The Offshore Voyaging Reference Site

Cummins Marine Diesel Engine, Problems

We are in the throes of re-powering Morgan’s Cloud, and as far as we are concerned, this unpleasant and expensive task has come about 4000 engine hours too early.

A couple of years ago we started to notice that we were getting much more soot on the transom from the exhaust than we liked. Then last year we started to see un-burned fuel on the water aft of the boat and coating the dinghy. Clearly all was not well even though the engine continued to start easily and run reliably, as it always had.

We pulled the injectors and had them rebuilt. We pulled the injection pump and had it calibrated and rebuilt. Still the problem got worse. Then we started to lose top-end RPM, a sure symptom of loss of power.

Finally we did a blow-by test in which we measured the amount of gas pressure in the crank case. Oh-oh, off the scale. Either the valves were not seating properly or exhaust gas was blowing by the rings. As the head came off, we hoped for the former—a  comparatively easy fix with a valve job.

But it was not to be. David, ace machinist at Billings Diesel and Marine, checked the cylinders for wear with a bore gauge and discovered that they were out of round. And that’s when it got weird: The cross hatch pattern that was etched onto the cylinder walls during manufacturing was still in almost perfect condition, indicating that there had been no appreciable wear. Further, we had never overheated the engine and the oil samples that we had analysed at every oil change had never shown excessive fragments.

A vague bell went off in my memory: When the engine was new we had a problem with fumes in the engine room. I measured blow-by at that time and had even written the result in an old log, which, miracle of miracles, we were able to find. The reading: 8 inches of water. At the time, I had called Cummins North East (CNE), the dealer that sold me the engine, to ask about this reading and was told that it was not a problem and that the maximum for a new engine was 12 inches. But the Cummins document we found last year seems to indicate (it is not as clear as one would like) that 3 inches is the maximum for this engine!

By the way, CNE  then sold me a crank case re-breather system to solve the problem of exhaust in the engine room, which never did work well until heavily modified by Billings.

All of the above leads to one inescapable, at least to us, conclusion: The engine was bored out of round when new—a lemon, pure and simple. Even if that was not the case, this kind of problem at 6000 hours on a diesel engine that has always been meticulously maintained is just not good enough.

This comes on top of another manufacturing mistake by Cummins (stripped bolts on the oil pan) that we experienced with the same engine.

As we always do with this kind of post, we offered Cummins and CNE the opportunity to comment. They wrote us a brush off email.

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My neighbor once brought home a brand new Alfa Romeo two seater that was full of engine problems from day one…after numerous inconvenient returns to the dealer for adjustments and so forth, the regional service honcho was called in only to discover the engine was improperly aligned with the drive shaft at the factory…they honored the warranty and refunded the purchase price of the car, but my neighbor laughed at the idea of replacing the car with another Alfa after contending with such a basic and catastrophic assembly error…moral of this story and yours: never accept even the slightest anomaly or doubts with anything new especially if it is mechanical…I know you are more than upset with this development especially after showering this engine with such constant tlc expecting only the best in return…now I have a better understanding of the wisdom in Lin and Larry Pardey’s long-standing decision to go engineless with its inherent drawbacks such as the urgent need to fend off from a lee shoreline or on a close collision course with another vessel, which, for them, is more likely to be much bigger and harder…on the other hand, Larry is the least likely person I can think of who would ever wind up in such situations…hang in there as I know you will…Richard in Tampa Bay (Cavu’s master and commander…26′ Bayliner pocket cruiser i/o)

David H

This is a sad tale, and one that every marine engine manufacturer needs to read. The problem is that barring few exceptions engines for boats are simply commercial engines fitted with unique cooling arrangements (Marinized). If the engine were in a static application (say a genset) there would be no reason to re-furb/replace. Being in a boat the fumes issue was always going to be a problem, and reliability is paramount. These low hours does suggest an early failure. I am certain that you have the fullest service records, and since the ‘hatching’ still shows on the bores I would address this problem with the US Cummins concession. Cummins have a vested interest in their reputation and should at least investigate this early failure.
Nevertheless I would still opt for a rebuild. This engine is a ‘baby’ and has many hours running available. A rebore to the next size piston is a certain cure, but if there is ovaling the bore dimensions will have to be checked once enlarged. Replace the bearing shells whilst you are at it and undertake a ‘top end’ rebuild. Will Cummins pay for this, or at least part of the job?
I guess there will be more yet to this story.

don lake

Hi John
Sorry to hear about your boat motor. I have a fleet of pickup trucks with Cummins motors and find them to be very good but, and that’s a big but, the last 2 that I have had are an ’06 and an ’08 and have had trouble with both of them. I don’t think I would buy another one. Love reading your blog; very informative. Newfy in Tucson

capt George Wall

I had a Cummins 6BT5.9 in a previous boat. Suggest you contact Tony Athens who runs the Cummins Forum on “”. I found him to be the best source for tech support on these engines and an unbiased, straight shooter on problems. Good Luck.

