Fuel return line flow

[230slhouston]

Hello,
How much fuel should be returning from the fuel return line? I cracked the return line from the IP, it trickled out. I cracked the line going from the filter to IP, there was a gush of fuel out?



Maistran (MB)
230sl

[Shvegel]

There is a restriction in the outlet fitting of the pump so what you are seeing is normal.

Basically the return helps to clear vapors in case of vapor lock and also helps to keep the pump cool.

[merrill]

something else to note.

the retun line from the inj pump, should leave the inj pump banjo fitting, into a metal L shaped line,  then to the cloth covered rubber hose.

the L shaped line should point to the front of the car then turn down.  supposidly this prevents vapor lock as the return fuel leaves the inj pump.

Matt
Austin Tx
66 230 sl - "white"
78 300 D - Blue
98 C230

[hands_aus]

I reckon there should be a fair flow of fuel not just a trickle back to the fuel tank.

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

[glennard]

Electric fuel pump is pumping about a gallon/minute, + or -.  Engine is using 0.06 gal/min, + or -.  So, 95% of the flow is on a merry-go-round, tank to FI pump and back to tank.  What goes around, comes around.  (I think the) gasoline looses about 20 or 30 octane from dizzyness.

[Shvegel]

glenard,
Keep in mind the pump will flow 1 GPM with no restriction of the outlet. Keep in mind recirculation flow is through a restriction less than 1/8" in diameter.

[CraigD]

Even with the restriction of a 1/8" line, does the 1 gpm output seem uselessly high?  Why do you think a pump with such high capacity would be chosen?



Craig
'70 280SL Euro, Manual,  Leather
Silver/Black

 

[glennard]

It's a centrifugal pump with a specific pump head curve.  Factor in the friction losses, orifices, nozzles, etc. etc.  The FI pump requires a good suction pressure, i. e. flow rate on the head curve.

[al_lieffring]

The flow rate of 1 litre per 15 seconds should be the flow rate measured at the return line. The high volune is needed for the fuel tank pick up to work properly. The flow from the return line goes into the fuel strainer assembly (flower pot) through a venturi port, this creates a suction that draws more fuel in and fills the flower pot up to a higher level than the rest of the tank. This helps prevent fuel starvation when cornering and braking at a low fuel level.

Al Lieffring
66 230SL

[230slhouston]

Al,
If I am understanding you correctly, you are saying that if I remove the the return line that goes back into the tank and check the flow, it should 1gpm returning from the IP? I did not check it that far back in the return line, I cracked the line going back into the dapmner, it was barley a trickle but the line into the IP had some serious flow. This could be some blockage in the IP? Is there a check valve somewhere between the filter and through the IP and return line?

Thanks

Maistran (MB)
230sl

[Benz Dr.]

There's a valve on the out put of the injection pump. It could be stuck. If you have proper fuel volume before the pump but little after it that would be a good place to look. Never seen this before but anything can happen.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[230slhouston]

Dan,
The check valve is not blocked (removed and and checked it out), I removed it and there was decent flow volume, same as the inlet. I measured the flow, it is almost 3 liters in 30 seconds. Could it be pressure? However, the return line line is a lot less, is it possible the pressure is to low to activate the check valve?

Does anyone know were I can buy a cheap pressure gauge to measure the pump pressure?

BTW, I did do a pump repair with a kit from George Des.

Thanks
Maistran

Maistran (MB)
230sl

[Benz Dr.]

You have more than enough volume. Did you check the line for obstruction? I've seen lines that were completely plugged by rust. You have to get another one if it is.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[230slhouston]

Dan,
I cracked the line from the outlet of the check valve and the flow was a trickle.

However, I did get the car started and drove it 15 miles, the farthest thus far and touch wood, it has not begun its downhill decent, of missing to not starting at all.

I have another thread as well on the fuel mixture.

Thanks
Maistran.

Maistran (MB)
230sl