Fuel pump pressure

[Allforjags]

Hello
Does anybody knows the correct pressure for short fuel pump  w113  280 sl and where i must put the pressure gauge to measure correct pressure. I have small misfiring problems at high RPM and a lack of power on every 3-4 seconds

Thanks

[WRe]

Hi,
the correct fuel pressure lies between 0.8 and 1.2 bar, but everything above 0.4 bar should be ok.The bypass-valve opens at 0.8 but could be higher because of dirt. The amount of return flow to the tank (from injection pump) should be ca. 1l in 15-30 seconds.
Attached picture from pagodenteff.de  shows the fuel flow and a possible measure point.
Did you check the filters (from tank to engine)?
...WRe

[Allforjags]

Yes i checked filter in the tank, filter in the fuel pump and changed gasoline filter under the hood … all like new

[Benz Dr.]

Fuel volume should be around 1 liter in 15 seconds. 1 liter in 30 seconds is only 500 ml in 15 seconds and that's really not enough. Measured in PSI, the car won't run well under 10 PSI and new pumps should produce 15 PSI.

Pressure and volume tend to work together so if you have low fuel pressure you will likely have low volume as well.

[cfm65@me.com]

Hi Dan,
The original question by Allforjags is with regard a 280SL.

I have just replaced a leaking “tall” pump on my 250SL with the recomended Pierburg E1F fuel pump. The filters have been cleaned and the return line is clear.
The Pierburg returns one liter of fuel in 30 seconds back to the tank.
Would this be sufficient for a 250SL?
Regards
Chris

[Pawel66]

Chris,

Have you also checked the filter/screen in the tank as well as the little "rectangular" hole in the tank that fuel needs to get through?

Also (this was my case): have yo umade sure the soft fuel lines are not bent or squeezed by anything? Fuel pressure dumper clean inside?

We will see what Dan says, but you may be low on 0,5l in 15 sec.

[cfm65@me.com]

Hi Pawel,
Yes the filters are clean and no kinks in the hoses.
I am not sure about the rectangular hole in the tank. Please tell me more.
Regards
Chris
Pawel, I did some more research and I now understand what you mean by the rectangular hole.
Thanks
Chris

[ja17]

Sounds like that pump may not be up to the task.  Check the fuel pressure while the engine is running.

[cfm65@me.com]

Thanks Joe,
I would be really upset if the pump is unsuitable for the 250SL or faulty.
I ordered it from one of the usuals in Germany. Advertised in their website as ‘alternate’ which I assume meant replacement.
I’ll rig a fuel pressure gauge on top of the filter housing and report back.
Regards
Chris

[Chris_ATL]

Have you tested the injectors?

That could be causing the issue if there is a speck of rust that somehow got in there...

[cfm65@me.com]

TKS Chris,
Much appreciated. I started from the rear, because of a leaking fuel pump.
I am contemplating having the fuel pump as well as the fuel injection pump rebuilt.
Unfortunately, being in Africa, I need to look abroad for assistance with these units.
Regards
Chris

[Benz Dr.]

1 liter in 30 seconds is actually 500 ml in 15 seconds and no, I don't think it will work well but then we don't know what your fuel pressure is. I feel that anything under 750 ml in 15 seconds is starting to cut it pretty close. This volume corresponds to around 10 PSI which is the minimum this system will work at effectively. Your mileage may differ and I'm speaking about what works in general but it is what I've seen.

[ja17]

You can just un-hook the fuel line going into the intake solenoid starting valve and clamp a pressure gauge on the line there for a pressure check. The high volume of fuel circulating in the system helps cool the fuel, the IP and the electric fuel pump. Lower volumes of fuel will cause starting problems especially when hot. Fuel pressure is a little easier to achieve from a substitute pump. Most are lacking correct volume.

[Benz Dr.]

You can just un-hook the fuel line going into the intake solenoid starting valve and clamp a pressure gauge on the line there for a pressure check. The high volume of fuel circulating in the system helps cool the fuel, the IP and the electric fuel pump. Lower volumes of fuel will cause starting problems especially when hot. Fuel pressure is a little easier to achieve from a substitute pump. Most are lacking correct volume.

I agree. This is a low pressure high volume system. You need pressure AND volume to make it work right. You wouldn't think it would make that much difference but everything has to be prefect or at least PDG for this system to work dependably under all working conditions. Most problems occur during starting, hot or cold, stop and go traffic, or really hot weather, and I'd say most of this revolves around low volume or blocked fuel lines.

