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Discussion Starter #1
2007 C350 4Matic

I'm getting raw gas smell at times in cabin and outside the vehicle; seems limited to when tank is full or nearly full, but I have only had a few instances so far.

While this smell is present, I have examined and sniffed around the engine compartment, esp. by the fuel rails and the metal fuel hose, and nothing there, and no leaks observed on ground. Ditto for the filler neck and cap; although there was a faint smell back there, it was not localized at the neck. I inspected and cleaned the gasket in the cap and its mating surface on the filler neck, and all was fine. No codes.

Reading here I saw that some W203 have had an issue with the gasket leaking on the Fuel Sender on top of the tank, and I am hoping to chase this now.

Is the sender accessible by removing the rear seat bottom cushion?

I see 4 half-moon shaped plastic covers under the forward roll of the bottom cushion, and under those covers are bolts. Will removing those bolts allow me to remove the bottom cushion, or is it more involved?

Any advice on getting to the Fuel Sender and replacing/repairing the gasket (dealer item, or fuel-grade RTV?) is appreciated, as well as advice on other things to check before chasing the Fuel Sender gasket.
 

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Went ahead with this, as snow is coming overnight and I have no garage.

Got the nuts off (not bolts as I thought), and pulled the seat out and the underlying insulation. Took the left inspection plate off, and what is there seems to match the sender. Things look quiet in there (see photo). No obvious traces of leaks, but then, gas does evaporate off quickly.

There seems to be a large strap clamp around the outside of large white plastic ring, but no obvious way to loosen that where it is now. Does that have to be loosened to pull the sender out, or does the sender come out differently?

On the partsgeek site I see there is a fuel sender gasket (looks like a beefy black o-ring; 07 2007 Mercedes C350 Fuel Sender Gasket - Fuel Delivery - Genuine - PartsGeek), and a fuel tank sending unit o-ring (a colored, more complexly molded gasket; 07 2007 Mercedes C350 Fuel Tank Sending Unit O-Ring - Fuel Delivery - Victor Reinz - PartsGeek). Are both needed/desired if I pull the sender?

Still have most of a tank of gas at the moment, so any remedial action will be waiting for an empty tank and cooperative weather.
 

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Your sender is leaking. The discoloration you see on top of the sender that looks like dirt/grime is actually signs of fuel residue where it has been leaking. The leak comes from a poor seal in part of the plastic housing. I once read where a forum member just completely gooped it up with epoxy, but the official (and best) repair is to replace the sender. When you do this, go ahead and replace both seals (sender and umop) as the new ones are thicker.

Your car appears to have the "California Emissions" option where the sender is attached with a large "hose clamp" retainer instead of the screw off retainer, so you don't need the removal tool. However, removing that clamp is a PITA. I used a "skewdriver" when I did the job on my CLK. You will need to remove the fuel pump when you do this because the hoses and wiring that run between them are permanently attached to the sender and disconnect from the pump. One word of caution: I ended up having to replace the pump, too, because the hose attachment on it would not release. The plastic nipple on the pump where it attaches was very brittle and broke.

Here's the WIS for removal of the retainer ring:

http://benzbits.com/w209/FuelPumpLockRing.pdf
 

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Thanks for this information and advice, Rodney. It is greatly appreciated. Interesting that you noticed residual grime on inner top part of the sender. I thought that it might just be normal dirt accumulation for a 10 year-old car in NE, but now I see your point. It must mean that some amount of liquid fuel (not just vapors) must be getting past the internals of the sender when the tank is full and the gas sloshes around during turns, backing up, etc. It must also mean that some fuel vapors must be entering the cabin even when the tank isn't full, but at a low enough level to not be noticeable.

The WIS is very clear about the removal and replacement procedure, save for a couple of typos (in the first image, 1a should be labeled 1b; in instruction 3.1, it should be 3a, not just 3). I thought an offset screwdriver would be needed to loosen that hose clamp, but with the very limited arc of motion possible in there, it surely would be a frustrating exercise. The flex extension mentioned in the WIS (is this the 'skewdriver' you mentioned?) would be much easier to get in there and turn.

It is also invaluable to know before starting this that removal of the fuel pump would also be required, so thank you for that, and the issue of likely breaking of the plastic nipples on the latter (seems these plastics do tend to embrittle when they live outside for 10 years).

Given the amount of work entailed, and the fact that my sender and pump are still working, I am inclined to take temporary measures until a proper fix can be made in the warmer months. Foremost I will not completely fill the tank, which based on the lack of fumes when the tank is about 2 gallons down, seems to eliminate the slosh over.

