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Onan 12,500 diesel generator runs very rough.

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Piros1

RVF Regular
Joined
May 3, 2021
Messages
6
So I have a 2015 Newmar Essex with a Onan 12,500 diesel generator. It starts easy but runs very rough similar to gas engine with the choke partially closed. There also is a lot of smoke coming out of the exhaust and leaves a black residue on the ground under the tail pipe. History, I bought the Coach used and it was running rough when I purchased it in 2020, I have no history regarding the previous owner or the coach but it was very clean when I looked at it prior to purchase. What I have done so for is to replace the air filter and check to be sure the intake system was clear of obstructions, oil and oil filter change twice, new belts, three new injector nozzles, and cleaned the radiator (also I have replaced the fuel filter twice). It is putting out the correct voltage but under hard load in high altitude it will not pull the load on my last trip. In lower altitudes it is fine as for as pulling the AC’s and such but still runs very rough. I have discussed with others and with a Cummins Onan technician and a couple mechanic that did not really have an answer. Cummins suggested bringing it in and let them put a hard load on it for a couple of hours. This is my second coach with the same generator and my previous generator ran very smooth.
Has anyone ran into this with their generator?
 
Did you happen to replace the fuel filter? I didn’t see that listed in your post.
 
Regarding High Altitude, the Onan Operating manual has a page outlining high altitude Ops.
 

Attachments

  • onan genset.pdf
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Assuming the smoke is Black, rather than Blue Or White, a few things come to mind other than improper fueling/air supply.
From what you've said good fuel and fuel metering (correct size injectors) is not an issue.
Possibly a mechanical issue such as low compression (bent valve, bad seat, Broken ring) or injector timing.

Any blowby should be obvious, typically accompanied by Blue Smoke. Injector timing might be worth looking into. I've attached a generic troubleshooting guide that might contain an idea for you.
 

Attachments

  • diesel problems.pdf
    2.5 MB · Views: 8
Thanks, definitely black smoke. The generator only had 250 hours on it when I got the Coach. It has 635 now. I do understand that high altitude typically makes the engine run richer. My previous coach with the same generator ran a little rich when in the high altitude but never bad enough to be concerned. Usually we would only be in the high altitude for a short period.
No blow by or oil consumption.
Thanks for the diagnostic procedure chart, I will check through it when I head up to work on the RV shortly.
 
Possible I am way off base here, my only experience with tuning is with gassers. My diesels have thankfully only required fluid changes.

Thinking back on your comment regarding the engine acting like a "choke" is stuck on.

I've been looking for a ECU schematic for the ONAN generator. No luck yet.

Does the ECU/Control board require a temperature sensor to adjust injector timing/dwell? Is there a thermostat to regulate temperature of engine?

You could properly test the coolant pipes with a thermal gun to assess temperatures. See if engine comes to temperature, and when the thermostat opens.
 
To answer your question I really don’t know if it requires a temp sensor to regulate the engine outside of a conventional thermostat. You could be onto something that I should dig deeper into.
I did have some conversation with a Onan technician and his thoughts were to put a hard load on the generator for a couple of hours. I think that is how they normally start their diagnostics in this situation. He did not mention any sensors but since this unit has been acting up since I have had it it may be in the system controller or related such as a sensor. Like I stated it only had 250 hours on it and now only has 635 I think.

Thanks for the thoughts, I will research this.
 
So went through a lot of diagnostics as well as checking everything that has been done. What we have found so for is the valves were adjusted to tight and the engine seems to have a lot of carbon build up. We have properly adjusted the valves and soaked the cylinders in Marvel oil for three days, also swapped one injector nozzle in one cylinder that did not appear to be running the same as the other two. It is running much smoother now and will know more after the trip I am on now.
 
Tight valves can be cause for lower compression as pushrods heat up and grow in length. More fuel being added to compensate for lower horsepower output would also make for more carbon build up. There's a good chance the carbon build up can take care of itself if compression is allowed to build correctly and you can get it up to full operating temp then load it up well for the biggest bang and max airflow through the cylinders, across the pistons and the combustion camber. Running them tight can also keep the exhaust valves from seating where they dissapate most of the heat they see, this can result in a burned valve, mildly where the seat and face are a little eroded to looking like a cutting torch was taken to an edge.

I would perform or have someone perform a compression test to confirm everything is where it's supposed to be after getting at least one solid running hour on it.
 

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