Greetings to all, new member here. Long time diesel owner but first Benz.

Well, my '96 E300D with 137k miles has a fuel leak (or seal problem) that I can't quite seem to find, or I should say my mechanic can't find. They are an independent shop, good people, experienced, have all the Mercedes equipment to do the job well.

Thusfar, we have replaced the o-rings (at or below the injectors?), fuel return line (had a bad visible leak), and the main fuel filter (bad seal). All clear fuel supply lines were replaced within the past 18 months. She's doing better now, but if I park her on an incline overnight it takes a long time to prime her and get started the next morning. There are significant air bubbles visible in the clear fuel lines going into and out of the fuel filter. If I back into my driveway, no problem starting the next morning whatsoever.

I assume that since this is the case, and that once started she runs fine, that my fuel pump is probably fine, no? But, that is what the mechanic suggested as the next most likely culprit. Is there some type of valve that if compromised, could be letting fuel drain back into the tank when parked on an incline? I have been told there is also a pre-filter on these cars, what about that? Sorry, I am no gear head. I change my own oil and oil and air filters, that's about it. Hey, it puts me ahead of 95% of the population, doesn't it? Also, now there is no visible sign of fuel leakage (driveway, under engine, smell of diesel), just unwanted air getting in.

Any words of wisdom?

My bet would be the pre-filter O-ring (29) which seals the interface between the pre-filter (26) and the fuel delivery manifold (5). Buy a new one at Fastlane and install yourself - takes about 5 minutes. NOTE: References in parentheses are mapped to the attached image.

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Scott C.

2006 E320 CDI (120k miles)
FOR SALE: 1998 E300 Turbo Diesel - Black w/Tan Leather - Euro delivery (236k miles)

Anyone know the part number for the rubber o-ring that is on the end of part 32 from the diagram. It's the tube that leads the fuel into the pre-filter.

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#dieselFLEET
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'97 E300
'99 E300

Thanks for the advice. Again, I am not too great under the hood, but this problem, if nothing else, is improving my education. I inspected the seal/ring where the pre-filter goes into the housing. It may indeed need replacement, I don't know. However, if that is the culprit, would there be signs of fuel leakage around the seal? There are no such signs.

I have read through searching that there is some kind of check valve in the fuel lines (I assume to maintain positive pressure while the vehicle is not in use?) Since I have no signs of external fuel leakage anywhere, is it possible this check valve is faulty and bleeding fuel slowly back into the tank? Since it only happens significantly when I am parked on an incline, is it possible that the line through which fuel could be bleeding back is not submerged at those times? Does anyone know the proximity of where such a line feeds from in the fuel tank? Or where such a check valve resides on my car?

I don't mind parking in my driveway backed in. However, I assume that later when I could develop some other more serious problem, this current problem could mask discovery and solution of the new problem, and so should prudently be fixed now as soon as possible.

This Benz brotherhood thing is pretty cool. I enjoyed reading and posting some on the TDI club forums for several years while I owned jetta and passat tdi's. Great cars, but this 10 year old mercedes blows them away!

No. The fuel in this area is under a slight vacuum so air would leak in instead of fuel leaking out. I believe the only fuel line that is under positive pressure is the return line to the tank(119).

I am not certain, but I think the fuel in the spin-on filter cannot leak back into the tank with the car off since this line is fed from a small hole at the top of the filter manifold.

How is the condition of the fuel line(32) from the heater(155) to the top of the pre-filter? It loses it's ability to seal very easily and if it is dark brown like a "tea" color, I'd say replace it (if you are running regular diesel fuel).

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Scott C.

2006 E320 CDI (120k miles)
FOR SALE: 1998 E300 Turbo Diesel - Black w/Tan Leather - Euro delivery (236k miles)

Hi

There is an o ring between the stop solenoid and the IP. The stop solenoid has 3 pipes going into it and is black on the side of the pump. there are 2no Torx headed bolts that hold it on. This o ring when it goes sometimes the bottom of the stop solenoid can feel damp.

Try it if all the other seals are good.

McCool300TD

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1988 190E Sold
1996 C230K Sport Sold
His: 98 E300TD Estate, 7 seater, Avantgarde, Silver, Black cloth, parameter steering, dipping rvm, folding mirrors, rain sensing wipers, pentagon tints, 17" Elegance wheels. 298K
Hers: 93 300TD, 7 seater, Grey MB Tex pearl blue metallic, 15" 8 hole alloys, OTG, Cruise, not much else 220K

If you try everything else and it doesn't work, replace the lift pump. This may be what your mechanic is talking about. Not the injection pump but the lift pump, which is the only pump your car has between the tank and the IP.

It's not that expensive, something like $140 or so IIRC, and not difficult to change if you pull the intake manifold off.

Changing this pump solved all kinds of issues with my car. I was getting air in the lines, and the car was stalling about 2-3 minutes after starting, and I'd get a huge burp when I opened the fuel tank. All solved after changing this pump.

Rgds,
Chris W.
'95 E300D, 217K

I do not know the part number, however I believe that the o-ring that goies inside the prefilter is the same o-ring that is on the other snap-on tubes.

1999 E300, 606
PRE-FUEL FILTER O-RING PART NUMBER--Note this is the one that goes on the outside of the prefilter
E1030-81362

Snap-on Hose seals
O-ring
13mm OD
8mm ID
2.5mm thick

Shut-off Valve seal
O-ring
17mm OD
12mm ID
2.5mm thick

I hope that this helps.

I replaced the pre-filter last night, so we'll see how that goes overnight tonight (last night I was busy rethreading the hole where the bolt attaches the pre-filter to the filter housing after "breaking one off" in the housing; I say it was a brittle bolt, and I'm sticking to it!)

I'll check the other seals if the pre-filter and its seal/ring didn't resolve the problem, and replace seals/pumps accordingly.

Still loving the Benz. Fourth different diesel I've had, and my favorite by far.

Thanks for all the tips, gang; keep 'em coming.