Fuel Pump Filter

[knirk]

I have tried to get the engine to be more responsive to gas pedal movement after the warm up period for the last 14 months. Began by adding a 0.5 mm self made shims under the CSV housing. Since then I have fiddled around with different sizes until now. I ended up adding two 0.5 mm shims. Finally there is no change in responsiveness when cold compared to when warm and the engine is really running great!
When I took it for a tough test drive I smiled all the way up to 4500 rpm when the engine started to misfire – and it happened again also at different rpm's, but always after a period with the gas pedal floored. I suspected fuel starvation and went to work.

- cleaned the fuel lines
- cleaned the fuel tank filter (not much dirt)
- changed the fuel filter
- did a fuel pump flow check (got 0.75 litre in 15 sec)

And finally today I cleaned the fuel pump filter. Not much info in the manuals so I took some pictures.

One bolt and two nuts holding the pump shield (10 mm spanner).
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I used a clamp on the inlet hose instead of emptying the tank.
This is the new / small type pump. The fuel outlet connection uses spanners 19 and 22 mm.
Remember were the electrical cables belong.
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The pump is suspended in three bolts. Unscrew the nuts at the top with a 10 mm spanner.
It may be possible to unscrew the fuel filter without removing the pump, but not mine.
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The fuel filter sits on the inside of the inlet/nut unit. 24 mm spanner.
This is how much dirt that came out after I tapped it a few times. I also cleaned it with compressed air.
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After installation I blead the pump by the little screw in the by-pass connection. According to the Haynes Manual there should have been installed a hose from this connection via a T to the  fuel return hose. I have read somewhere that if not installed you need to bleed the pump with the inserted screw (accessible through a small hole in the shield).
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Finnaly I did a new fuel pump flow test and the amount went up from 0.75 to 0.8 litre in 15 sec. I used a hose with a clamp to blend off the fuel tank return inlet. I managed to swap between the hoses while holding my finger over the opening – without much spill.
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I then went for a new test drive. And even if I may still have a slight blockage somewhere or a not so good fuel pump (should be 1 litre in 15 seconds) the engine misfire under heavy load disappeared – and this time I kept on smiling.


Per G. Birkeland
69 280 sl aut 834
Norway

[ja17]

Hello knirk,
Thanks for the nice information and photos. This screen is often neglected. I assume you have  checked the fuel screen in the fuel tank also?

Joe Alexander
Blacklick, Ohio

[glennard]

re:Rust and crud in the fuel system.   'If there's a dirty Shroud of Fuelin, there will be no Pagoda Tourin'.  i. e. clean that mess of plumbing in the tank.

[knirk]

Hello Joe,
Yes I followed one of your excellent tours last week and got to make myself the 22 mm tool.

http://www.sl113.com/forums/index.php?topic=1712+tank

After emptying the tank and spilling fuel all over I was actually a little disappointed that the filter looked like new with no dirt at all.


Per G. Birkeland
69 280 sl aut 834
Norway

[George Des]

I've disassembled many of these small and large style pumps. I've been surprised at how many I have taken apart where the filter is actually missing. This filter is essential in keeping crud from the bottom of the fuel tank getting into the works of the electric fuel feed pump where it can jam the vane causing the pump to burn out or in the case of the large style pump destroying the surfaces of the sliding ring seal which in turn will cause this pump to leak fuel out the leak off tube. After working with these pumps over time, I've concluded that dirty fuel left standing for long periods in these pumps-both large and small- is their number one enemy. I have not been able to find these filters new from any of the usual suppliers, but have been able to salvage when I need one from an otherwise un-rebuildable pump. You just need to be very careful when you remove them for re-use since they are very fragile

George Des

[Ricardo]

Hello Per
I'm going through the same steps that you have taken, trying to stop my engine from sputtering, and like you I didn't find much crud in the tank or lines. I'm going to check again the filter in the pump, which I did 3 weeks ago, but not again yesterday when I drained the tank. I removed one of the shims from under the injection pump slide valve and this seemed to produce a slight improvement, but I think I'm running a little too lean now, so I will put the shim back. I would like to add another shim and I wonder what you made yours from? Will a thick paper work or does it need to be steel?
My car ran great when I first drove it this year, but after 120kms. it started sputtering on acceleration. I have changed everything but the hood ornament with no real improvement. Recently I thought it was the intake manifold gasket but it wasn't. Then I checked fuel delivery and like you, I only got approx. 3/4 litre in 15 seconds, but cleaning things didn't help. Very frustrating  :evil:
Richard
'67 250
originally 834 now 501

[BHap]

I had a similar problem when running at highway speed for an extended period of time...  also was the fuel pump inlet filter getting clogged with sediment from the gas tank...    my fix was to use an inline fuel filter between the tank and the fuel pump - from an Audi 4000 I think.....

