onan generator problems

[wshelton]

I have a 7.5 diesel onan with about 1000 hours. It has had its share of problems over the years, however a new one has poped up. It will start and run fine under moderate to heavy loads, however, resently it will suddenly bog down and barely even run or die. To offset this I quickly turn off what ever is close so it will power back up. Then reload it and it runs ok, but will bog down again soon enough. I am not overloading it either. When it bogs down, black grayish smoke comes out exhaust and soot or some other substance is on top of water.

 

[REBrueckner]

It sounds like you are describing that "...the generator runs normally, then will periodically bog down..".
The fact that is runs ok for a time confirms that it is NOT overloaded UNLESS a load comes on that you haven't identified, say a water heater thermostat calling for power. Or a room electric heater (That's what I most often forget.) Assuming that's not the case, does it run ok when cold, but begin to bog down when hot? That could be valves needing adjustment.

Otherwise if it sometimes runs ok when at normal operating temperature, I'd suspect a fuel problem. When it bogs down is it running smoothly or irregularly, as missing? If irregularly, as in some cylinders not firing, I'd guess it's being fuel starved. If smooth, I'd suspect overloading.
For what you describe, I'd first change fuel filters and be sure you drain the engine mounted cannister,if any, to remove any water. It's cheap and easy. The next alternative to have the fuel pump checked, then injectors...You can remove a return fuel line hose, stick in in a can, and see if bubbles are present, indicating an air leak, not my first guess of your problem.

Let's wait and see if others have had your specific problem...If you have additional knowledge of operating characteristics, post them since remote analysis is even more difficult than on site analysis.

 

[wshelton]

This is a beginning anyway.You might have a point on operating temperature. I noticed this the last time I used it. It did this soon after start up. One thing however. Several weeks ago it did the same thing after running for some time. At that time I was way low on fuel and later filled up. I did not have the problem after that till just resently. I thought it was starving for fuel that time. Now, when it bods down, I would asay it is running smoothly, not missing. Its like just runs slower and slower to way less then idel, and at that point I would say its missing and running irregularly. My fuel is clean I assume, the engines run fine from this tank. I have only one fuel filter on the engine no others with the exception of the filters on the mains. Temperature could be a factor although I did feel the waters exhaust durring this and it was not to hot to the touch.(Iwas in the Slip). More info needed about the fuel or valves?

 

[Will]

Black soot in exhaust can be a symptom of restricted air intakes. Is there anything like a loose insulation pad from the sound shield? Is there an air silencer or filter? A broken valve spring is another possibility but look at the easy things first.

 

[Nonchalant1]

It sounds like it could also be the governor. If you can replicate the problem, you can check to see if it's the governor or not. Get down next to the running engine where you can see and put your hand on the throttle lever. When it starts to bog down, look to see if the governor is giving it more fuel. If not, move the throttle to increase it manually. If the governor was not giving it more fuel when it started running slower, the governor is your problem. If you can move the throttle by hand and increase engine speed to overcome the slowdown, the governor is your problem.

However, if you and/or the governor give it more fuel and it doesn't speed up, it's an engine or fuel problem. The black soot and sheen on the water could simply be from too much fuel for the engine speed when the governor is working properly and trying to get the RPMs back up.

Doug Shuman

 

[DickB]

When this happened to me, I finally traced the problem to a build-up of snot just ahead of the racor filter. Why this gunk didn't make it into the filter, I don't know. It didn't obstruct the fuel flow enough to run under no load, but the minute I turned on a load, the engine would nearly die. My guess is that you have a similar problem.

Dick

 

[Nonchalant1]

Racor snot. I hate it when that happens.

 

[wshelton]

I did you silve the snot problem?

 

[DickB]

I disconnected the fuel line at the other end and blew it out. After priming, gen ran fine again. It seems that the fitting on the uphill side of the filter is a smaller diameter than the fuel line, thus, the snot wouldn't pass through. Must have picked it up from a low fuel tank. But it's never happened again.

Did you solve your prob?

Dick

 

[wshelton]

Have not been back to the boat yet,but will review all the input here,thanks to all. I hope it is the snot problem, that seems the least $ to fix. Any other suggestions welcome.