One cylinder mis-firing on QSC-500, low RPM only

Hampton

Air Defense Dept
TECHNICAL Contributor
Nov 26, 2006
7,628
Panama City, Fl
Boat Info
2008 44 Sedan Bridge
Engines
Cummins QSC-500's
Straight Drives
My 2008 Cummins diesel engines have just under 800 hours. They get used regularly, though not as much this past year as we have been building a house. We have used the boat more often in the last couple of months.

The #2 cylinder on the stbd motor is not firing at idle, but runs great above about 50% load.
1) Can hear the odd idle sound in exhaust - missing
2) Placing finger on fuel line provides very little vibration on #2, all other cylinders kick the fuel lines a little
3) At 1000 RPM, out of gear, cylinders measure about 165 degrees, #2 measures about 150
4) In gear low RPM, port engine load is about 30 and stbd is about 45
5) As you advance the power, it feels like the stbd prop is bent - pulled boat and tuned props
6) After about 50% load, the loads level up (stbd vs port), vibration goes away.
7) Tested wide open 2680 RPM each (same as always). 25 gph each. 33.5mph. 98% load.
8) After wide open test, load at idle in gear was closer to port, about 30/40 instead of 30/45.

Yes, I need to change the fuel filters - I have them, and am definitely behind on this.

So, I assume it is an injector issue. I'll change the filters and over-treat the fuel.

1) What do I treat the fuel with?
2) Will I do damage to treat the fuel and run the boat?
3) If no change, what are my next steps?

Thank you.
 
Hey John - Welcome back! FrankW has posted the fuel treatment plan on the home page here. Sure does sound like an injector. On my Common Rail MANs, the only solution is call the dealer and have him bring the laptop - I bet its the same for your QSCs...

Are you still flying the Bus...?
 
Hey John,

With the new Forum software the Fuel Management Article has been moved to the section entitled "Tutorials". All the additives recommended except the Stanadyne Lubricity additive are available locally in PC......Amazon sells Stanadyle Products so you can get a case by the weekend.

None of the chemicals recommended will over treat your fuel, but if you have not been using additives or Valvetech fuel you may have some sludge in your tanks. If that is the case, it may take several sets of Racors to clean up the sludge after you treat the fuel.

But, I don't think that is the problem. If it were, the issue wouldn't be localized to one cylinder and you would run out of fuel and couldn't reach your normal WOT. If these were 6CTA's the solution would be to send the bad injector to Pensacola and have the one bad one cleaned and pop tested to see if that remedied the problem. But, with QSC's this could be an injector problem or an ECM issue. It does sound more like an ECM problem than an injector since the miss goe away and clears at higher rpms. This means you will have to get a Cummins tech on the boat.....not a general diesel guy but a Cummins certified mechanic that has access to the Cummins software so he can run diagnostics on the engine and reflash the computer if necessary.

Finally, I wouldn't run the engine with the miss. If there is a problem other than fuel delivery, the cylinder that is missing can "wash out", meaning that the fuel injected but not burned will wash the lube oil off the liner walls and cause rabid and premature wear.

Hurry up with the house.......we miss seeing you guys on the water.
 
And, get an oil analysis; if you are down on compression it will show up there.
Low compression will not fire at idle but the engine will seem to or possibly run fine at higher RPM's.

Tom
 
Thank you both. Not the fuel. I have always treated regularly. I have gotten oil analysis on every change since we bought the boat. Time for another one.

Frank, do you have a name of a guy I can use?
 
Give the tech a s/n and have him bring an injector, connector and cover gasket. Prob save a second trip.
 
John,

The marina has used Jodie Kennedy (Marine Maintenance) and John Hice as well as Cummins South East. If Roland uses them, they must be decent. Cummins is too far away so multiple trips get expensive since you are paying them to come from Mobile. John Hice drives a $90K Tesla to work and it bothers me when the mechanic drives a better car than I do, but that is a personal thing. I'd use Jodie Kennedy if he can get to it.
 
But, I don't know wht you don't get an injector from Cummins and change it out yourself. They are easy to do. On your engine the injectors don't have to be timed; they just plug into the wiring harness and don't need to be registered in the ECM.

I'm pretty sure that is what you are delaing with and a new or remanned injector eliminates the most likely cause of the low speed miss, given that your oil samples are ok on a regular testing basis. If the injector doesn't clear the miss, then you deffinitely do need a Cummins tech involved.
 
