I finally got around to replacing my fuel filter today. For those of you unfamiliar with the M35A3, here are a couple pictures showing where the fuel filter is, the filter being yellow and toward the rear driver side of the motor, and the fuel/water separator valve being red and accessible under the truck behind the front axle:
The fuel filter in the picture looks damaged because I was already working at it with a pair of large channel lock pliers.
So here's what I did:
1) I set up my workspace. I anticipated a mess and put a large oil pan under where the fuel filter is mounted, mostly from unscrewing the valve from the bottom of the fuel/water separator.
2) I opened the drain valve on the fuel/water separator. As I expected, nothing would come out, so I started to unscrew it from the fuel/water separator housing. I used an 11/16" wrench to loosen it, wrenching it to the frame one time, then got it off the rest of the way by hand. Once I got it off, diesel fuel just poured out. It made me realize that my oil catch/storage pan was too shallow and fuel was splashing around the surrounding ground, so I got newspaper to help absorb it.
3) Thinking I drained all the fuel out of the filter, I started loosening the filter. Like many have reported with their trucks with spin-on filters, it was on really tight! I tried loosening it by hand, but that didn't work, so I used a large pair of channel lock pliers to get it loose. It took several bites, during which I scraped some paint off the filter and even crushed a portion of the housing. Once I got it loose enough to spin off by hand, even
more diesel fuel gushed out from the filter, splashing to the pan below. Even after that, there was
still some fuel trapped in the filter, and it took some tilting of the filter to get the rest of the fuel out.
Here's the fuel/water separator from the top without the filter:
Looking down into an oval hole in the fuel/water separator, I could see there was still diesel fuel in there. I didn't try to flush it out as it looked pretty clean despite the dirt that was blasting out of the valve any time I opened it with the engine running. My guess is that dirt was actually coming from the fuel filter. But I don't have a complete understanding of the flow of fuel through the filter and how the water separator catches the water during engine operation.
4) I prepped the new fuel filter. I bought a Cat 7E-9763 filter as the parts TM specified. I dabbed my rubber-gloved finger in some clean-looking diesel fuel from my catch pan and rubbed it on the filter's o-ring. I also wiped the mating surface of the fuel/water separator clean with a paper towel.
Here's a picture of both filters, the new one being on the left:
It's a different filter than what was installed, which was a Cat 1R-1740. My guess is that this is a filter specified for the FMTV, and that it works for this engine too. I'm not sure if the Army put it on or if the auction winner did it. Considering this filter is at least 10 years old, I want to get it cut open to see what its insides look like after all those years!
5) I spun the filter onto the fuel/water separator, using only my hand. The Cat pictograms on the filter show to do it in this fashion. Like a whole-house water filter, it seems you don't have to tighten it like crazy with a tool like the Army or the other guy did. I did tighten it as best as I could, again using only my hand. This will make it easier to get off with a regular spin-on filter wrench the next time I need to change it.
6) I then examined the drain valve I took off earlier. It is a little ball-type valve. In the open position, I couldn't see the sun through it, so it was definitely blocked and probably the source of my no-drain problem. I started with using a 0.5 mm allen wrench to see if I could clear the blockage. The wrench passed through the valve easily, but it wasn't bringing out much with it. I then tried a pipe cleaner meant for my kid's crafting projects, but the bristles were too soft to get anything out
So I rolled up a small piece of paper towel and spun it through the valve like a drill bit. And here's the result:
That collar that formed on the right end of that piece of paper towel was the gunk causing my blockage! I looked through the valve and was satisfied with how clean it looked inside, so I screwed it back onto the fuel/water separator. I snugged it with a wrench, careful not to make it too tight so as to strip any threads.
7) I started the truck. It ran for a minute with the fuel between the fuel filter and its discharge line. I was surprised it started, but then it died a minute later. To prime the fuel system, I followed the directions in the operator's manual with a little extra care. The manual says to crank at most 15 seconds, and then let the starter motor cool for 30 seconds. Having not received any taxpayer money for the work I've done with my truck, I was a little more careful and cranked the motor for 10-12 seconds at a time, and resting for 30-60 seconds after each cranking session. After about 8 minutes of this process, fuel made it to the motor and the truck roared to life!
I checked for leaks at both idle and about 2,000 RPM. Regardless of engine speed, I saw no fuel leakage at the filter or the drain valve
9) After I shut down the engine and let the truck cool down a little bit, I tested the drain valve. Upon opening the valve, fuel poured out for maybe 3-5 seconds, then stopped. I waited with the valve open to see if more came out, and none did. For kicks, I closed the valve, waited 10 seconds, then re-opened it, and more fuel came out, and in this case and every subsequent try today, it would stream for 3-5 seconds, then pause with a "GLUG GLUG GLUG" sound like air was being purged from somewhere, and more would come out. If the stream really stopped, I would close the valve, wait 10 seconds, then open the valve. The same thing happened.
I'm not sure if this is how the fuel/water separator is actually supposed to work because the operator's manual makes it sound like the fuel should just drain out without much fuss. Perhaps there's more gunk in the fuel/water separator, but I didn't feel like taking it out of the truck today, so that might be a project for the next fuel filter change. That being said, I haven't seen any water come out of the fuel/water separator, and now there's no dirt coming out of it either
I hope other truck owners find my documentation helpful. I like how the filter does mount upside down to discourage pre-filling the filter. Just do it by the book and you'll be okay!
