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  #1  
Old 03-01-2007, 01:20 PM
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newbie fuel filter question '96 E300

So I'm going through and changing all the filters I can find on my new-to-me '96 E300. The fuel filter includes a little baggie with two O rings in it. The O rings are different sizes, one is smaller, one is larger. Where are these two rings meant to go?

Second question, I have an apparent air leak into the fuel lines. Air bubbles are only flowing in the line that connects to the top of the pre-filter (I changed the pre-filter and also the O ring for it), and one other line that's on the back side of the housing, roughly at the bottom of the pre-filter. (Anyone have a diagram with all these fuel lines?)

Since the IP and all these fuel line parts are under the intake manifold, I assume the manifold has to come off to get to most of these? Easy procedure to remove it?

Thanks in advance! I did do a forum search but didn't find just what I was looking for.
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Old 03-01-2007, 01:22 PM
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Could it be for the banjo bolt that holds the spin on filter?

Is the car running rough? Hard starting?
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  #3  
Old 03-01-2007, 01:31 PM
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Quote:
Originally Posted by aklim View Post
Could it be for the banjo bolt that holds the spin on filter?
The larger O ring looks like it's for the banjo bolt. Smaller one still a mystery.

Quote:
Originally Posted by aklim View Post
Is the car running rough? Hard starting?
Car is new to me, only two weeks. Been running perfectly until last night, started to feel some slight engine hesitation on the highway. Parked it on a hill last night facing up-hill and wouldn't start this morning.

The fuel lines going to-from the main fuel filter have no bubbles at all. It's just the lines for the pre-filter. Since both the pre-filter lines have bubbles (I'll take a photo tonight to point out what I'm seeing), I assume the leak is before the pre-filter. I'm not familar enough with it yet to know what is before the pre-filter.

I'm a long-time VW diesel guy so I know diesels and know about air leaks. I do all my own wrenching. This is my first MB so I'm still learning about the beast - thanks all for your help!
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Old 03-01-2007, 01:37 PM
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Sounds like a leak at the pre-filter o-ring (big one). Has separate part number and doesnt come wiht the pre-filter.

The fuel line from the fuel thermo (in the head) to the top of the pre-filter should drain back at shutdown. Is normal.

Air in the line at the bottom of the pre-filter filter housing is usually the big o-ring. If new, check to make certain it is sealed up good.

Park nose down in mean time to help wiht hard starts.
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  #5  
Old 03-01-2007, 01:44 PM
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Originally Posted by TMAllison View Post
Sounds like a leak at the pre-filter o-ring (big one). Has separate part number and doesnt come wiht the pre-filter.

The fuel line from the fuel thermo (in the head) to the top of the pre-filter should drain back at shutdown. Is normal.

Air in the line at the bottom of the pre-filter filter housing is usually the big o-ring. If new, check to make certain it is sealed up good.

Park nose down in mean time to help wiht hard starts.
The pre-filter has a new O ring on it, checked it to make sure it's seated properly and sealed. There is air in the line at the bottom of the pre-filter housing, but that makes sense since there's also air in the line at the top of the pre-filter housing. Air in = air out. The air must be entering prior to reaching the pre-filter.

Just to clarify, when I say there's air in the line, what I mean is there's a steady stream of large, fast-moving, bubbles visible when the engine is running.
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Old 03-01-2007, 02:18 PM
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If there is air in the line at teh top of the pre-filter, the leak is at the pre-filter (most common) or preceding it (ie: fuel thermostat in head). I've had to use some silicone grease and also tweak the hold down metal clip to get it sealed again when replacing the pre-filter.

The oring you replaced was about ~3cm D x 3mm (~1.25" x 1/8") and was tucked up inside the underside of the old plastic pre-filter right? And, you ordered it separately for the pre-filter?

Since you are new, the little white horseshoe clips on the fuel lines should all be loose and floppy currently. THey are pushed tightly close (a ringer) to remove the lines. If pushed closed now they could be the source allowing air to enter.
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Old 03-01-2007, 02:25 PM
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Quote:
Originally Posted by TMAllison View Post
If there is air in the line at teh top of the pre-filter, the leak is at the pre-filter (most common) or preceding it (ie: fuel thermostat in head). I've had to use some silicone grease and also tweak the hold down metal clip to get it sealed again when replacing the pre-filter.
Good tip, thanks. Which direction is the fuel flowing through the pre-filter? Does it enter at the top and exit the bottom?