I am also considering attempting the fix by filling that inner recess on top of the sender, as you mentioned another did with epoxy. I'll have to look up the temps required for curing, and perhaps try one of the putty types, which I could press into place and keep the connectors on top free. If the temps are too cool at present, I might try pressing some rope caulk in there as a temporary measure. I doubt the rope caulk is very resistant to gasoline and might degrade if in long term contact with it, but might survive the occasional slosh of gas from below (hopefully none if I make sure not to fill the tank).

Please let me know if I am crazy to consider any of the above.
 

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Now that I know that the fuel sender is involved, I searched benzworld.org more completely tonight and found lots of threads, including one by our beloved Rodney. I was taken aback at how common this issue is, esp. for the older E-class (W211). While the seals have been redesigned to be improved, it is somewhat disheartening that for non-seal related leaks (i.e., failure of the plastic housing of the sender) like mine probably is, the fix is just replacing with the same parts (e.g., high density polyethylene, or HDPE) that will degrade again.

I'm probably going to give some J-B Weld product a try on top of the sender later next week, when we are in the 40s. Regular epoxy does not bond particularly well to polyethylene, so I might try one of their better plastic bonders. But before I do, I am going to inspect the top of the sender to see if I can identify any cracks that might be the source of the leak. One option is to "weld" the cracks with a soldering iron. I have brought polyethylene tubing to the gel point in the lab with a heat gun, and once you get the hang of it, it is pretty easy to work.
 

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On my CLK, first it was the seals leaking in cold weather, then it was where the pressure sensor attaches (B4/3) when the wank was filled. I am not sure if the pressure sensor can be removed and repaired or not - I didn't try.

As for removing the clamp, this is what I used:



I was able to get it into the recess between the access opening and the sender, then maneuver it around to get to the screw. It wasn't easy, but it worked.
 

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Thanks for the heads up on that, as I might have leaks in both places as well. I will check the area outside the large white ring carefully for evidence of residue from dried fuel there, to assess if the gasket is leaking. As you so helpfully pointed out above, the staining inside the large locking ring on mine shows there definitely is an issue where that pressure sensor attaches, presumably immediately beneath it in order to make the stain where it is on the top surface.

I picked up some J-B Weld Plastic Bonder yesterday, which seems to be the best suited of their products for an attempt at this repair. I hope to give it a try mid-week. I'll try to ease off the connector on the sensor first, and try to see if I can tell how it attaches to the white plastic. I won't try too hard, due to the likely brittleness of the plastic.

I don't have terribly high expectations for the success of this, but will report back when I have some results.
 

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It was warm enough to work outside today, and I opened up the access hatch on the sender. The grime that is there wipes up very easily (1st photo is after the first swipes on part of the sensor housing and the locking ring), and is both inside and outside the locking ring. At this point I am uncertain what it means, as the grime is also in the recesses on the ribbed top of the lock ring, and I’m finding difficult to believe that liquid fuel would have flooded all the way up there too.

I saw the black electrical connector for the sensor was held into the sensor housing by two small integral clips that are part of the while plastic top of the housing, plus another small piece of white plastic on the side acting as a channel way, presumably to keep things aligned. I could carefully ease back the two clips and raise the back end of the connector off the top surface of the sensor housing, but the other end was not coming along and I did not want to risk breaking it; perhaps there is a screw under the little black cap on the connector, or the wires are fixed to the inside of the housing. The 2nd photo shows how far I could lift the rear of the connector.

I decided not to use adhesive on this now, preferring to go with something easily removable and undamaging. I applied some rope caulk around the base of the sensor connector while raised and then pressed it down onto the caulk until the two clips reseated, and then pressed some more caulk around the edge between the top plate and sides of the sensor housing (3rd photo), which look like two separate pieces. I left the rest of the sender assembly alone. I then reconnected the wiring harness to the connectors and sealed up the access port.

There have been no more fumes once I used up a couple of gallons a few days ago. It will probably be a couple of weeks before I refill the gas tank again, so I will have to wait until then to see if any of this makes a difference.
 

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Had a fill up over the weekend, and then drove around taking some swooping curves at speed to make sure the gas had the opportunity to slosh around in the tank. No gas smells, so that rope caulk was enough to stop the flow.

Today I removed the caulk so I could replace it with J-B Weld Plastic Bonder. When I removed the bits of caulk, I sniffed the pieces, and those around the edge between the white plastic cylinder and top plate of the sensor housing smelled a bit of gas, but the pieces that had I placed directly under the black sensor connector itself did not. So the leaking must be occurring at the edge.

I do have some concern that the leaking from the edge might just be the proximal expression of the leak, and the real cause is internal to the sender; in fact, the edge does not look like an engineered seal, but just the mating of two plastic surfaces. If true, I am just plugging up the outlet for the leak, not actually fixing it, and may have issues down the road.

