Prefilling Fuel Filters on Volvo Equipment

The Issue with Prefilling Fuel Filters

No maintenance technician wants to spend more time than needed on a service job — and certainly no construction equipment owner wants his machine offline and unproductive longer than necessary. But sometimes technicians take short cuts to speed things up, and one of the more common shortcuts I hear from our dealer technicians is customer technicians prefilling fuel filters. The damage it causes is immediately noticeable.

Why do some techs prefill fuel filters? Normally, when a new fuel filter is put on, a hand primer is pushed to bleed the system. Whether you pump by hand or electric, it’s time-consuming work, but it’s the only way to bleed the system with absolute clean fuel. If a tech prefills the filter before he puts it on, he theoretically can start up the machine right away without having to prime the system and, in essence, saving time. But what he’s more likely doing is kick-starting the process of damaging bearings and destroying injectors.

Here’s why: With a fuel filter, the fuel enters from the outside of a micron filter, travels through it to the inside, and then passes into the fuel system out of the center hole (shown here). When most techs prefill a filter, they pour the fuel into the large hole in the center because it’s the easiest place to do it (versus the smaller holes along the outside). But the second he starts the engine, the non-filtered fuel goes directly into a high-pressure fuel system, and these systems won’t tolerate even the smallest amount of contaminant.

It only takes one time to start causing damage to an injector — it’s a non-forgiving action. If a tech does it just one time, he could start to depreciate the injectors, leading to very expensive repairs that likely include an initial diagnosis trip plus a follow-up trip to replace the damaged injectors. Depending on the engine type, this could take six to eight hours to complete. Tack on machine performance issues that are likely occurring before the service trip, and you can start to see the costs add up.

Some techs assume they can be very careful and prefill a fuel filter without introducing contaminants, but the likelihood of that happening is incredibly low. Fuel filters have micron measurements that are smaller than the human hair. Even if a tech tries to cap the center hole and let fuel drip into the “dirty side” of the filter, it’s virtually impossible to keep contaminants out of the “clean” inner side of the filter. A fuel can, a funnel — even bare hands — will likely introduce some kind of contaminant.

If you’re an owner of equipment, take some time to fully understand how your service techs install your fuel filters. If they’re prefilling to save time, talk to them about the risks, both from a machine downtime and a repair cost standpoint. At Volvo we recommend installing filters dry and pre-priming them for contamination control — it really is worth the time to do it properly. And if you have any questions about prefilling fuel filters, leave a comment below, and we’ll follow up with you.

By Al Drake, Volvo Field Technical Support Specialist (FTSS), Western U.S.


  • Harley Costantino

    does this article also apply to engine oil and hydraulic filters?

    July 11, 2019
  • Al Drake

    Fuel filters are the most critical. There have been debates if pre-filling engine oil filters is a good practice. Majority say no. Hydraulic filters most commonly filter oil returning to the tank. Hydraulic systems with filters installed in the inlet are below the oil level in the hydraulic tank. Gravity forces hydraulic oil into the inlet filter.

    July 15, 2019
  • Allen

    I have been doing that for 15 years. Never knew. Guess I was lucky. Won’t be doing that anymore.

    August 27, 2019

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