First post here and need some help!! I have a 1996 E300D that i call the white demon (car is white and is demon possessed, lol). I love the car, however, i cannot keep it running long enough to even use it anymore. I've had the car to 2 different shops with no luck and now i am diagnosing the problems myself. I have air in the fuel lines that is causing the car to die when taking off from a stop sign or red light. After the car is running it's fine, but when taking off under a load it dies the same as if you turned the ignition off and then does not want to start back (air lock at that point maybe). here is what I have done so far.....

Checked K40 relay, all looks good (no cracks and looks to be newer unit)

SOV clicking on and off with ignition switch


Replaced.....

Pre-heater gasket

Line #1 Pre-heater to Pre-filter

Line #2 Pre-filter to SOV

Line #3 SOV to lift pump

Line #4 Lift pump to fuel filter

Line #5 Fuel filter to IP

Pre-filter and Pre-filter O-ring driven all the way down and flush flat against filter base( cannot go down any tighter).

Fuel Filter and new O-ring

To start, I have replaced the o ring on pre-heater along with gasket. This cleared up the bubbles from heater to pre-filter, but on line # 2 coming out of the bottom of the pre-filter, I had TONS of air bubbles. I then got on Pelican parts and ordered all new lines (1-5), pre-filter, and fuel filter. Did not see a reason to change lines 6-8 because they are so far down the line from where the air leaks are occurring I do not think they are a problem. Changed them all out and cleaned up the leak areas of fuel. I now have MORE air bubbles than before with absolutely no detectable fuel leaks at all. How on earth is this system continually sucking in TONS of air with all new parts and o-rings? I now have air back in all 5 of the above mentioned lines and before, i just had air in line # 2. The rubber fuel line feeding the pre-heater looks good. Had a little fuel on it at the lowest point, but I believe that was from one of the leaking clear lines. Largest concentration of air is coming from #2 line. Has anyone ever had to replace the filter base from a crack allowing air to enter into pre-filter and go through # 2 line? I have also bypassed the #1 pre-heater line. I removed it and took #2 line from pre-filter outlet and put it into the pre-heater outlet. The result was then TONS of air bubbles coming from the pre-heater into the SOV. I put the #1 line back on and it's has just a few small bubbles here and there. I have used grease around all o-rings to see if there is a small air leak and there is no difference when using grease or just o ring by itself. This is why I HATE working on Mercedes Benz cars, one step forward is always 3 steps back Any ideas? What is causing the air in Line #2? If I can stop that air leak i believe the whole rest of the system will clear up. Thanks everyone!

Josh

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Sounds like your air leak is at the Line entering the pre-filter, Pre-Filter itself (possibly cracked or not sufficient downward pressure provided), pre-filter Oring, or the line exiting the pre-filter housing.

Confirm you have a large quarter sized oring up under the lip of the plastic pre-filter, have to remove it and turn it upside down to see it. Is a separate part number and doesn't come with the new pre-filter. You wouldn't be the first person to throw that away when changing it.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

OK, here is the rundown on what you just mentioned....

Line entering prefilter is new with new O-ring and i made sure it is completely flush down from tab. You cannot move the line at all side to side once installed.

Prefilter is new Febi, and new O-ring installed with it. Initial install had air, I removed, and used grease around the top, still had air. I removed again and re-installed by lightly hammering the unit into place. No gap, no cracks, and it is as flush as it can go. Still had the air in 2nd line.

2nd line has the same amount of air if installed into outlet of prefilter or outlet of heater (bypassing 1st line). Line # 1 has just an occasional air bubble after revving engine. Line # 2 is new and has new O-ring. Even tried changing to a different oring and still same effect.

assuming all is well with the items you described:


all the rubber hose connections to metal and plastic pipe on the fuel supply from the tank use a very 5hitty crimp type clamp.

replace them all with adjustable hose clamps and cut off the ends of the hose if they are rotted or cracked.

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This is a common problem on w210 diesels. It's not always air leaking in, rather the very powerful lift pump is pulling vapor out of the diesel fuel causing the engine to stall.
Read this
1997 W210 606 Fuel Flow Mod..

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CENSORED due to not family friendly words

Usual culprit is the connection at top of pre filter as it is the highest point in the fuel system. It can let air in but not leak fuel out.
My thinking is the system struggles to get fuel from tank (maybe strainer problems ) and upon stopping engine the suction in lines draws air back at pre filter connection.
I have on my 97 w210 two 'o' rings on the pre filter as with only one it was letting fuel draw back down the line.
I also recently replaced all my clear fuel lines with new and using viton rings and this was when it started to draw back down the fuel line !!!, I find the 10mm bolt and clamp affair a bit weak as you can still move the fitting on the opposite side if you press down hard enough.