But for now, I have pasted that edge with plastic bonder, as shown below, and I'll have to see how it goes. The whole job took only 30 min. including removing and reinstalling the rear seat, now that I know what I am doing.

From the when it rains, it pours department: climate control malfunction this weekend, no flow through the ducts that does not correct with an ACC test/reset. I will start a new thread on that. Until a few weeks ago, this was my wife's car. We got a 2014 E350 for her, a CPO just off a 2-year lease, and I took this one. I think she planned it this way...
 

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The J-B Weld Plastic Bonder applied to the edge of the sensor housing on the fuel sender assembly seems to have done the job. Filled up a couple of days ago, and with a good amount of driving around, no fuel smell in the cabin or outside.

I will update this thread if this fix fails over the coming year.
 

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The J-B Weld Plastic Bonder applied to the edge of the sensor housing on the fuel sender assembly seems to have done the job. Filled up a couple of days ago, and with a good amount of driving around, no fuel smell in the cabin or outside.

I will update this thread if this fix fails over the coming year.
Is that sensor housing on top of the fuel sender part of the fuel sender? If I were to glue the sensor housing, will I run into trouble if I were to replace the fuel sender (i.e. removing the sensor housing if it's not part of a new fuel sender unit)?

Thank you
 

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If you look carefully at the last photo I posted, in post #9, you can see where I applied the J-B Weld. It was where the horizontal small white plastic "plate" (that the black sensor body is directly attached to) meets the vertical white plastic walls of the housing that comes up from the base of the large sender unit.

I do not have information on the internal construction of the larger sender unit, or how the sensor is attached to it, or if it can be disassembled by the user. All I know is that the fumes were coming from the joint where that small white plastic plate under the sensor met the vertical walls coming up from the main sender unit (see earlier posts explaining how I figured that out). That joint did not appear to me to be an engineered seal, and I assume the gas smell was due to a broken internal seal or part deeper inside the sensor body. I believe Rodney mentioned that the internal seals are probably the source.

I also don't know how easily the J-B Weld I applied would come off, so I don't know if it would interfere with attempts to separate the sensor from the sender body, if indeed that could be done. Ditto for epoxy.

Lastly, I note that before I applied the J-B Weld, I was able to stop the fumes with application of grey rope caulk (Mortite, specifically), a product intended for sealing air leaks around windows. This is a somewhat sticky and moldable material that does not harden in its normal intended use, and can be easily removed. I did not want to leave Mortite in place because I suspected it was not considered long-term stable in the presence of gasoline and therefore I had concerns about its safety. I suspect the same could be said of the J-B Weld, but I perceived less risk. Other options might include a product called Coax-Seal, and the like, but like the other materials, I found it difficult to find chemical resistance information on them.
 

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The original sender is attached to the unit. The replacement has a removable unit. I found that the JB weld fix is temporary in most cases and should only be used as a stop-gap till a replacement can be had.
 

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Well, I understand your point and respect your perspective. But with a car that is now 11 y.o. with 125K miles on it, and as a second car in our family and not used for longer trips, I'll likely deal with patches for this particular issue when, and if, needed. This car is in great cosmetic and driving/general mechanical shape given its age and mileage, but between this issue and an air blockage of some sort to the heater core which kills the availability of heat, and which would take 1 to 2 kilobucks to fix mostly because of the need to deconstruct the dash to get at it (something I have not warmed up to doing myself), I don't see much reason to alter my current path. Outright failure of the sender to measure fuel level might change my mind, although I am not beyond counting miles between fills...
 

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Well, I understand your point and respect your perspective. But with a car that is now 11 y.o. with 125K miles on it, and as a second car in our family and not used for longer trips, I'll likely deal with patches for this particular issue when, and if, needed. This car is in great cosmetic and driving/general mechanical shape given its age and mileage, but between this issue and an air blockage of some sort to the heater core which kills the availability of heat, and which would take 1 to 2 kilobucks to fix mostly because of the need to deconstruct the dash to get at it (something I have not warmed up to doing myself), I don't see much reason to alter my current path. Outright failure of the sender to measure fuel level might change my mind, although I am not beyond counting miles between fills...
I understand where you are coming from but this is a real safety issue. I would not drive the car till it has been repaired. The overflow for the unit dumps out from behind the tank and could easily come in contact with the exhaust system. Just be careful.
 

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Actually, liquid gasoline is not the issue. It's the fumes that ignite, and it's the fumes that you smell. I'll second the advice that this needs to be fixed ASAP. Assuming it's the typical leak on the sender unit housing, I'd clean it off and coat it with epoxy.
 
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