Peter Bateman

I would be very happy with the rebuild route proposed. Bigger/oversize pistons/new x hatch/new big and small ends, grind valves, pump and injectors. But then we rebuild these engines beside the road in East Africa and get 250,000 kms plus from a makeshift repair. Cummins is a good block with what should be many hours of life. I wonder if this is an “Indian Cummins” as we had Ingersoll-Rand deliver compressors from India with a Cummins motor that died rather early….. As a boat rebuilder it is easier to remount the same engine on the engine bed after a rebuild and this would be a factor in my decision.


Cylinders out of round without wear – my guess would be that somehow this was a rebuilt engine, not a new one.

Capt George Wall

John: This probably comes too late, but I thought I’d pass it along. I have had a 6bt5.9 in a previous boat. I was fortunate to discover “” and the Cummins moderator Tony Athens. A wealth of information and an incredibly invaluable resource in everything from basic operation to troubleshooting problems of all types. Any marine diesel operator can probably benefit from this website.Check it out if you haven’t already.
Capt George
Schooner Irena

David Wells

I hope this is way too late to help your case. By now I hope you have it resolved.
I am a professional diesel technician, practicing for the last nearly fifty years. Yes still learning.
No I do not have any connection with Cummins or any other specific diesel manufacturer other than I work on all of them in my own buisness for the last 35 years.
In the OLD days they used to cast the cylinder blocks and throw them in a field in the back of the farm for four or five years to let the cast iron settle down dimensionally. All the best engine manufacturers did this. Rolls Royce on down.
After “settling” the cylinder block castings were machined to their fine tolerances and were as dimensionally stable as they could be.
Modern manufacturers do not have the luxury of time to do this practice. If the dimensions were out by much you might have had a green block.
Back to your specific case. I have to say that the BT5.9 engine series, along with the C series 8.3 liter cousins are some of the MOST rugged and very best engines for their size that Cummins has ever built. When I was doing my apprenticeship in the 60’s, 220 hp weighed 3500 lbs, was most of five feet long and four or more ft high, and two and a half feet wide.
In those days an engine with 6000 hours might easily need overhaul, but with wet liners and modular cylinder components it was an easy one or two day fix to overhaul unless the engine is buried deep inside a machine or very inaccessible in a boat.
The BT5.9 Cummins is capable of this kind of horse power in the marine environment…easily. The more power you take out of any engine the lower the expected life.
My question to your observation about “out of round” and still have the crosshatch in the cylinder walls is…HOW MUCH out of round? There are allowable tolerances.

I would first look at a number of “other” aspects outside the engine FIRST.
Such as engine space ventilation.
Overheated engine spaces (above 120F degrees) WILL cause modern diesel engines (and old ones) to smoke even when the engine is in fine shape.
In the old days, if a diesel was not smoking it was not working right!! Things have changed for the better.
Additionally exhaust back pressure and restricted air inlet air will both cause engine smoke and accompanying lack of power.
If an engine smokes for an extended time the turbocharger may get plugged with smoke and carbon. This will cause it to get stiff and it cannot pump enough air into the engine to let it run properly.
My guess is that you have, or had, a stiff turbo due to carbon plugging. This by itself can cause unusual blowby past the piston rings.
The “power loop” in modern diesels involves a number of intricate dynamics, one event depends on the next. The injector pump produces a certain amount of measured and correctly timed fuel to the injectors. The turbocharger is a “trick” device that uses the speed and energy of the exhaust gas to pump more air into the engine so that more fuel can be burned…and so on…
Turbo charge rates are typically about 40% above normal atmospheric pressure. In the rational world, without other influences like overheated engine rooms, back pressure and other “external” influences, the engine can then burn 40% more fuel. This will trick the engine into being not 5.9 liters in size and weight, and output…but 8.26 liter power capacity…WITHOUT ANY INCREASE IN WEIGHT…get it? Add after-cooling and the numbers are even better.

The “power loop” involves a neat little device called an aneroid. This is a sweet little thing on the outside of the fuel pump that is connected to the inlet air plenum between the engine and the turbocharger.
In its static or basic (no turbo pressure) state at low engine loads the aneroid is set to allow the fuel pump to deliver the correct fuel for a non-turbo engine. As the boost pressure rises it allows proportionally more fuel to the injectors until full boost and power are achieved at WIDE OPEN THROTTLE. At this point the air and fuel rates are balanced and there should be very little if any smoke…and lots of “umph” going on.
At varying speeds and loads between idle and WOT the fuel pump and turbocharger are designed to work in-step, together to produce the designed power.
Cummins are a very special company. They do not make trucks, fridges, air conditioners, cars or anything else that I know of. They make ENGINES, so they had better be good.
My guess is that you had or have “other influences” outside the engine that caused your engine to appear to fail at 6000 hours.
My thought is that you might have been better off looking at the turbo first rather than the fuel injection system.
I would be interested to hear what happened AFTER your rebuild, and how the engine settled down afterwards. Some or all of the “other” influences might still be there. These could lead to another “early” overhaul.
How about engine room insulation? Is there ant fiberglass in the insulation? This can wreck havoc with engines in boats.

