Prime fuel pump..everytime (72 w114)?

[Jub]

Hi everyone,

Keep running into this issue: let my 72 sit for a few weeks, then when I go to start up it doesn't. I check the fuel pressure going to dual carbs, and it reads zero (FYI: replaced fuel pump, changed fuel filter).

Only way to get it going, is to take fuel from bottle and squeeze it into line just after the fuel filter (retrograde kinda way; pushing it backwards towards pump). Once it gets going, its fine...until I let it sit again.

Lastly, probably unrelated it why it sits so long. While running it one day, it just cuts for no appearent reason and won't restart. Thinking its electrical (inline fuel filter has 2.5 psi pressure).

Anythoughts/theories?

Thanks,

Jub

[JeffreyNMemphis]

Have you cleaned your fuel tank and replaced the fuel strainer in the tank?

Fuel Strainer 1114700686 - Genuine Mercedes-Benz - Genuine Mercedes-Benz - 111-470-06-86-M22 | Pelican Parts

2.5 PSI sounds about right for a carbureted engine. Do you have breather tubes connected to the filler neck of your fuel tank? Are those rubber lines intact?

[Tomguy]

It sounds like vacuum in the tank sucks fuel from your carb bowl to the tank. You can try an inline check valve @ the carb but that may make it worse (as in your fuel tank might collapse if the vacuum gets too high). Are your tank vent lines plugged in any way? You can also see if you can get a vented fuel cap (that allows air in but does not allow vapors out).

[Frank Reiner]

Originally Posted by Tomguy

It sounds like vacuum in the tank sucks fuel from your carb bowl to the tank. You can try an inline check valve @ the carb but that may make it worse (as in your fuel tank might collapse if the vacuum gets too high). Are your tank vent lines plugged in any way? You can also see if you can get a vented fuel cap (that allows air in but does not allow vapors out).

Tg:

A bit of objection must be raised to the above hypothesis.
1) The inlet to the float bowl is above the float level.
2) The fuel pump contains two check valves that permit flow in one direction only.
3) The tank vent system must be operating, otherwise once the engine is running, it would soon starve for lack of fuel flow.

[97 SL320]

I've had engine driven diaphragm / plunger pumps fail to move fuel after an extended period of non use but apparently be "fine" after dumping some fuel in the carburetor bowl. . Air is less viscus than liquid so any leak will look large during cranking but small if liquid is involved. Same goes for a plunger type electric fuel pump.

Put a pressure gauge on the fuel outlet, run the pump, turn off and see if it holds pressure. This test is only valid if you don't have a bleeder return line type fuel system. ( this is where a small amount of fuel is run back to the tank in an effort to purge vapor )

If the fuel pump is failing and is electric, aftermarket ones exist. Carter , Airtex make a " looks like a fuel filter " pumps and Facet makes a square unit.

[Jub]

So seems like consensus is leaning toward breather valve issue:

Does anyone have diagram of things that may need to be replaced (specific tubes/lines).

Lastly, I will note co-incidentally: when I bought car, PO had a fuel cap on that did not fit (figured PO drove off one day and lost original). I did buy a OEM MB cap later on, and didn't think anything of it....humm... could misfitting cap allow effectively funtion as a breather?

And I will order fuel strainer (wonder what tools I will need, hopefully nothing too specific..)

[Idle]

If you are talking about the fuel strainer under the tank then all you really need is a pair of pliers. This is not in there that tight as it is too flimsy to take a lot of torque.

For something like this I would start at the strainer in the tank and move towards the carbs, cleaning not only the strainer but also running a brush through the steel gas line along the bottom of the car.

I usually run a piece of steel cable through the line since it will go all the way through. Then I tie a piece of fishing line to the cable and pul it through. Then tie a pistol bore cleaning brush of the correct size to the fishing line and pull it though.

Then at least you are clean up to the fuel pump.

But I think the trouble is in your fuel pump. As in a check valve has failed due to a piece of dirt in it. These mechanical pumps are not expensive and are also not hard to install. At one time you could buy a rebuild kit for them and that's pretty simple, too.