I have a '98 E300td and got air in my fuel lines from a crack in the shut off valve. That probably is not your problem, because you would most likely also have a fuel leak which you do not describe. I'll describe my experience anyway.

The previous owner took my car to a dealer about 10 or 11 years ago to get rid of "that diesel smell." The dealer changed the rubber lines, the plastic lines, and the fuel filters and charged about $1,000. That did not get rid of the diesel smell, and I think that is why the previous owner sold the car. There was still a leak, but it wasn't very bad.

About five years ago, I had to change the glow plugs and, since the intake manifold was off the car, also changed the plastic lines, the o ring between the shut off valve and the injection pump and shut off valve and the filters. That still didn't fix the leak, which remained small.

Then about two years ago, the leak got worse. I parked the car on an incline with the front wheels up a bit for about 20 minutes. The car would not start, and I had it towed home. I change the lines and filters again. I inspected the shut off valve this time and discovered the crack. Replacing it fixed the diesel smell problem I had had for so many years.

I figure that in parking on the incline, diesel fuel drained pretty much completely out of the shut off valve and allowed air to enter and the no start problem.

Check in tank fuel filter if not done so already, many come out looking like this.

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David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

I don't think this is the problem but for the sake of completeness . . . make sure the o-rings on the secondary fuel filter and its bolt are present, undamaged, and sealing to the appropriate surfaces. I had trouble with mine some years ago.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

ok, update time.....

Thanks for the input everyone! Here is what i have done today......

Changed the factory clamp on the rubber line from tank to a SS worm clamp. Much tighter than the OEM.

I also put 2 O-rings on the fuel pre-filter.

Line # 1 now has no air whatsoever!!

Line #2 still is full of air

Line #3 has even more air than line # 2.


I just remembered that the SOV O-ring was replaced about 1.5yrs ago. So I'm still trying to figure out why line # 2 has TONS of air in it when everything has been replaced. Could I have a cracked prefilter metal housing? Could I also have a cracked SOV?

I have not had any luck changing the cheap parts and I do not want to go throwing more bills at this money pit machine. Any suggestions?

also, in reply to ESchwab, my car does the same thing that you described. It starts great downhill, but has never liked starting when parked uphill. In winter I would back it in an uphill driveway so that it would start when below 32 outside. The area under the SOV is a little wet, but i cannot tell if that is from a genuine leak or just where fuel drained from changing all the clear lines out.

For some reason my main reply did not go through.....

I have replaced the rubber hose clamp from the tank at the pre-heater with a SS worm clamp. I then went and put 2 o-rings on the prefilter.

Results.....

No air in line # 1 now at all!!

Line # 2 is still full of air and now line # 3 has TONS of air in it. Car runs horrible now that the air bubbles are gone from Line # 1.

Also remembered that the SOV o-ring was changed about 1.5 years ago.

Also, let me make a clarification. I installed 2 o-rings on line # 1 going into the pre-filter, not the plastic prefilter itself.

Lines #1,2,&3, are all ahead of the lift pump and are under suction, so any bad o-rings let air leak in. Lines after the lift pump will leak fuel but will not let air in, except the return lines #6,7, & 8 combine at a fitting on the back of the filter housing. There is an o-ring on that fitting where it plugs into the housing that should be replaced as when the engine is off air can get in and cause the car to be hard to start. There is usually air in the pre-filter that gets sucked into #2 when running and shows up in #1 with the engine off. This is normal. If you have a plugged fuel tank filter, the air in the pre-filter will expand under the suction and will appear to be leaking when it is not. I would drain the tank and check the tank filter.

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'97 E 300 D

The tank filter was suppose to have been replaced less than 200 miles ago(if the shop actually did that). Only one way to find out and i guess that is next on the list.

So tonight, I've checked the pre-filter and it fits in so tight that it is very hard to get out without breaking the plastic. it has a super tight lock on it when installed. Still air in lines though. I have went as far as putting epoxy on the inlet and outlet of line #2 and on the outlet of line #1. Same amount of air in lines even after I put epoxy around O rings The only thing I know to do is to make my own line from the pre-heater straight to the SOV.

Also, the front most metal hi pressure line is now leaking very bad. I guess while working on line # 2 I have leaned or put pressure against the first Hi-pressure line and it now has a huge leak. Metal nut that tightens down is super tight fuel is leaking around the line coming out of the top. So to recap, i started with one line full of air bubbles and after replacing all the lines and new o-rings, i now have 4 air leaks/fuel leaks to contend with. This is why the car is called the white demon. The more things you try to fix, the more problems it creates.

Any ideas? I've ran out of options and have done just about everything i can find listed on the internet to do. Only things I have not done yet are check strainer in tank(should be practically new) and replace filter head(in case of crack). Other than that, i have no idea where the air is coming from between the pre-filter area getting into the 2nd line.