280SL Running Rough when Fuel Quantity is low

[Nicolas Aristodemou]

My car starts to run rough when fuel level is low, below 1/4 tank full, but well before low fuel warning light comes on. Roughness is felt at low engine speeds and at idle.

When I fill up with fuel car runs normal again. It’s almost like a reminder that I have to top it up. What could cause that? Defective fuel pump, blocked tank and/or fuel pump strainers? Something else? Engine fuel filter prior to FIP is new.

[hkollan]

Hi,
This sounds very much like an issue with the fueltank. The returnline in the tank could be blocked possibly by rust, preventing the "flowerpot" in the tank from being filled with returnfuel.
As long as the fuel level is above the top of the flowerpot, fuel will overflow into the flowerpot allowing the car to run.
A new fueltank is probably the solution.

Hans K

[Nicolas Aristodemou]

Thanks for the tip  Hans, I hope I don’t have to go to such an expensive fix...... maybe I could unblock the return line?

[MikeSimon]

Don't go out to buy a new tank. There are plenty of places that clean and restore fuel tanks. It is a common job necessary in restorations and with the ample amount of U.S. classic cars around, jobbers cater to this.

[CJHenderson]

I agree with Mike, I'm getting ready to send out my tank for cleaning and if needed applying a rust inhibitor. The only thing that you might have to do after that is to repaint the exterior of the tank.

[Nicolas Aristodemou]

Thanks for the advice. All that presumes that the problem will be internal to the tank, hence the need for cleaning and refurbishment.

Living in Cyprus though, I do not think that such a refurbishment can be offered locally and if I am to send my tank to mainland Europe for refurbishment I would surely be paying the same if not more than getting a new tank (postage costs alone would be €150. So maybe get a new tank and have piece of mind?

What do you think of this eBay (Germany) offer?

https://www.ebay.de/itm/JP-Kraftstoffbehalter-Fur-MERCEDES-W113-1134700701/333583050820?fits=Platform%3AW113&hash=item4dab196844:g:n4oAAOSwCStftpMu

I think for less than €400 I could ha e a new tank. Seems tempting to me unless there is a catch....

[CJHenderson]

Here in the states it's $420 from Parts Geek and on eBay it is all over the map from $446 to $1430.

[hansr433]

Thanks for the advice. All that presumes that the problem will be internal to the tank, hence the need for cleaning and refurbishment.

Living in Cyprus though, I do not think that such a refurbishment can be offered locally and if I am to send my tank to mainland Europe for refurbishment I would surely be paying the same if not more than getting a new tank (postage costs alone would be €150. So maybe get a new tank and have piece of mind?

What do you think of this eBay (Germany) offer?

https://www.ebay.de/itm/JP-Kraftstoffbehalter-Fur-MERCEDES-W113-1134700701/333583050820?fits=Platform%3AW113&hash=item4dab196844:g:n4oAAOSwCStftpMu

I think for less than €400 I could ha e a new tank. Seems tempting to me unless there is a catch....

Are you sure that you cannot get this fixed in Cyprus?  Check with the motorcycle repair shops, or places that rebuild radiators.  The Ebay price is tempting, but the reviews in general are mixed.  Citric Acid or a molasses solution in the tank will dissolve rust and other deposits, but the process takes 2-7 days.  It can be sped up by throwing some smooth stones in the tank and tumbling the tank by attaching it to a cement mixer or similar.  This is what we do when we clean motorcycle tanks.

[Nicolas Aristodemou]

I took the fuel level sender off and looked inside the tank and flower pot using a cheap camera attached via usb to my laptop. Amazing but everything looked as good as new..... I also took the tank filter from the tank bottom off, was so clean that I just put it back..... so no luck there in finding the source of my problem.... also disconnected the return fuel line after the fuel damper connection and blew it clean with compressed air having opened the fuel tank cap first.... again no blockage.... could hear the air bubbling into the tank. So looking for other possible causes of my problem. Electric fuel pump flow well within specs. Could it be a problem with my FIP?

[Pawel66]

Just curious: did you have a chance to examine that passage under the flower pot - this is how the flower pot gets fuel when the fuel is low. There is this window underneath - maybe this got clogged?

[Nicolas Aristodemou]

Thanks for the tip Powell. I didn’t check it to be honest. But will do that today. Will update if I find anything, thanks again

Nicolas

[Pawel66]

I found this place particularly difficult to check. I also did it with endoscope, through the strainer opening.

[ejboyd5]

A new fuel tank is probably the solution.

It's very easy to paint with a broad brush if someone else is buying the paint. Suggest you follow the investigative leads supplied for a more economical solution.

[Nicolas Aristodemou]

It's very easy to paint with a broad brush if someone else is buying the paint. Suggest you follow the investigative leads supplied for a more economical solution.

I agree with you ejboyd5. Besides don’t we all enjoy this troubleshooting process? It is the satisfaction of finding the cause of the problem and if possible fixing it afterwards that is most rewarding.

Simply replacing  a part that could be fixed is always the easy way out. It’s like doing a multiple choice test with the correct answer highlighted 😀

Having said that I think that a rather large dent on the tank caused by a previous opener (hitting something?) could be causing my problem since it is  between the “flower pot” and the tank fuel outlet, bending the fuel lines inside the tank. What I propose to do is to take the tank out of the car, drain it, let it dry -it’s very clean inside- block all the fuel outlets, secure the tank so that it doesn’t “fly away” and then slowly pressurize it with compressed air in an effort to push the dent out restore the tank’s original shape. If that succeeds without splitting the tank open all will be good. Otherwise I think that I will be ordering a new tank.

The correct answer to my multiple choice question would be then “none of the above” 😂😂

[GM]

Or you could try the Paintless Dent Removal (PDR) technique - https://youtu.be/-Y-WedpjlB0

[ejboyd5]

Not having seen a picture of the dent, it is very difficult to advise, but I seriously doubt that increased air pressure will restore the original configuration. If the dent is sufficiently deep and in an exact spot, it is possible that the dent has constricted the entry port that allows fuel to flow from the bottom of the tank into the pot when the fuel level is low. Good luck with that problem.  As suggested, PDR may help.

[Nicolas Aristodemou]

I have included a photo of the dent. You can see the entry point of the supply and return lines and the fuel drain point (filter removed) at opposite side. Difficult to get a more clear picture.

[Pawel66]

It might have elevated the whole flower pot with its bottom opening and this is why you car runs poor. Each centimeter of the tank height is quite a lot of liters of gas....

[Nicolas Aristodemou]

Thanks Powel, the tank is now drained so 3 fixing screws out and disconnection of 4 hoses, supply, return fuel lines and the two vent pipes. Then I will try the compressed air pressurization trick.  If it works it works, else will be getting a new tank since old one will be already out.

I understand that these reproduction tanks do not have the flower pot arrangement inside. I presume they have some kind of internal baffles to prevent fuel from running all in old place -in turns for example- and creating temporary fuel starvation. I’m I correct or is there another way that such starvation is prevented in these replica tanks?

[Pawel66]

I am afraid I do not know how the aftermarket tanks are built...

I think that I would try and visit some chassis shop and try to make this tank straight with the dent repair system - there are sets composed of bits that you glue to metal sheet and some kind of puller. I think every chassis shop has it. That would be my primary attempt to straighten the tank, rather than the compressed air.

It is, I think, quite easy then to check if the fuel passage under the flower pot works. When you have the tank empty, just pour a little bit of cleaner - you should immediately see a flow of it from the tank outlet nipple if the passage under flower pot works.