Pair of Diesels fine in morning, won't rev past 1,500 in afternoon - SailNet Community
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post #1 of 25 Old 06-06-2015 Thread Starter
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Question Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

I have two Cummins 7L Turbo Diesels (6BTA5.9-M3). Everything was fine in the morning, cruising at 2,500rpm. Tied up for 2 hours over lunchtime. On return trip both engines refused to rev past 1,500rpm. Would go to 1,500 at one third throttle position, but moving throttle after that would not result on any more revs or speed. The only common component is the fuel tank (I have twin tanks and run with them connected). I did not refuel during the day. Tank was still half full at end of day. Weird. Any ideas?
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post #2 of 25 Old 06-06-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Something plugged up
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post #3 of 25 Old 06-06-2015 Thread Starter
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Forgot to mention: engines professionally serviced last week.
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post #4 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Possibly fuel filters clogged from stirred sludge in tank. Wouldn't be the first time a yard installed the wrong micron size either.


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post #5 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Also check for an air leak in fuel line. Same thing happened to friend (also right after diesels were serviced). Air leaks are sneaky and hard to trace tho:-/

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post #6 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Try again on a full Tank, (eliminate that or not) was it fine before service, what was serviced. Most likely starvation but many other things as well. How old are fuel lines? separate filters for each engine I suppose. If both are doing it, it is a common problem. Tank vents OK? What does your Pro say. Most likely leaks would be in recently opened filters, check with a paper napkin its best way to find a small leak you can see it suck right into the paper when you find it. A small suction leak will show up with a lower Tank level and shutting down could introduce an air pocket. it may have been OK if continued to run at that low level without shutting down. Keep results posted, interesting problem.
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post #7 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Fuel tank vent obstruction?

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post #8 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Ask what they serviced and check what they did. most likely they install to fine a filter and it is getting plugged up

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post #9 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

Throttle linkage slipped?
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post #10 of 25 Old 06-07-2015
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Re: Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

If the fuel filters are not clogged, check the delivery tubing and 'dipstick' and its small inlet 'screen' for blockage. Usually such line blockage is due to the fibrous 'mats' of bacterial colonies .... looks like fine 'strings'. These 'bacterial strings' are deformable; and when in high flow and lots of vacuum/pressure from the lift pump, will slowly 'tighten'/compress the mass of 'strings' slowly slowing down fuel flow; when the engine is stopped and pressure/vacuum will slowly decay and the 'strings' will expand their 'mass' and fuel flow will be back to 'normal' ... until the engine begins to consume fuel and the clog process starts all over again. Since 'both' engines are affected, it would suggest that the blo-blockage is somewhere in the common delivery line all the way back to the tank's dip tube and from junction of the individual fuel 'branches' that feed each engine.
What's your fuel filter(a) vacuum gage reading when you get this fuel starvation; and, what is it when you cant get full rpm?

Use the proper amount of biocide and examine your tank system for water accumulating in the tank sump ... usually due to the o-rings on the tank fill port cap are 'gone' or broken .... OR long term storage of a FULL tank. The less amount of water the less amount of 'bio-growth'.
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