[cfm65@me.com]

Thanks gents, much appreciated. Pawel mentioned a rectangular opening which I assume is under the coffee pot.
I have removed the tank drain plug as well as the fuel gauge unit in the tank. However I cannot see the rectangular opening. Although there seems to be about an inch deep fuel remaining in the tank, fuel is only dripping out of the main drain hole.
My car has a bit of a low nose attitude.
Any suggestions will be much appreciated.
Chris
Hi guys, the pump pressure is only 8 psi

[Pawel66]

Whe I struggled with my system:

I was going through it twice, but in essence when I did it the second time:

1. Having discovered low flow at the end of the return line, I decided to check the flow after each "stage" of the system. First: was the fuel stream solid just from the fuel supply hose from the tank before fuel pump and additional filter I had (I just poured gas through filler and observed if there is a lot of gas coming from the hose or it is just dripping). A lot came out as I poured so I assumed the opening under the flower pot and the screen inside were ok.
Second: after additional filter, before fuel pump. That was ok.
Third: flow measurement right from the fuel pump. It was ok.
Fourth: fuel flow measurement before the main filter - that was low. I followed the fuel line and I found out I sgeezed the hose with the fuel pump protecting "bucket" edge when I fitted it (took me some time to confess I did that...).

If the fourth measurement was ok, I would have measured after the main filter, then if ok - after FIP , if ok - after dumper. If all these were ok - the blockage would be in the return hose.

When I did it the first time, I figured on the second stage my additional filter was clogged.

2. Please note: the hole under the flower pot is needed when fuel level drops below the flower pot wall height. If the fuel level is above and the screen on drain plug is ok - it will be ok. But if the fuel level is low and you have that hole clogged - fuel will not get to flower pot and will not get to hose through the screen.

If you have the drain plug with screen removed, if you have just an inch of fuel in the tank and if you pour gas through filler neck - you should get solid flush of gas down the drain plug opening. If fuel is just dripping - there is a fair chance your hole under the flower pot is clogged. You will get fuel through with full tank, you will get no/limited fuel when fuel level is low.

The opening under the flower pot is shown here, but it looks like you got there already: https://www.sl113.org/wiki/Fuel/Tank

You can also test the fuel pump itself quickly: inlet hose to fuel canister with fuel, outlet hose to a container where you can measure flow. Our fuel pump Guru, Bob, says that for the Bosch pumps you should let it pump for 10minutes so that it can warm up before the measurement (performance may fall when pump worms up). Not sure if this applies to your pump. If so - both hoses to a bit bigger canister with fuel for 10 minutes.

All safety rules while working with fuel apply.

[cfm65@me.com]

Tks Pawel,
Much appreciated. I think the gap under the flower pot is clear, I’ll have another look.
Thanks
Regards
Chris

[ja17]

The hole is directly under the return line going into the flower pot. It starts off square in shape but the actual hole is round on the inside. It is very low in the tank and can easily be clogged from dirt or debris.

[ja17]

Here is a good photo from the BBB of pressure test specs and connection point...………….

[cfm65@me.com]

Tks Joe,
Much appreciated. Yep, I noticed the rectangular intake below the return line, poked a wire through both inlets, seems clear.
Fixed the pressure gauge on the other side of that line, where it Tees off just before the fuel injection pump.
Many thanks. Clearly the pump is not up to scratch on pressure or volume.
I have had a chat to the supplier. He’ll follow up and come back to me.
Cheers
Chris

[Benz Dr.]

There are more places than the usual listed in this thread.

1) The hole in the flower pot turns into a channel at the bottom with a walled off section on the inside edge of the main pot. This area can also plug up with stuff and restrict fuel flow. So, just because the hole in the bottom of the flower pot is open, doesn't mean it's open to the inner reaches of the flower pot.

2) the return line inside your fuel tank can become plugged. I shoot a bit of compressed air into the return line at the engine bay end and then I listen for air bubbles coming up through the fuel inside the tank. If everything is good you will hear that it is. If not, figure out why. I've used a bass guitar string on a hand held power drill to sort of roto-rooter my way up to the flower pot. Worked OK but again, you need to be careful doing it.

Note: 2 safety considerations. Remove the fuel cap from your tank before you start adding any air pressure. Pressure inside the tank can and will blow back at you. This will also happen if the line is plugged. For this reason I wrap a rag around the air wand and fuel line to avoid a face full of fuel. Do this job outside and well away from any sources of ignition. Don't find out the hard way..... 

[cfm65@me.com]

Thanks Doc,
I managed to blow through the return pipe from the front and heard the bubbles.
I’ll give it another go.
Chris