August 12, 2008

Changing the fuel filters on a 300D.

Since the week I bought the car, I have had both the main fuel filter and the pre-filter bouncing around the rear footwells of the car. I turn, they turn too, it's all fun and games until I hit a hill and the car essentially goes nowhere. Clearly the hour to change the filters had rang. So I naturally put it off for a bit. I got around to it this morning, it's a fairly simple job.

I changed the pre-filter first. It's right by the injection pump on the driver's side of the engine. It has a line on either end held on by a ring.

Unscrew the ring, take the filter out, put your finger on the end of the line and quickly put the end of the new filter in there. Set the old filter down above the headlight. Realize it's leaking diesel and toss it on the ground where it will continue to leak. Make sure the new filter is in all the way and tighten the ring. Repeat the process for the other end of the line.

You've now changed the pre-filter. The main filter is a bit different. It looks just like a regular oil filter:

That said, you might be tempted to grab your oil filter wrench and attack it. That would not do much good. On the photo above, look at where my key is pointing-- that's the bolt that holds it in. Now, as much as I'd like to tell you what size that is, I have no idea. It's larger than a 19mm, I didn't have anything larger on hand and didn't fancy digging through my garage to find one so I used an adjustable wrench, worked fine.

Undo the bolt, you'll see the filter slowly start to drop. This is full of diesel and, much like the last one, will leak. Let it leak onto the pavement for a bit until you decide to use the roll of paper towels you brought down for this very purpose.

Once the top bolt is loose you can unscrew the filter. Toss it next to pre-filter where they will leak in harmony, nicely complementing the oil stains from your valve cover gasket.

Now that the filter is off it gets interesting. You need to prime the car because air gets into the injection system. The injection pump has a little knob that you use to prime it. Alternatively, Ian told me of a way that involves cracking open the injector lines until gas comes out.

I did none of that. I filled the main filter with diesel until it was almost to the top, screwed it back on, tightened the bolt. Then I got in the car and cranked it until it started. And it did. And it died. So I cranked it again until it started, this time it started faster and ran fine, albeit with a shoddy idle, my treat every morning when I start it up.

Make sure everything is tight and that you haven't left any diesel-soaked paper towels in the engine bay, close the hood and start the car. Drive it around to see how it runs. While you're driving, you'll notice a mighty diesel smell. You'll likely worry about that diesel smell, you might even be tempted to turn the car off until you realize it comes from your hands. Park it, open the hood and inspect for leaks. If it all checks out okay, go inside and take a shower.


James said...

So does it run better with new fuel filters? I know that my friend's 2006 Jetta TDi (which is Unitronic Chipped and quite quick) ran like absolute crap with a clogged fuel filter. Then again, he frequently runs home-brewed B100 biodiesel! But with the clogged filter, it would spool really hard till about 1800 rpms and then fall on it's face cause the injectors had no hope of keeping up.

Did this remedy your driveability issues?

-James at TCB

Ronan said...

You know, it didn't have the effect I was hoping it would have. It's slightly peppier but I still lose power when I go up hills. Perhaps not as much as I used to. I also cleaned out the ALDA, etc.

Out of curiosity, what is TCB?

I. R. Rothwell said...

You BASTARD!!!!!! how dare you not bleed out the lined like i told you!!!

haha, just kidding, glad it worked out anyway -

James said...

TCB is my own random car-related blog, which can be found at

I think i stumbled upon your site on Jalopnik? not sure. Somewhere.


Anonymous said...

Hey Ronan - This is interesting stuff. I'm hoping to learn more as you document refurbishing the 300D. I'm particularly interested in hearing about switching from auto to manual transmission. If you ever need a know-nothing to hold wrenches etc., give me a call. Reed