Changing fuel filters

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Well-known member
Dec 23, 2010
Washington State
I am going to change the fuel filters on my 450 Cummins both the primary and secondary sides.  Is there anything that I need to take into consideration before attempting this other than don't fill the cartridge before installing and letting the fuel pump do the work to purge the system?  If the motorhome has been sitting for a while is there any pressure on the system?
Do the primary filter first, then prime using the electric lift pump (cycle the key several times) and start. Once you are sure it starts and runs ok with the new primary, shut her down and do the secondary. Repeat the process to prime and start. Some people like to fill the primary with fuel first (I don't), but never do that with the secondary - too much risk of introducing dirt.

I wouldn't change the secondary yet on a one year old coach - should have lots of life left in it if the primary is doing its job.
Thanks for the reply.  The only reason I am doing the secondary is it is recommended once a year or 40,000.  So I am wasting money replacing the secondary?
I have always changed both filters every year.  Cheap insurance.  And we carry a spare set on the road.
I think so, but you have to make your own call. One year of motorhome usage is rarely enough to dirty up a secondary, even if you picked up a bad tank of fuel. Most dirt still gets caught in the primary. Note that the mileage recommendation is 40k.

My coach doesn't even have a secondary. I replace the primary annually.  Once I did not and didn't make it half way through  the summer before a change was required for performance reasons.

Just for clarification, on mine I have one right at the engine, and another with the water separator back behind the rear radiator.  Which one is the primary and which is the secondary?  I assume the fuel flows through the one at the engine last, but I didn't really check when I had it open last week.
The filter with the water separator is the primary filter and will typically be a 10 or 20 micron filter.  The secondary will be a 2 micron filter and is after the primary.
Great!  Thanks, Ned.  That's what I thought, but I was a little uncertain because when I had problems with losing my prime last week it was suggested there might be a hand pump on my primary filter.  Turns out it was actually on my secondary filter.
The "primary" is the first filter in the sequence, i.e.the one closest to the tank. It usually has a water separator and, in a two filter system, will typically have be a coarser (higher microns) filter than the secondary, which is close to the injection pump (at the engine). In a single filter system, which was widely used from (roughly) early 2003 thru late 2004, the primary is the only filter and it combines a water separator with a low-micron value filter. "Low" is usually considered to be 2 micron, while high is typically in the 10-20 micron range.

The micron value of a filter is the smallest size particle it [mostly] traps. I say "mostly" because no filter is 100% effective. Some may be as low as 50% but most are more like 90-96%%.  A better quality filter will generally trap a higher percentage of the specified size and the very best will be about 98.5-98.7% effective.
My Cummins manual states that on my 450 the one with the water separator is the Secondary,,Suction side and the other filter is the primary, pressure side.
Since the filter on the engine is the only one that the engine manufacturer installs, they typically call it the primary fuel filter. This is generally where you'll find the primer pump for the brands that have a hand operated factory primer pump, (whether installed initially or sold and added later) and most "add on" filters don't supply such an item. The coach manufacturer typically decides if and where the "additional" filter is placed. They also are the ones that decide if an electric primer pump is installed in the system.

And that my friends is why one always has to ask and clarify which filter you are talking about. While one coach may have one system, another may have something entirely different. If/when you ever talk to the engine factory guys, they will think their filter is the primary, not having any idea of what other filters may be inline on the crawler tractor, OTR truck, MH, diesel powered generator, or whatever other use that engine is set up for. And if you're involved in a warranty claim they want to know that the correct factory specified filter has been used.

You may decide in your own mind which is primary and which is secondary. And you can certainly tailor your filter choice to work as you wish, 20 or 30 micron in the first filter the fuel goes thru and 2 micron in the next. (good luck with deciphering the part numbers into micron ratings by the way) But every time you talk to someone new about your particular rig, I bet you'll talk about the one mounted on the engine or the one mounted on the chassis/bodywork somewhere.


I suspect bucks2 has the right of it. Chassis manufacturers seem to call their "first" filter the primary, meaning the one closest to the tank. Cummins obviously has a different view. Plus, some engines come with an inline filter attached, while others may merely specify what filter(s) must precede the injector pump. There probably isn't a universal answer.
Both of my filters are mounted on the engine about one foot from each other and very easy to get to.  Thanks for the interesting discussion.  Everyday that I work on the coach the more I learn.
Not a big deal if they are both easily accessible. Just be super-careful with the filter closest to the injector pump, cause the slightest fleck of dirt in the fuel can destroy the pump.
I have a Cat C-7 2004 engine with the 2-filter system.  The secondary filter on the engine was changed for the first time way back in 2005 by the Freightliner Oasis shop in Tulsa and again about 4 years ago by the Cat shop in Wichita when the valves were adjusted (I changed it myself inbetween).  Both shops pre-filled the filter since there is no primer pump on the engine and since they did it that way I assumed that was the correct way to do it and have been doing so myself.  I couldn't see exactly how they did it but I have a plastic plug to screw into the center hole of the filter and am very careful to be sure  the fuel only goes into the outer small holes.

Since I don't have a primer pump is there a different procedure I should be using?  And if so, how do I keep the shop techs from pre-filling the filter?  Most shops won't even let the customer remain in the coach let alone be in the stall area where the work is being done.  They always claim it pertains to insurance liability but I kind of feel they just don't want the customer seeing some of the things they pull.
Pre-filling is still common practice but risky if not done very carefully (as you do). Back in the day, it was standard practice to pre-fill both filters, but problems happened from time to time and engine manufacturers started to add priming pumps so it would not be necessary to take the risk.  At first they were manual pumps, but the introduction of electronic engines brought electric lift pumps that are capable of priming the entire fuel system.

Anyway, I doubt if you can prevent a tech from pre-filling if that is their practice and many oldtimers still insist it is the only proper way to change filters. Don't trust no steenkin' lift pumps! The best solution is to keep on doing the filter changes yourself. At least for the engine side filter.
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