But, I don't know wht you don't get an injector from Cummins and change it out yourself. They are easy to do. On your engine the injectors don't have to be timed; they just plug into the wiring harness and don't need to be registered in the ECM.

I'm pretty sure that is what you are delaing with and a new or remanned injector eliminates the most likely cause of the low speed miss, given that your oil samples are ok on a regular testing basis. If the injector doesn't clear the miss, then you deffinitely do need a Cummins tech involved.

Now that is the kind of advice I was hoping to receive! I'm losing sleep over the compression comments above - that doesn't make them wrong, just not good news. It is time for an oil change, so, my historical samples don't mean a whole lot at this point.
 
Now that is the kind of advice I was hoping to receive! I'm losing sleep over the compression comments above - that doesn't make them wrong, just not good news. It is time for an oil change, so, my historical samples don't mean a whole lot at this point.
Oh-but they do. Before the oil change, as long as that oil has been in the engine when the miss was occurring get the samples; you will be able to do a comparative analysis with samples before the miss and during the miss. Leave no stone unturned on your root cause evaluation. Not to cause you loss of sleep; but should the samples come back comparatively high in iron, aluminum, and chromium have the engine scoped. There also is the case where a valve seat dropped but that would cause all kinds of chaos that you would definitely hear.
 
I understand. My point is, until I do the new samples, the old ones don't help. Your points are well taken.
 
Mike from Detroit Marine came out and hooked up the computer last week. It seemed like #2 injector was bad. He changed it out today, and the problem is solved. They will be coming out to remove the after coolers for servicing after the first of the year. The old injector had zero signs of damage or corrosion.

After re-starting the engines, they ran very well (at dock - never was a problem under load). The starboard fuel flow came down after running it a while, but is still consistently 25% higher than port and the loads are similarly different - a little higher on stbd (stdb injector replaced). There may be air in the system still. He thinks I need to take it out and run it for a while and then check the loads/fuel flows.

I'm wondering if the computer isn't still self-correcting from the bad injector or something like that. I'm hoping it figures it out in time. I'm wondering if it really is an accurate reading. Mike said he thinks the fuel flow is calculated from the load. I didn't ask how the load is measured - I thought it was calculated from the fuel flow.
 
Why not a Cummins tech? Can you get a boost reading from your engines? They will directly reflect the true output of the engines. if the boost is also higher on that engine then it truly is working harder. Also, did he perform valve/bridge sets? HPCR systems will bleed themselves out quickly as the high pressure fuel is supplied direct to the injector which is the highest point in the fuel system. Unless you are having problems on the supply side, air should not be an issue. Fuel can be lost between the injector and connector which will bleed out of the fuel return line at the back of the head. There should be no fuel coming out of that passage during operation. I deal more with QSL's and have found noticeable performance gains with simple valve and bridge sets.
 
He is a Cummins tech, I think. Don't know about the rest.

Mike Owens is a Cummins, Caterpillar, Detroit and MTU certified mechanic with 30+ years experience on marine engines. He happens to work for Florida Detroit Diesel because they acquired his previous employer's local branch location. Mike is so good that there is a 6-8 week waiting list to get him. I wasn't there, but Mike is one of those mechanics that leaves no stone unturned and I can guarantee that he did any and all testing he felt was necessary to fix John's engine.
 
John, I thought my old Cummins service tech told me the fuel flow was an algorithm with RPM as the prime input. I'm not sure how engine load is derived.
 
Mike Owens is a Cummins, Caterpillar, Detroit and MTU certified mechanic with 30+ years experience on marine engines. He happens to work for Florida Detroit Diesel because they acquired his previous employer's local branch location. Mike is so good that there is a 6-8 week waiting list to get him. I wasn't there, but Mike is one of those mechanics that leaves no stone unturned and I can guarantee that he did any and all testing he felt was necessary to fix John's engine.
That's great that he is getting one of the best. Hope my comments didn't come across as questioning his work. Cullen/Detroit is also the VP marine dealer in this neck of the woods. We do Cummins and VP off highway/industrial engines. Lower horsepower ratings with higher duty cycles.
I also made a mistake above in reference to the HPCR fuel system. It is used on the QSL's. The QSC's in our applications use the Bosch rotary style VP44 fuel pump.
Hope you can get your problem sorted out quickly.
 
Should also add that if you haven't replaced your fuel filters yet as mentioned in your earlier post, then may want to do that before further testing. A restricted supply to the VP44 pumps can retard injection timing.
 

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