[knirk]

Ricardo,
I used a 0.5 mm steel sheet to make the shims. Started with the big hole in the middle. Drilled many small holes around the edge and grinded it circular afterwards. The two small holes are about 6 or 8 mm. It’s a lot easier to work with if you wait until the holes are made before you cut out the final shape of the shim.
For testing purposes you could use thick paper, but as a permanent shim I would go for a solid material.

Does anybody know that the fuel pump flow should be minimum 1 litre in 15 seconds, or is this the flow normally obtained if the published fuel pressure is within limits?


Per G. Birkeland
69 280 sl aut 834
Norway

[hands_aus]

Per
I read somewhere that if the by-pass hose is not installed on the new style pump, the diaphragm of the fuel damper on the return fuel line should be replaced with one with a hole in it.

George Des, JoeA am I correct? Could this be a cause for low fuel pressure/flow?

Ricardo, The shims were used on fuel injected sedans of the same era. You could try buying some used ones.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[George Des]

Bob,

I'm not familiar with the internals of the fuel dampner and why there should be a different insert when the bypass is used, but somehwere, I've read that same thing. I've also read that the bypass connection does not have to be used on the older cars that were originally equipped with the larger style fuel pump--in this case a 4mm screw and sealing washer are used to blank the connection point at the delivery pump. My understaning is that this connection is not a bleed screw as has been referred to in this set of postings, although it certainly can be used for one. The connection was added to these pumps to allow a bypass connection that is supposed to help prevent vapor lock. Again, not sure what the mechanical connection is between the bypassa and the dampener,but there must be some reason for the change over.

George Des

[A Dalton]

My take on the by-pass hose is that the newer [smaller] pumps would be more prone to Vaporlock cuz the fuel goes through the pump motor to aid in cooling the motor, but in doing so, that also heated the gas,[specially at idle]. So, they send it through a loop/by pass for cooling.
 Whereas , in the early pump, the impeller and gas flow never saw any pump heat from the actual motor..
 The openimg at the damper on early models leaves two loops , so that may effect high rpm starvation.
 Not really sure ..my preference , as many others I know, is the early style pumps and the complete fuel line loop.
 Many claim that 113 vapor lock problems are more likely attributed to the fuel filter assemby being hard mounted to the engine mount than any other cause. The engine heat transfer here is high due to the aluminum to aluminum mounting and can be rectified by using an insulator mounting block between the two...
 

[George Davis]

All,

the fuel pump bypass hose situation is this:

With early "large" pump, there is no bypass hose from the pump to the return line, but there is a hole in the injection pump outlet valve (not in the fuel dampener).

When the "small" fuel pumps were first installed, they have the bypass fitting but it was not used, it is blocked with a screw and these also have the hole in the injection pump outlet valve (so the arrangement just like the large pumps).  Note that if a small pump is used to replace an original large pump, the bypass hose does not need to be connected.  (And as noted, the screw in the bypass fitting is a handy way to bleed air out of the pump if it's airlocked).

Later, around the beginning of 1970 (or thereabouts???) the bypass hose was installed, connecting the small pump bypass fitting to the fuel return line.  At the same time, the hole in the injection pump outlet valve was eliminated.

The situation you don't want is a fuel pump with no bypass hose fitted together with an injection pump outlet valve with no hole.

George Davis
'69 280 SL Euro manual

[A Dalton]

<<The situation you don't want is a fuel pump with no bypass hose fitted together with an injection pump outlet valve with no hole.
>>

 ... and the other is the smaller pump with the by pass hose connected with  the IP valve  with the hole ..
 This is where you would have two loops  , which will result in lower fuel pump capacity to the IP at higher rpm..

[ctaylor738]

It's the 280SE guy again.

I have my new style pump out and apart, and really need to clean the inlet filter.  How do you safely secure the pump so you can get some leverage on the filter fitting.  It is in there VERY securely.

TIA.

Chuck Taylor
107 Addict
Falls Church VA

[George Des]

Chuck,

This fitting is tough to undo. What I do if the pump is disaasembled is to very carefully clamp the lower body into a padded vice with the fitting facing upwards. I then take an appropriately sized deep socket on a long breaker bar to turn the fitting. This usally provides enough leverage to remove the fitting. Just make sure the vice is anchored and it is padded so you don't damage the housing. Some soft wood on each side of the housing against the faces of the vice will usually suffice.


George Des

[ja17]

Hello,

These fittings can be removed easily with an air impact gun on or off the car. It is especially difficult to remove it anyway else, especially when in the car.

Caution: avoid using an electric impact because of danger of fire from the electric motor and  gasoline.

Joe Alexander
Blacklick, Ohio