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Pair of Diesels fine in morning, won't rev past 1,500 in afternoon

3K views 27 replies 15 participants last post by  Galo 
#1 ·
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?
 
#6 ·
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.
 
#10 ·
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'.
 
#12 ·
Hi. Quick update.

We have checked fuel screen and filters, which are OK and correctly specc'd
Confirmed there is no electronic control unit
Checked throttle mechanisms, which are OK
Engines are turbo-charged and we did have issues with one of the turbos not spooling up so we have overhauled both turbos (was on my list for the fall so just accelerated the schedule)
Sea trial after all that revealed same problem: engines perform great up to 1,500rpm/9kts at around one-third throttle position but no additional revs after that. Reducing throttle on one engine does not affect the other. No noticeable smoke. No weird vibrations, noises or smells.

Next step is to trace back through fuel system for anything causing starvation - pumps, injectors.

Poor support from local Cummins dealer not helping.

Thanks for all the suggestions.
 
#16 ·
Hi. Quick update.

We have checked fuel screen and filters, which are OK and correctly specc'd
Confirmed there is no electronic control unit
Checked throttle mechanisms, which are OK
[snip]
Next step is to trace back through fuel system for anything causing starvation - pumps, injectors.

Poor support from local Cummins dealer not helping.

Thanks for all the suggestions.
Repeat - What is the vacuum gage reading on the fuel filters when the engine(s) decreases rpm ??????????
 
#14 ·
Being as I was just talking to a member of my YC with twin Cats, and same issue.....turned out to be an oil fill sensor that go out after x years. A couple of other members have had the same issue with there larger diesel motors. Part was about $100 plus labor. I got the impression easy to R&R the sensor.

Marty
 
#17 ·
Being as I was just talking to a member of my YC with twin Cats, and same issue.....turned out to be an oil fill sensor that go out after x years. A couple of other members have had the same issue with there larger diesel motors. Part was about $100 plus labor. I got the impression easy to R&R the sensor.

Marty
THIS!! Might explain if it started after recent service. Low oil switch could cause a "limp" mode, check oil level first then this sensor. Is there OBD to retrieve codes?
 
#22 ·
Update

After consulting a second mechanic and taking the boat for a sea trial it seems that the problem is not with the engines. After running at full throttle for 10mins the turbos eventually kicked in and the engines made 2,800 rpm. It seems the engines were overloaded and had trouble achieving enough speed for the turbos to be activated. The vessel has large swim platform with a 10' dingy at the stern. We took the dingy off and the boat got up to speed much faster. Also, a quick underwater scan showed some barnacles on the props, shafts and hull. The boat was pressure washed 3 months ago, but the antifoul paint is old. It's possible that the heavy rear end coupled with barnacle build up could be straining the engines. Next stop is a lift and pressure wash.
 
#24 ·
Update

After consulting a second mechanic and taking the boat for a sea trial it seems that the problem is not with the engines. After running at full throttle for 10mins the turbos eventually kicked in and the engines made 2,800 rpm. It seems the engines were overloaded and had trouble achieving enough speed for the turbos to be activated.
sigh.... turbos do not take 10min to spool (activate) even in a worst case scenario they should be spooled in less then 10 seconds at full throttle. RPM is not as much a factor as heat, and at full throttle under load heat is massive. I fear you may have turbos that are on the way out. Somehow at the same time. Could be flooding of the turbines.
Take off the exhaust, and look into the turbos. The new may be very bad.

BTW who the heck spec'ed twin turbo engines on a sailboat? Thats just stupid.
 
#25 ·
Let's use some common sense. Does it seem plausible that both engine's turbos would have the exact same defect and be cured of it as the exact same time? For a second, you had me wondering what kind of wastegate you have on your turbo (the device that spills exhaust so your turbo doesn't overboost your engine). Maybe it's been stuck open and just freed itself. However, both at once?

Did this recovery to full rpm come after the boat came up to speed? Perhaps it's just overloaded or underpowered, until you get up to plane. Perhaps the prop pitch is wrong (this would be most likely consistent, for both sides). Or, you need adjustable stern planes.
 