[Mike D]

Originally Posted by Jub

And I will order fuel strainer (wonder what tools I will need, hopefully nothing too specific..)

I use an old spark plug socket, reversed so the large "hex" is on top. This fits into the large hex/allen head on the strainer. I use a largish pipe wrench with about a 2-3 foor "cheater" bar to break the strainer free. Those suckers can be tight! I usually have to apply a couple applications of "boot power" before it comes loose.

There's a couple of posts on this forum regarding the removal of the strainer.


http://www.peachparts.com/shopforum/vintage-mercedes/14401-diy-removing-fuel-tank-drain-plug.html

[Idle]

Originally Posted by Mike D

I use an old spark plug socket, reversed so the large "hex" is on top. This fits into the large hex/allen head on the strainer. I use a largish pipe wrench with about a 2-3 foor "cheater" bar to break the strainer free. Those suckers can be tight! I usually have to apply a couple applications of "boot power" before it comes loose.

There's a couple of posts on this forum regarding the removal of the strainer.


http://www.peachparts.com/shopforum/vintage-mercedes/14401-diy-removing-fuel-tank-drain-plug.html

Caution: They are tight mostly because the gasket has frozen and has sealed up tight.

It might take a lot of force to break them loose but don't overtighten when going back. And use a new gasket with a light coating of oil to get a good seal without binding up the gasket.

[Mike D]

Good point.

[Squiggle Dog]

My roommate's 1965 W111 220S has the same problem. If it hasn't been run for a week or two, the fuel filter will be completely empty and it takes about 5 minutes of cranking (with pauses in between), nearly draining the battery before it will start.

The fuel tank is clean. The tank strainer is clean. All fuel lines are clear. The fuel hoses are new. The fuel pump is new and the carburetors are newer Webers.

I don't get it. Makes me love my diesels.

[Idle]

Could this be something more like a problem with the carbs and somehow the fuel in the bowls draining out or vaporing off?

[Frank Reiner]

Originally Posted by Idle

Could this be something more like a problem with the carbs and somehow the fuel in the bowls draining out or vaporing off?

Originally Posted by Jub

Hi everyone,

Keep running into this issue: let my 72 sit for a few weeks, then when I go to start up it doesn't. I check the fuel pressure going to dual carbs, and it reads zero (FYI: replaced fuel pump, changed fuel filter).

Only way to get it going, is to take fuel from bottle and squeeze it into line just after the fuel filter (retrograde kinda way; pushing it backwards towards pump). Once it gets going, its fine...until I let it sit again.

Lastly, probably unrelated it why it sits so long. While running it one day, it just cuts for no appearent reason and won't restart. Thinking its electrical (inline fuel filter has 2.5 psi pressure).

Anythoughts/theories?

Thanks,

Jub

When all else fails, read the whole thread.

The flexible fuel line between the chassis and the fuel pump inlet is a likely location for a small air leak; as noted in an earlier post, after sitting for a few weeks, when pumping is re-initiated air will be drawn in more readily than fuel. Once the fuel supply system is refilled, even with a small amount of air being drawn in with the fuel, pressure at the pump inlet will be low enough that fuel will continue to flow from the tank, until the next period of inactivity.

[Idle]

Originally Posted by Frank Reiner

When all else fails, read the whole thread.

The flexible fuel line between the chassis and the fuel pump inlet is a likely location for a small air leak; as noted in an earlier post, after sitting for a few weeks, when pumping is re-initiated air will be drawn in more readily than fuel. Once the fuel supply system is refilled, even with a small amount of air being drawn in with the fuel, pressure at the pump inlet will be low enough that fuel will continue to flow from the tank, until the next period of inactivity.

I once had a small crack in the rubber fuel line going from the tank to the steel line. This line is very short but it is there.

When the car heated up it would die like it was running out of gas. It turns out the hose was expanding and opening up the tiny crack. Then it drew in air.

Replace these hoses on both ends of the steel fuel line. They are cheap and easy to replace. And who knows? This could be the source of all the trouble.

[Jub]

Thanks for input/direction.

Day off tomorrow and weekend, will muck around and see what I find.