quote:
Originally Posted by never monday View Post
What fuel system is on this 6BT? there were several. Post a pic of the pump if needed.
are you making excess smoke?
Also what fuel filters, and when were they last changed ??
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Old 26-06-2010, 05:32   #4
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just tried posting and it didnt go through...

The fuel pump looks similar to the pic shown below only mine has 6 fuel lined coming off the back.

There is no smoke coming out at all.

I am thinking now maybe turbo lag? Does the turbo have a waste gate, i dont remember seeing one, the boat is more than 1/2 hour from my house so it is hard for me to remember now without being able to see it.

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Old 26-06-2010, 10:58   #5
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with that old mechanical system. I'd change fuel filters first. If it doesn't clear then pump work might be in order. I don't remember a wastegate on them. But, if your making less than ideal boost you'ld have excess smoke.
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Old 26-06-2010, 11:04   #6
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The filters are brand new, the fuel is clean, I also am running a seperator. I have read on other forums that the lift pump maybe bad. Also I too the feed lines off of the selector and noticed that there was no fuel in the lines even minutes after shutting engine down. Should I have check valves at both tanks to keep the feed right at the ending, would this allow to lift pump to not work as hard. My next test will be checking preassure after the pump and vacumm before
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Old 26-06-2010, 12:38   #7
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if you suspect lift pump, definetly check it's output pressure. IIRC, it should be around 15psi.
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Old 02-07-2010, 05:41   #8
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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 preassure and vacumm 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
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