#26 ·
OK. Final update. New sea trial with new mechanic checking pressure differential across aftercoolers to check for blocker there. At full throttle the boat now sits at 1,500 rpm for 20 seconds, rpms creep up slowly (starboard leads port engine) and then turbos rush in at 30 second mark. Boat is planing and bow levels off by around 40 seconds. This is slower than before my problem began, but I might just declare victory with this performance.

The only changes since last sea trial is that we scraped a few barnacles off the shafts, which were spotty but not encrusted. My best guess is:

- The initial problem - boat sticking at 1500 and not planing at all - was with the turbos, which we overhauled;
- The boat is heavy and possibly over propped (will look at changing pitch next), which means it struggles to get over its own wave even when engines are working properly;
- My trim tabs have intermittent problems with both actuation and position indication, which is not helping and might be the cause of some of the mysterious behaviour.

Up on plane the engines make 2,900 rpm and sound great.

As disclosed before, not a sailboat. My experience is that sailors know about diesels, hence the post here. Thx.
 
#28 ·
OK. Final update. New sea trial with new mechanic checking pressure differential across aftercoolers to check for blocker there. At full throttle the boat now sits at 1,500 rpm for 20 seconds, rpms creep up slowly (starboard leads port engine) and then turbos rush in at 30 second mark. Boat is planing and bow levels off by around 40 seconds. This is slower than before my problem began, but I might just declare victory with this performance.

- The boat is heavy and possibly over propped (will look at changing pitch next), which means it struggles to get over its own wave even when engines are working properly;
- My trim tabs have intermittent problems with both actuation and position indication, which is not helping and might be the cause of some of the mysterious behaviour.

Up on plane the engines make 2,900 rpm and sound great.
Okay, having been a stinkpotter all my life (transitioning to sailbutts soon) and having had 1000+ hours on several diesels, I can tell you the problem is NOT solved.


First, the fact that you are (eventually) reaching 2800 RPMs means the boat is NOT overpropped. U are (eventually) reaching close to full rated RPMS, so....propping seems to be right.

Trim tabs...ahhh....no. U would need to have a trim tab the size of a window and pointed > 45 degrees down to cause the amount of drag that it would take to spend 20 seconds struggling to get on plane. And if that were the case, you would be over on your ear or with the bow completely buried when you did come on plane, so....nope...that's not it.

U don't say what year/model of Cats u have, ie, whether they have the traditional mechanical FI or they are the newer common rail, electronic FI system -ACERT system in Cat-speak.

The newer, fully electronic engines' CPUs monitor boost pressure build-up and adjust injection parameters using boost pressure data to avoid smoking. Essentially, even though you 'firewall' the throttles, the CPUs compute that you initially don't have enough boost to consume 100% of the 'full throttle fuel' and they don't provide 'full throttle fuel flow' into the injectors. As boost builds, the CPU gradually increases injector 'duration' and timing to provide fuel appropriate to what the engine can efficiently burn, hence, no (or very little) smoke.

Please check that you don't have an 'engine synchronizer' engaged, because that will make both engines rev as one. The synch feature will make the good engine rev like the one that may not be good.

If the engines are not smoking badly during this protracted, engine-lugging 'reach plane' situation, it means that there is something dialing back the injection map(s) such that boost is building veeeeeeryyyyy slooowwwwly....

PM me if you need more thoughts/ideas...
 
#27 ·
Having owned many powerboats in the past, I can assure you that if those trim tabs were down, you would be hard pressed to get up to speed. Additionally, do have a single fuel tank vent, or one for each tank. I experienced a similar situation while in the Florida keys when mud daubers constructed a nest in my tank vent. The vent wasn't completely blocked, therefore, when the boat was left for a few hours, the air gradually seeped into the tank and the engine operated just fine. After about a half hour, though, it began to bog down and was starving for fuel. I took me a week to determine the source of the problem.

Good luck,

Gary :cool:
 
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