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Maintaining a Diesel Generator

Buying a diesel generator may seem like the easy choice, especially given their reputation for reliability, but choosing diesel requires a commitment to proper maintenance.

The reliability of any generator is dependent on it being well maintained, so on that account a diesel generator is no different. The type and amount of maintenance is what sets diesels apart, so only maintenance that applies specifically to diesels will be listed.

Ultra-Low Sulfur Diesel (ULSD)

diesel generator maintenance The worry of microbes growing in a storage tank has become more and more pertinent in recent years. Since the US government mandated that diesel engines only run on ULSD in 2007 the likelihood of finding microbes in a fuel storage tank and the need to have an aggressive treatment plan has grown.

Preventing growth and treating it if it occurs is not difficult. Most maintenance companies, National Power included, suggests fuel sampling every six months and treating the fuel with biocides whenever new fuel is added, or at least once a year. If microbes are found, depending on the size of the tank, the fuel can either be polished and cleaned or entirely replaced.

All of these maintenance protocols are easy to complete, but they do require diligence. Microbes can grow in as little as three months in untreated or insufficiently treated diesel and the risks they post to a generator are great if they get drawn into the engine.

Replacement Parts

Parts for diesel generators are usually more expensive than parts for natural gas generators. When you also take into account parts, like fuel filters, which unlike natural gas, are required in diesel units, the cost of parts maintenance for diesel generators is higher.

Wet Stacking

The buildup of fuel vapors and soot in the exhaust system ? known as wet stacking ? is another problem that is easy to prevent, but that requires diligent maintenance. Wet stacking occurs when a diesel generator is run at significantly reduced loads, which means the exhaust does not reach a high enough temperature to burn off all the fuel vapors. This can happen when a generator is poorly sized for the load or the set is routinely exercised without transferring the building load to the unit.

To prevent wet stacking the NFPA 110 requires that units be transfer tested monthly with at least 30% load for half an hour. If during regular testing optimal exhaust temperature is not reached to burn off the buildup, it is also recommended that a diesel generator be load bank tested once a year. This requires that, under the supervision of a trained generator technician in case the buildup ignites in the exhaust, the generator run at 75% load or higher for several hours with the goal of raising the exhaust temperature enough to burn off the unvaporized fuel and soot.

Trustworthy Fuel Supplier

This isn?t a maintenance issue, per se, but having a fuel supplier that you trust makes all the difference in the reliability of a diesel generator. First and foremost, someone has to fill the tank regularly. This means that your fuel supplier needs to be prepared to deliver fuel as promised regardless of the weather conditions. Suppliers should be able to maneuver a tractor trailer over icy roads into a specific space on your property. They should have in place contingency plans for delivering fuel in the event that roads are closed due to hurricane, earthquake, flooding or other natural disasters. And they should definitely have a backup generator on their tank farms to ensure they can fill their trucks.

How full does the supplies keep their tanks? It is generally recommended that fuel tanks be kept at least 80% full, because a lot of empty space inside a tank encourages the collection of condensation. If the supplier?s tanks are regularly below 80% full they could be transferring contaminated fuel to your tank.

For generators located in colder climates, you will also need to know whether or not a supplier treats the diesel to prevent gelling. Gelling refers to the gel point of diesel, which is the final phases in a three phase process ? the first two phases being the cloud point and the pour point, respectively ? of the paraffin wax that is present in diesel crystalizing. At the cloud point, which can occur around 40F, the wax crystals coat the fuel filter and rapidly reduce the flow of fuel. From there, as the temperature drops, the diesel continues to thicken and cause more problems for the engine, ultimately leading to the shut down of the generator if left untreated.

Gelling, like the growth of microbes, is easily prevented, but requires that a fuel supplier is diligent about treating the fuel.


This is not all the maintenance needed to keep a diesel generator operating reliably, but it should provide some insight into the extra care and attention these units require.


Call John Dixon at 800.790.1672 or to decide if a diesel generator is the right unit for your job. To learn about a service contract to simplify maintenance for your generator, contact Molly Logan at 919.256.3942 or

By Andrew Randall, Ron Towery, and Kara Odell