Is 10 PSI and 2 lpm too little fuel?

[twistedtree]

I'm trying to get a car together to bring down the the P-50 event, and working on an early '64 230.

Right now I'm trying to verify the fuel delivery system which seems to be reading low.  Measuring at the CSV, I get 9-10 PSI, and think I should see 12+.  I also measured flue flow in the return line at the tank and get about 1/2 liter in 15 seconds, or 2 lpm.  I think I should be getting 1 liter in 15 seconds, or 4 lpm.

Now here's the puzzling part.  The fuel tank is new, so the usual accumulation of crud in the flower pot is not an issue.  Both the fuel delivery pump, and the fuel injection pump are freshly rebuilt by H&R.  The fuel filter is new, and I have replaced all of the fuel hoses.  Although this doesn't exonerate any of these components, it makes them far less suspect than usual.

I did two other tests which seem informative.  I measured fuel flow right at the outlet of the pump and got 1.5 liters in 15 sec.  This seems like good flow, suggesting the lower flow that I get from the return line is due to back-pressure in the system.

I measured pressure at the filter inlet line with the pressure gauge blocking the line, i.e. zero fuel flow.  I got about 12 PSI which seems OK, though on the lower side of the range.

One other potentially interesting observation.  With the pressure gauge on the CVS, I see an initial pressure build up to a little over 10 PSI, then it drops to a little below 10.  At the same time I hear what I think is the relief valve opening.

From all this the best I can conclude is:

1) My fuel pump can just barely generate 12 PSI

2) My Injection pump relief valve is opening around 10 PSI

3) At 10 PSI I'm getting substandard flow volume, probably related to (1)

But my real question is whether this is just all on the low side of an acceptable range, or it it something I need to fix?

[ja17]

Hello Peter,

How is the engine running?   Did you check the intake fitting of the electric fuel pump? There is a fine screen filter built in the fitting.

Keep us up to date.

[twistedtree]

The engine seems pretty good, but I can't say for certain because I've only been able to drive it around the yard.  But I had to richen the main rack by 10 clicks or so to get it running like this after reinstalling the rebuilt FIP.  That's a lot of clicks on a pump that would have expected to be spot on.   Maybe this is due to the marginal fuel flow/pressure, or rmaybe not.  I just don't have enough experience with these cars to know how sensitive they are to the pressure.

I'll check the intake screen.   I would have expected it to be disassembled and cleaned as part of the pump rebuild, but will double check anyway.  The pickup screen in the tank is new.

[ja17]

Was the pump adjustment you made on the main rack or the idle thumb screw?

[twistedtree]

Main rack.  Yesterday I quickly hooked up an AFR meter and took some measurements.  All are just reving up in neutral so have limited meaning, but I'm seeing numbers around 12 which I gather is on the rich side.  In response, I just leaned out the main rack by 2 click CW and will see what that yields.  My AFR "installation" needs a bunch of work before it can really be useful, but at least those initial readings give a gross picture of where things are.

Keep in mind that I'm working with an engine with no reliable history and significant things that were set up wrong and/or messed up badly when I started, so nothing is off the table as a possible problem.  The good news is that I'm progressively circling in on a car that's starting to run correctly.  Right now I'm rigged up to test the auto trans pressure settings in the 3 solenoid positions to be sure that's correct.  This, after fixing the wiring that was all wrong for the solenoid and idle compensator pressure switches.....  The car is a bundle of surprises, that's for sure.

[Benz Dr.]

Fuel volume and pressure are so closely related that they are basicaly one in the same on our cars. No pressure = no volume in a low pressure, high volume system. 10 PSI is just enough to run most engines but you may find it won't resart after you shut it down once it hot soaks. I had a 280SL in that would start when cold but would not restart once it was shut off on a warm day. A rebuilt fuel pump cured that problem.
We found that 10 PSI wasn't enough pressure to open the pressure regulator on the outlet side of the IP. I installed a 230SL regulator plate with the small hole in it which allowed enough fuel to bleed through which it kept vapour locking problems to a minimum. Since this was a temporary set up, we replaced everything we changed with proper parts when we installed the replacement pump.

Your findings indicate possible blocked or restricted fuel lines. Although we usually take a pressure reading at the CSV this line is also the most prone to blockage which can give lower readings. Try doing a fuel volume test at the CSV which may show this to be a problem. Working backwards through the system will usually help you to find any problems or restrictions. Your fuel pump is working OK and it's more likely the parts you haven't replaced that will be at fault.