Quote:
Originally Posted by TMAllison View Post
The oring you replaced was about ~3cm D x 3mm (~1.25" x 1/8") and was tucked up inside the underside of the old plastic pre-filter right? And, you ordered it separately for the pre-filter?
Correct.

Quote:
Originally Posted by TMAllison View Post
Since you are new, the little white horseshoe clips on the fuel lines should all be loose and floppy currently. THey are pushed tightly close (a ringer) to remove the lines. If pushed closed now they could be the source allowing air to enter.
I will take a look at them, thanks.
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Old 03-01-2007, 02:46 PM
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The fuel "source" does enter the top of the pre-filter.

I think you should pull the little fuel line from pre-filter to thermostat and check those orings next. Maybe lube wiht sili grease.

Again, your leak should preceed the pre-filter.

If the fuel lines are yellowed they are old and should ALL be replaced. The entire set is something like $50 at dealer prices.
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  #9  
Old 03-01-2007, 04:18 PM
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Quote:
Originally Posted by TMAllison View Post
The fuel "source" does enter the top of the pre-filter.

I think you should pull the little fuel line from pre-filter to thermostat and check those orings next. Maybe lube wiht sili grease.

Again, your leak should preceed the pre-filter.
That's what I was thinking, except that I can't reach the thermostat end of it. Do I need to pull the intake manifold to get to it? Or get at it from below? Or just jam my hand in there and apply the band-aids later?

Quote:
Originally Posted by TMAllison View Post
If the fuel lines are yellowed they are old and should ALL be replaced. The entire set is something like $50 at dealer prices.
Sounds like the way to go. Thanks again for the helpful responses!
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  #10  
Old 03-01-2007, 04:28 PM
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TMAllison is right on the money with this. The big o-ring goes on the prefilter and the car will not start without it.
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  #11  
Old 03-01-2007, 05:07 PM
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TMAllison is right on the money with this. The big o-ring goes on the prefilter and the car will not start without it.
The big O ring included with the spin-on fuel filter does not go on the pre-filter. I do have a new prefilter O ring on the prefilter.

Sounds like the leak is upstream from the pre-filter, that's what I'll dig into this evening and I'll take a photo too - tough to describe some of these parts!
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Old 03-01-2007, 05:22 PM
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Most folks dont have to remove the intake to R & R the fuel lines, but, some have.

I think it has a lot to do with the size of your hands and flexibility.

I can do all of them in about 15 or 20 minutes.

The last time I did them i did bark a knuckle doing that thermostat line. I used the handle of a screw driver to help click,click the line back in place as it is a little tight in there to get leverage.

Pull your windsheild washer bottle to gain acess to get to the rest of them when you're ready to tackle the rest of the job.

Remember to push the white clip closed until you've made a game of horseshoes "ringer". The lines are still held tightly in place by the oring and it still takes a pretty good tug to pull it up and off.
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  #13  
Old 03-02-2007, 10:43 AM
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Just wanted to show some photos of what I'm (poorly!) describing. The green arrows indicate the flow of bubbles that I see. The line that enters the top of the pre-filter is very dark and discolored. In addition, it feels loose and wobbly at the thermostat end like it's not attached well there.

I'm going to the MB dealer today to order replacement lines. Anything else I should pick up while I'm there? New thermostat? Any additional O rings that I need?

Thanks again
Attached Thumbnails
newbie fuel filter question '96 E300-bubbles1.jpg   newbie fuel filter question '96 E300-bubbles2.jpg  
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  #14  
Old 03-02-2007, 11:41 AM
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That line going from the fuel heater (I see it being called the 'thermostat' as well in this thread) to the pre-filter is usually the first one to go and based on the color of yours, you are definitely in need of a new one. I had practically the identical symptoms as you and it ran great after replacing that line - the old O-rings had a practically square profile!

I'd recommend unbolting the whole fuel filter manifold from the engine - I think it's held on with 2 external Torx bolts. With that out of the way, you can easily get to all the other fuel lines except the one from the IP 'overflow' which I think you can safely skip.

Get a new shut-off valve O-ring and consider changing the M6 bolts that hold it on to socket head caps. Check for leaks around your delivery valves that have plagued us OM606 folks since the ULSD switchover. You can replace them - don't buy the 'you might need a new injection pump' BS that you might hear from the dealer.

Good luck.
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  #15  
Old 03-02-2007, 12:25 PM
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I have had the same issue. Throw the smaller o-ring away, its not needed for anything.

Unless some bright spark had the idea of including an o-ring for the fuel lines. But I haven't checked that.

The larger O-ring sits at the top of the main filter, the banjo bolt screws down through it.
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