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Changing fuel filters on a diesel engine

by Tom Jones

I asked about the correct method of changing fuel filters on a diesel engine. Forum members set me straight.


When I changed the secondary filter on the coach, I read in both the Monaco owners manual and the Cummins manual that I should not fill the fiter with fuel before re-installing it. They went on to say that, after changing the filter, I should expect the engine to take some time to start, and that it would stall several times. (It doesn't run on air after all). The filter also had a statement that it shouldn't be filled if used on an ISC engine. Sure enough, the engine started after some cranking, then died several times before finally running continuously.

I checked the manuals before installing the secondary filter; Neither said anything about filling or not filling the filter, but the filter itself said to fill it. What to do? I got a can of diesel and, just as I was about to fill the filter, I changed my mind. I screwed it on empty, fired up the engine, and sure enough it stalled several times before finally running continuously.

Anyone know why the secondary filter should not be filled, and whether the primary should or should not be filled prior to re-installation?


Bernie Dobrin
The filters used flow from the outside to the center and the oil flows from the center of the filter to the engine. Now if you fill the filter, you pour the oil down the center, so the oil you pour into the filter is introduced to the engine unfiltered. That is why the engine manufacturers say not to put oil in the filter.

Leo Everitt
Filling fuel filters in engines built prior to new series was the recommended way and can still be done. However, the newer engines have fuel lifters and pumps built to much tighter tolerances and any contamination could be fatal. Therefore, the change in recommendation. I think it would be a good thing for older engines as well, Too many variables like fuel condition and container cleanliness involved.

My sources at Cummins have told me that if its a new electronic engine then don't fill the filter even if manual says its ok to do, as it did in early (99) electronic series engines. This issue came up because people were not using clean containers and problems ensued , They'll say to someone they know on an unofficial basis that if your using a clean container than its ok - but then in next breadth say why not let the lift pump do it (assuming one has a lift pump).

Terry Brewer
I fill both my fuel filters before installing them even though the manual says to install them dry. I do go to the gas station & get fresh fuel though. I also strain it before I put it in, using a semi-permeable, reverse woven, highly sterile medium...Cheesecloth. Not trying to filter out Guardia only the big chunks if any. I'm sure though someone will now say I am introducing fibers of some sort into the fuel & should have my diesel driving privileges taken away. I will be willing to bet you no diesel mechanic installs them dry. Before I started doing my own I watched them fill the filters at a Cat dealership before installing them, same with the oil filters.

Steve Pally
Having learned about the direction of oil flow in oil filters a while back, when I change the oil filter on our coach, which holds nearly a gallon of oil, I carefully fill the filter from the outside holes. I would feel uneasy to allow that size filter to be installed dry.

Wayne Malone
I fill my fuel filters with mineral spirits direct from the can. Always clean. Seems to work for me. Exhaust smells kinda different until it is all used up.

Bob Zambenini
When I got my diesel a few years ago I heard this discussion about filling or installing dry. I had to change my filters because I got service at a dealer in San Diego and a short time later realized he did not change my flters! (he is no longer a Newmar Dealer!)

So I got my diesel fuel in a clean can from a local gas station and got one of my wife's many jars she cleans, runs through the dishwasher, and poured the diesel in the jar. Held it up to sunlight and low and behold it was absolutly clear. So I poured it into the filter and all was well. If it had had any visible specks I was going to install dry. Someone might argue that my eyes are not good enough to see back actors in the fuel.

I have talked to Freightliner places since and they all have claimed they follow the Freightliner direction of installling dry but you never know.

Also on the service station issue, I always try to use Loves or FJ. A neighbor who had a diesel motorhome for years told me if you see a truck filling the tanks drive on by because that stirs up the junk in the bottom of the tanks. That I have found to be totally impractical because at a FJ there is nearly always some tanker there filling the tanks and here I am in line and supposed to drive on!!!! I quit worrying about that advice.