I have a 1982 300SD that I recently picked up.

I have experienced this problem twice now.

When the fuel guage reaches 1/8 tank, there seems to be a significant chance that I will start drawing air into the fuel system. Lesson Learned, fill up before the tank reaches 1/8.

I am curious if this is common problem, or if my fuel level sensor is on the fritz.

I have also heard of a problem with one type of fuel pump becoming vapor locked, is there a possibility that this is what I'm experiencing?

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1982 300SD 300k+ miles

more than likely, somebody switched your feed and return lines to the tank... swap them back. I bet you solve the 1/8 tank air suck...

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Let us know if you have the lines swapped. Top line is return. Cigar hose, if you have.

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83 SD

84 CD

Please describe, in detail, the symptoms of "drawing air into the fuel system" and explain how you arrived at that conclusion.

Further description:

When the car reaches 1/8 tank, the car starts acting like it is starved for fuel (hicoughing, sputtering, etc.) When I opened the hood, the primary fuel filter, (clear plastic, elbow) has an excessive ammount of air in it, so much so that air is being introduced into the fuel distribution system. The attached photo is of the primary filter with a "normal" amount of air in it.

Primary filter and hose.
http://www.4shared.com/file/17714847/3eb3325/_2__DSC00072.html

My secondary fuel filter (looks like an oil filter) is attached to the cigar hose at the top of the engine.

Secondary Filter and hose.
http://www.4shared.com/file/17714840/9d8fa686/_2__DSC00073.html

The fuel lines are correct, top line is return and lower line is supply.

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1982 300SD 300k+ miles

Yep, that's what I figured.

You don't have any excess air in the system.

Your problem is a plugged tank screen. It's very noticeable with the symptoms that you described when the tank gets below 1/8.

You'll probably need a 46mm socket (or 1 13/16) to get the screen out. It can be cleaned, however, the screen is probably quite thin at this age.

As I am rather new to the Mercedes arena of vehicles, where is the tank screen located, is it at the top or the bottom of the tank? Will I need to drop the tank to get to the screen? Where would be a good place to look for a replacement screen? As there is a good probaility that this screen has deteriorated due to age.

Thank you in advance.

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1982 300SD 300k+ miles

Sapperbmet,
Before you get into draining your tank and cleaning your fuel outlet lines screen, please clarify why you indicated that air was getting into the fuel delivery system... was this because you actually saw bubbles in the the clear pre-filter you showed us a photo of? IF in fact you have seen bubbles in or entering this pre-filter when these rough running symptoms occur, then I think that you might have a cracked fuel line... most likely up near the engine but it could also be back near the fuel tank... and what's happening is that with a mostly full tank there is enough fuel head [aka pressure due to height of the fuel level in the tank] to over come pressure loss in the line and thus prevents the 1st stage fuel pump from drawing air into the line... but then as you draw down the fuel, the head pressure from the tank becomes less and less until you reach a point that the pump will begin to suck air into the line through the small crack. Look closely for signs of fuel leaking out onto the driveway or garage floor after sitting all night! This will help diagnose this as a possible cause AND tell you where the leaking line is, IF this is the cause!
Regards,

The tank screen is accessed from beneath the vehicle, just above the axle, slightly to the right side of the vehicle. You don't need to drop the tank but will need sufficient containers to drain the tank. There is a supply hose that is attached to the fuel screen. The supply hose is removed first and then the fuel screen can be removed. Sometimes, the supply hose is on so tightly that the fuel screen rotates with the supply hose fitting. If this occurs, you won't need the 46mm socket.

The screen gets packed with debris over the years. The mesh deteriorates with time since it's steel, but, it can be cleaned in most cases. You can get a new screen right on this site........press "buy parts" on the top of the page.

Thank you for the sugesstions.

As I have not yet located any fuel leaks, I'm going to have to say that the fuel strainer is the most likely culprit. I tested the leak theory both on a clean concrete slab and also with a -large- sheet of carboard. The cardboard revealed a few insignificant oil drips, but no fuel. After allowing the car to sit all night, I started her up and let her run on the cardboard for 30 minutes. This did not reveal any additional leaks of significant porportion.

I will be replacing the transmission filter/gasket and the oil pan gasket as both of these have developed leaks. As the car is 25 years old, I feel that both of these are due.

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1982 300SD 300k+ miles

Once again let me give you a hint on testing for fuel leaks - this p.m. before you turn in, take the fuel cap off the tank to allow atmospheric pressure enter inside the fuel tank. This releases any partial vacuum that might have formed on top of the fuel and thus might improve your chances of discovering any leaks in the fuel delivery system!.
Regards,

Yep, having a vacuum in the tank may not help in finding fuel leaks. But there should not be a vacuum if the vent is not plugged up.

If the problem is a plugged screen, how about disconnecting the fuel line in the engine bay and blow out the screen with compressed air? That should clear the plugged screen temporarily. Drive the vehicle to to if your symptoms go away.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Funola,
I believe these cars have a "check feature" in their vent lines that allows some vacuum to form as the fuel is withdrawn but then it releases to prevent starving the fuel pump. I think this is an early pollution control requirement on most cars and so IF I'm correct, then removing the fuel cap would significantly improve the chances of finding what might be a very small crack in a fuel line. I like and have used your idea of blowing back through both the supply and return lines from the engine compartment... and before I would seriously consider removing the supply strainer, I think I would even use a rag to form a crude seal around an air hose and try to apply a small, controlled, positive pressure on the tank just to see if I could induce any leaky fuel line to show it's ugly self!
Regards,

Sounds like I have a large number of good sugesstions to try this weekend. Thank you all!

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1982 300SD 300k+ miles