I recently completed a replacement of the DEF filter on our 2018 Cornerstone (X15 Cummins engine). This is a relatively simple job, but one where it’s possible to make some significant mistakes if you’re not properly informed. This post will describe the process.
The first step was to determine the location of the DEF filter. On my coach, that turned out to be pretty much adjacent to the aft inboard corner of the bottom of the DEF tank, with the bottom of the filter housing a few inches above the bottom of the tank. Raising the tail end of the coach with the leveling jacks provided a bit more room to slide under there and have a look.
The filter housing is in a rather confined space, so there isn’t room to use a normal filter removal tool. Fortunately the filter housing has a hex head on the bottom of it, but unfortunately the head is larger than any socket most of us are likely to have in our tool boxes. If you want to check, look for a 1 & 13/16” or 46 MM socket. I didn’t have one, or anything even close.
I tried loosening the filter housing by hand and got nowhere. I finally improvised a solution to loosen the housing using a large Crescent wrench, putting the jaws of the wrench on the hex head vertically, and using another Crescent wrench on the handle of the first wrench for rotational leverage. That worked, but if you’re going to do this job yourself you’ll be far happier if you acquire a 1 & 13/16” or 46 MM socket before you start. More on that below.
With the filter housing broken loose, I did the final loosening by hand and dropped it down. To my surprise, there was no DEF inside the housing. That turned out to be by design.
With the housing removed, I expected to see the small DEF filter. No, all I could see was a black neoprene (I think) rubber cup, which turned out to be covering the filter. Pulling downward on the cup, it came loose along with an ounce or two of DEF.
With the cup removed, I could see the DEF filter. I tried pulling down on it, but it didn’t want to budge and I couldn’t tell what was holding it in place. Fortunately I found a relevant video that indicated it was necessary to apply a significant amount of downward pull. That worked and at last I had the old DEF filter in my hand.
Before going any further, let’s have a look at the parts described above. The first image below, to which I've added some annotations, shows the parts that came in the DEF filter replacement kit I purchased. Except for the “BIOLUBE" oil, the parts you see in that image are the same parts I removed – a filter housing with a sealing O-ring on top, the “rubber cup” referenced above which is called a “Frost Compensation Membrane,” and the DEF filter itself which has a sealing O-ring near its top.
Looking at the filter I had just removed, I determined it was manufactured by Mann Filters, a German company I’d never heard of:
hwww.mann-filter.com/en.html
The filter I removed was Mann part number U58/7. To obtain the replacement filter and associated parts, you can purchase a Fleetguard kit which is part number UF106 or Cummins kit which is part number 4388378. The best price I could find on either one of those kits was significantly north of $100.
Doing a search for a Mann U58/7 filter, I learned that part number has been superseded by Mann part number U58/9. I found a listing on Amazon which showed a Mann U58/9 kit for about $97. The seller was listed as Southwest Diesel & Electric, also known as Diesel Forward, Inc., located right here in the Phoenix metro area. I called them, verified they had the kit in stock, then drove over to their facility to make the purchase. They were kind enough to give me a bit of a break on the price for showing up in person rather than ordering online. The second image below shows the top of box containing the Mann Filter U58/9 kit I purchased.
Before installing the new filter and associated parts, I set about to acquire a 1 & 13/16” or 46 MM socket. I tried several local stores and came up empty handed. Went to Amazon and found a fairly large selection of 1 & 13/16” and 46 MM sockets. None of them were cheap and virtually all of them were 3/4” drive, meaning I’d have to buy a 3/4”-to-1/2” drive adapter in addition to the socket. I finally found a 1 & 13/16” socket with 1/2” drive and ordered that one. The third and fourth images below show the socket and the packaging it came in.
So now I’m ready to install the new DEF filter, right? Well, not so fast. By sheer luck I came across a Mann Filters document that describes in detail the proper procedures for removing the old DEF filter (too late for me on those steps) and for installing the new DEF filter. An electronic copy of that document is included below. The installation instructions start on Page 9 of the document and continue through Page 12. Several things to notice:
On Page 9, note that the sealing O-ring on the DEF filter (not the O-ring on the DEF Filter Housing) is to be lubricated with DEF, not the BIOLUBE oil supplied with the kit or any other lubricating product. I put a small amount of DEF in a bowl, then dipped the head of the filter in the DEF just before installing it. Installation of the filter is as simple as positioning the open end of the filter (the end with the O-ring on it) over the protruding aluminum rod and pushing up solidly on the bottom of the filter.
Page 10 covers the procedure for installing the Frost Compensation Membrane over the DEF filter. No BIOLUBE oil is to be applied to the membrane until this step is completed.
Page 11 specifies where to apply the BIOLUBE oil on the membrane and on the pump housing threads. I also applied the BIOLUBE oil to the threads inside the top of the filter housing.
As a side note, the instructions and photos show a spray bottle of BIOLUBE oil, but what came in the kit I purchased was a small tube, similar to the tubes Loctite comes in. No big deal – I’m sure it’s the same light oil either way.
Page 12 indicates the filter housing should be torqued to 80 Newton-Meters, plus or minus 5 Newton Meters. Those numbers translate to 59 foot pounds, plus or minus 3.7 foot pounds. That seemed pretty darn tight to me, but I went ahead and tightened the housing to 55 foot pounds using my newly acquired 1 & 13/16” socket. From there, I fired up the engine, let it run for five or ten minutes, checked for any leaks and didn’t find any.
That’s my story and I’m sticking to it. Any questions or confusion, feel free to ask!
Marine parts houses use to sell those sockets. Also someone mentioned they found one at Lowe's 
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