I'm working with an NGO in Guinea.  There is an SDMO J30 (30KVa 1500 rpm, John Deere Engine with a Makalte Alt.) unit at an office up country.  I have a lot of troubleshooting experience with gas engines but little with Diesel engines.  I've yet to see it but I'm headed out this weekend, so now I'm trying to figure out what it could be.  I've been told it runs fine but every 15 or 20 minutes the power output fluctuates (up or down not always the same) but the engine speed remains constant.  My first thought was the alternator acting up (bad bearing or the like)  But after reviewing the maintenance manual it seems to be a pretty hands off on the alternator end (sealed bearing).

So that leads me away from the alternator.  That leaves the engine.  And if its engine trouble here its almost always contaminated fuel or problems associated with contaminated fuel.  The fuel supply here is dirty at best and full of dirt, rocks, sticks, and leaves at worst.  But aside from a plugged up fuel filter or a clogged pump / injector what else could go wrong.  Most everything I've found with a Google search lists contaminants but not problems associated with the bad fuel.

Another thought I had was a big load kicking in every 15 minutes or so.  But the biggest thing they have a AC units, and it could be one of the units' thermostats kicking in about every 15 minutes, but if that were the case the power would fluctuate more frequently in the heat of the day not the semi-constant 15 minutes between drops or spikes.

Another issue is maintenance or lack there of.  This unit was bought used from another NGO in the area and was "in perfect running order" when sold.  So as far as I know its always had this problem.  There is little or no documentation about what has been done to this unit, so I assume aside from an air filter or a fuel filter every now and then nothing has been done.  I am unsure as to the units hour count but I'm assuming its not new but also not completely worn out, I'm going to attempt to find out tomorrow.

So my question is could dirty fuel be causing the power drops/spikes?  

Am I overanalyzing this too much?

To answer your basic question, yes, dirty fuel can cause all kinds of grief.

Let's step back a bit,  Are the fluctuations in frequency (hertz) or in voltage?  And engine problem with varying frequency will drive some voltage instability, speed drops, voltage drops, speed goes back up, so does voltage.

Voltage regulator problem could look similar, voltage drops, unit unloads a bit, speed goes up, votlage raises, load increases, speed drops a bit.  Hard part is seeing what is leading what with limited tooling and equipment.

In your situation fuel is the most likely culprit, units operating in those conditions should have large properly sized primary fuel filters with water seperation, preferably double element style with isolation valves so filters can be changed while unit on line.  Racor or Dahl or similar type centrifugal filters may not work properly because these type filters depend on a minimum fuel rate to effectively remove the water and gunk from the fuel.  Smaller engines with fairly low fuel flow rates work best with coalescing type primary fuel filters.

The primary filters should have a higher micron rating than the secondary or final fuel filters on the engine before the pump.  Most older and smaller engines have secondary fuel filters with a nominal micron rating of about 10, newer engines can go down as low as 2 microns with "high efficiency" fuel filters. Most primary fuel filters should be between 15 and 30 microns, and fitted with a restriction gauge to give you an idea when they need to be serviced.

The problem could be triggered by a load change, and a poorly responding engine makes the issue appear worse.  Start with the basics, good clean fuel, proper supply pressure, and try to make sure all the cylinders are firing.  Sometimes you can tell by cracking a fuel line at an injector loose, and seeing if the engne speed drops a bit, indicates that cylinder is firing, if you crack a line and no change, that cylinder is likely playing nice with the others.  Whe you crack the lines, observe if there is air bubbles, sometimes when engines plug filters, the fuel system cavitates and generates bubbles in the fuel system, causing poor performance.

Once you feel fairly comnfortable with the fuel system, make sure the air intake and exhaust system is ok.  Make sure the air cleaner is ok, sometimes removing it breifly while unit is under load gives you an indication if the restriction gauge is gone or bad.  Check the exhaust, small units runnng lightly loaded can cause exhaut system to plug, affecting engine breathing.

If it has been a long time, or you're just not sure, the valve lash adjustment being incorrect can also cause performance problems,  This is fairly easy to check if you have a manual and some basic tools.

I'd avoid messing with the fuel injection pump unless you have checked everything else, all in all in the small John Deere engines have pretty robust fuel injection systems and they usually survive a good bit of abuse.  It's not impossible, just not usually the first thing that fails.

If you get lead to think it may be the tail end, check the field output with a meter (F1 and F2 or F+ and F-), it will be a DC voltage between 10 and 20 VDC usually, see if it is steady, jumping around a lot, or seems real high.

That's pretty much the basics for a unit that size.

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