Listening for a reply


Irving Braverman

good luck

Brian Taitt


I am presently building a 60′ V hull fiberglass boat to do commercial fishing. (Twin screw preferred).
I was tolled that Cummings make a good engine that burns less fuel than most.
If you were in my position, which pair of Cummings engines would you choose given the size of the boat? This boat has a 9′ draft.


You say that you believe the cylinders were not bored out correctly during manufacturing and that they were lemon shaped. Interesting. I was wondering which side of the cylinders were out of round. Would this not effect the power stoke and increase wear? Would the rings still rotate under operation? Im suprised if the cylinders were out of round that you would not see issues with the power cylinder componets when u completed a teardown. You say you have seen an increase in blowby pressure from engine installation but no wear on the components. Other casuses of high blowby are injector washers and turbo failure from the others pointed out. If the cause was power cylinder related you would have high oil consumption and your oil analysis would also highlight issues.

Regarding the loss of rpm, is this torque also? Sounds like a engine de rate to me.I would look at your turbo and check for an air leak. Engines dont derate because of increased blowby.

denis buggy ireland

i have owned dennis dart buses with cummins 5.9 engines for some time — when the bracket on the rear of the injector pump loosens you will find that the timing cover plate which is made of aluminum will fail as it depends on this bracket to remove stress from the front of the engine when it fails you have to remove the camshaft as it is forged to its gear to do this lift the cam followers and tie them up with cable ties to remove camshaft otherwise you have to remove the engine and turn it on its side .
regarding your bores —the essential information is the wear on the rings and pistons and crucially you should have checked the boost pressure of the turbo before dismantling as diesel can wash the oil off the bores if timing of fuel or turbo is wrong —i have also come across this problem when an engine was assembled with the carbundum dust and fine metal was left on the bores and filled the gaps in the rings and acted like grinding paste in the bores —it is possible to determine what caused your engine to fail it merely requires examination and measurement and oil anylisis .regards denis buggy


I have a 1996 36′ Riviera and I am having problems with engine blow-by on one of the Cummins 6BTA5.9-M2 it has only 825 hours and looking for someone to look at it in the Los Angeles Area. Anny suggestions?


Greetings John,
I commercial fish on the westcoast of Canada.Cummins has a good reputation on the westcoast.dependable engines and good customer service. I haven’t heard of any situations like yours out here.
It is really aggravating have to deal with the on going problems. I’ve being fishing with Cummins on this boat for 15 years. 400 to 500 hp. Never had a problem. sounds like you’re right on with the maintenance.
The Engine Is The Heart.
My buddy just repowered his 65 trawler with a 400 hp Scandia. Great service, and less fuel consumption. Maybe that is the way to go.
Hope your problems finally work out without too much BS
Westcoast trawl captain.

John Schuchert

The motor you had wasn’t bored out of round. The problem that you have is somebody used truck pistons in the marine engine, and probably like an FP diesel, or ADA kit which uses cheaper Chinese rings. I know because I did the same thing when I rebuilt one of my engines to see what would happen and everything that you are describing, including the 3000 hours and dead, happened to me. Somebody saved money rebuilding your motor by not using cummins parts which are 3-4 times more expensive, and now you are paying for it! If you still have any of the parts that came out, compare them to the cummins original parts and you will notice the rings are a different shape and everything.

Hope this helps,


You never stated the year and model number of your Cummins Engine?

Bob Lablaw

What were the out of round readings? I know this is an old post but for the rest of you reading, 6000 hours out of any 6bta (unfortunately unstated hp and whether it’s after cooled or not) is exceptional. Even if it was a 210, still exceptional. If the rings hadn’t seated from new the oil samples would have read a measurable amount of diesel fuel yet you say they were normal. I’m gonna make a guess at what happened here: you used some other oil than Valvoline or Shell Rotella 15w-40. Again, information for those reading: loaded rpm variance at break-in is the most important step to avoid excessive cc pressure because it seats the rings correctly. But if you use crappy oil (yes there’s a difference nowadays) then you will lose. Don’t idle a new engine for long periods of time, don’t low load or lug a new engine and don’t point fingers if your measured blow by on a new cummins engine is 8″ of water or more and you continue to operate it under those conditions.
To review: follow the cummins manual break-in schedule (if you’re reading this comment you can find it, have faith) and use the correct oil and filter (fleet guard or Baldwin, never fram). Zoom zoom