Fuel pump low volume/pressure - will it cause car to not start

[scoot]

I've tried to ask this question a variety of different ways, with different variables, and each time I get answers about timing, WRD, etc and never get the answer I want.  So I will leave out all of the background information and other factors and reduce this to the most simple question I can.

If the (correct bosch) fuel pump in the Mechanical Fuel Injection car is NOT pumping the volume of fuel or pressure of fuel that is within specification, but it IS pumping enough to cause fuel to return to the gas tank, will the injection pump function for purposes of running/idling the car.  Assume I'm using starting fluid and the car is firing and that timing and plugs and all that are ok.  The only thing I'm asking is if the injection pump needs the proper input pressure and volume for most basic firing of the injectors.

I am not concerned about the circulation needed to keep the pump cool.  I am not concerned about the pressure needed to spray the CSV.  I am only asking if an under-spec fuel pump will cause the injector pump to not pump gas to the injectors.  I only want the car to start.  Not drive.

thanks
Scott

[Benz Dr.]

I've been told you need about 16 lbs but even new pumps won't put out that much in most cases. I kind of go by volume as much as anything. If you have enough volume you should also have enough pressure.
I liter in 15 seconds is the standad and I've seen more than that and also less. I figure 750ml is the cut off point where you will have running problems but it could be more or less for you.
If you are not getting enough pressure to make the CSV work poperly, you probably don't have enough to run anything. I would expect the needed idle amount to be less but I'm not sure given all of the variables present in any given application.
On most cars, it takes about 8 psi to make the fuel pressure regulator open. The idea is to flood the chamber around the injection elements with fuel that's slightly pressurized so that the fuel will get picked up by the elements.

Others may have a better explanation.

[scoot]

I've been told you need about 16 lbs but even new pumps won't put out that much in most cases. I kind of go by volume as much as anything. If you have enough volume you should also have enough pressure. I liter in 15 seconds is the standad and I've seen more than that and also less. I figure 750ml is the cut off point where you will have running problems but it could be more or less for you. If you are not getting enough pressure to make the CSV work poperly, you probably don't have enough to run anything. I would expect the needed idle amount to be less but I'm not sure given all of the variables present in any given application.
On most cars, it takes about 8 psi to make the fuel pressure regulator open. The idea is to flood the chamber around the injection elements with fuel that's slightly pressurized so that the fuel will get picked up by the elements.

For whatever reason I was assuming the pressure regulator was AFTER the injection pump.  Is that not the case?  I don't care too much about the CSV - I'm trying to figure out if I need my injection pump rebuilt.  Starting fluid followed by car will run is fine with me.  Is plugging the return line going to increase pressure in the injection pump (in case the regulator diaphragm is crap)?  I know that I don't have 1 liter in 15 seconds.  I have a "new" pump in another car that I'm going to switch for test purposes...

thanks
Scott

[Benz Dr.]

Pressure regulator is on the outlet of the pump. You should have enough pressure if you have that much volume.

[scoot]

Pressure regulator is on the outlet of the pump. You should have enough pressure if you have that much volume.

I don't have 1 liter in 15 seconds.  I know that.  So having said that, would that cause the injection pump to not have enough pressure to fill the injection lines...?   I am testing the volume at the outlet from the pressure regulator.

[ctaylor738]

When the Attractive Asian Woman's 280 arrived for its latest repair it was almost impossible to start when warm.  After at least 30 seconds of cranking with the pedal floored, it fired one cylinder at a time.  When it started to fire I had to keep cranking the starter to keep it going.  When it got to three or four of six I released the starter and after a few seconds, the remaining two kicked in.  Then it would run above 2000 rpm, but wanted to stall when the clutch was let out.  I did manage to get it into the garage and did a fuel volume check.  The 1.5 liter Sprite bottle I use only has a 1 liter mark on it.  So my estimate was that it pumped maybe a tenth of a liter in 15 seconds.

But with that pitiful flow, the car did start (eventually) and run (barely).

I also observed a 250 that would fire run briefly and die with what turned out to be a blown Fuse #4, fuel pump not running.

[scoot]

 I did manage to get it into the garage and did a fuel volume check.  The 1.5 liter Sprite bottle I use only has a 1 liter mark on it.  So my estimate was that it pumped maybe a tenth of a liter in 15 seconds.  But with that pitiful flow, the car did start (eventually) and run (barely).

  Thanks.  That was what I was afraid of.

[scoot]

Since fuel volume being pumped to the return was less than half of what the specs say (and even though I rebuilt this pump, apparently not well enough) I decided to swap in the fuel pump from the 1971 280SE that I bought as a parts car.  It had a 2 year old NEW NEW NEW fuel pump.  So I figured it couldn't hurt and it wasn't much work.  Next step after that would have been having the injection pump rebuilt probably...

And the results surprised me.  The damn thing started.  Sounds like crap and has some backfiring (through the intake) which really crude timing adjustment didn't fix but I'm still thrilled as can be that the car that hasn't run in probably 15 years actually ran on gas today.   Just one small step in a HUGE project.  But I'm pretty happy.

Thanks as always to the members for their suggestions.
Please look at my WONDERFUL 23 second video:
https://www.youtube.com/watch?v=SjkiDoxOjIw

Scott

[Cees Klumper]

Watched it and Liked it!  Congratulations!