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Cummins M11 - loss power & fuel mileage


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1999 Volvo VNL 610 Cummins M11 celect plus.


Truck had been running good getting 8.5 mpg. Truck started feeling a little different, not running as smooth and mpg has fallen off to 7.5 mpg. Also a slight knock.


Changed out both fuel filters and no change.


Any ideas on what to check next?

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The answer(s) to that really depends on how far you are comfortable to venture in the mechanical world.


The easy points to check is condition of oil and coolant? Do you have a pyrometer or turbo boost gauge? If so what, if any, changes in their indications prior to and now.


Prior to this were you on an oil analysis program? If so what were your last few reports like?


Some of the potential things could be:


FUEL - injector, injector line or fitting leak, blockage.


Head Gasket - unlikely, but depending on where one blew, you could get a miss.


Bad Cam Lobe


Besides checking fluids, if you don't have access to electronic diagnostics, you can take something like an old school auto stethoscope (Harbor Freight - Cheap) and touch the probe to the head at each cylinder and listen for a dead cylinder.


Get that info and it will help folks give you a more educated guess, which is all that anyone can do remotely.


There are other possibilities though it is all grasping at straws without more data.



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How many miles are on it?

Is it making any smoke- white, blue, really dark grey?

Has it lost boost?

Is it making oil? When running is the oil level increasing from a faulty injector.

These engines are simple enough that anything that goes wrong is usually Air related or fuel related.


Is your truck singled? I have the same setup and bobtail I get 11-12mpg and towing I'm getting 10.5.

Just wondering the difference in mpg.


M11 tuned-400/1450

rockwell 10

singled short with either a 2.94 or 3:08 rear. It turns 1425 at 55mph.

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450K on truck,


no smoking


noticed boost little over 20 running 55-60 mph


have checked oil level frequently and no change in level


coolant appears ok


when I purchased and drove home bob tailing mpg was 11.5 (think my gearing is 2:90 something)


We made it to our friends near Chattanooga, luck would have it he is friends with service manger at local Cummins shop.

Plan to take it over to them and have them work it on.


Replies are much appreciated.

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Does your M11 still have the EGR system intact?


If it has EGR...... have it checked out ASAP.


Our old Freightshaker with a M11 had a odd ting in that the engine often would being at 1375 RPM on level ground 55 MPH and I would notice that the fuel flow would increase and boost would creep up to 15 to 20 and the engine seemed to be under more load. Overall fuel MPG dropped almost in half on long trips down from 13.5 to less than 8 MPG. It was nothing wrong with the engine......it was the spring brake control valve in progressive failure and any time our main system pressure dropped below 110 PSI the spring brake would slightly start applying until the air compressor would build up system pressure to 120 PSI and then the spring brake was fully released........This likely can only happen to me but it does show that sometimes the engine is actually not the problem but other systems are loading the engine and it is only powering up to carry the additional load.


Good Cummins shop will likely get to the problem in time....


Good luck.


Drive on.........

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Wish I known you were in town.... The best diesel shop around is just south of Chattanooga by 6 or 7 miles right off of I-75....


Either way, since you are going to be in town for a bit, feel like hoisting a couple in the next day or so?

Friend of ours from Trenton is friends with service manager at Cummins power south. Still waiting on call back. Will not be in town, we are heading to Tuscumbia tomorrow. May slip back over to truck shop to pick up a few things this afternoon.

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Okay, #6 injector nut backed off causing injector tube to fall into motor.

Sounds like parts of it may have got to cam.


I had overhead run before trip (1100 miles) by a shop in Joplin.

Obviously they did not get it done correctly!


I have been in-touch with Joplin service manager. He said they would stand behind their work (we will see!).


Joplin service manager is to contact Chattanooga. Waiting for return call, Trying to stay positive and keep my cool!

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Overhead was $315 but may end up being $7k plus!

Ouch.......Like I indicated at the end of my post a good shop found the problem in short order.


It's a real bummer when you have service done to keep your rig in good condition and the "service" turns into more problems than you were trying to prevent.


When you indicated that you were running 20 boost on the level it was a good indication that the engine was highly loaded all of the time.....if you consider that you had a six cyl. engine running on Five Cly it is amazing it ran as well as it did.


When I was just out of high school I had replaced a leaking front crankshaft seal on one of "Grumps" cummins 335 KW log trucks.......I did not properly torque the harmonic balancer and.........in short order the crankshaft BROKE at the #3 cyl crank throw.........What a mess......


My mistake was VERY costly and was very much a family hardship.......Grumps could of said a lot of bad things but he said ......"when you get out of school you feel like you have the "answers"........but once your out of school the REAL lessons begin..... a mistake remains a mistake unless you learn the "lesson"........


I hope your problem is resolved in a reasonable manner and soon.


Best regards,

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The only fun spot to this entire saga is that he is staying in my hometown with friends (pop less than 2300) and his truck is sitting about 4 miles from where I work..... What are the odds....


Like I said, if you need anything, just give us a ring...

David, great talking to you the other day. It is a small world. Thanks for offering to help.

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Well finally some good news. NO damage to the engine block. NO coolant in oil!


They did say the CAM took a pretty good hit from the injector tube.


New CAM has been installed and they are putting the rest back together.


They did recommend that I replace the rocker arm bushings, (2) rocker shafts and one of the other injector push tubes.

Said these items have nothing to do with the injector nut backing off. I will be responsible for cost of those parts but not labor since they are already in there.


Should be done by the first of next week. Now have to figure out how and when to get over to pick the truck up.

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Rods and main bearings, as well?

X2 on that........It's "Almost-fun" (and cheap) to spin-in a new set of rod & main bearing when the pan is off and it is a cheap way to fresh'n-up the ole engine while you have a chance.......


On the other hand if the oil pressure is up and the filters look good there is something to be said for .........IF it aint broke DON'T fix it.........

It seems that the injector debris problem was caused by good intentions gone ......bad


Darned if ya do.......darned if ya don't........


The good news is that the "fix" did not involve scraping the entire engine.....


Drive on..........

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