Fuel Pressure question

[Pinder]

Hi everyone
 
   Im investigating a slightly rough idle when car has warmed up, RPM round 800. I have a new 123 ignition installed and coil. (the rough idle was present with the old distributor too. Car seems to run pretty well with the new distributor but the slightly rough idle  is bothering me. engine compression on all cylinders is between 175 to 180.   Im suspection maybe its fuel pump as I had rebuilt it.

Fuel pump pressure I measured at the fuel filter canister (using the plug port on top) and its 16 psi at idle but when I press on the gas and as rpm goes up it goes to 20 psi. Is this normal? should it not stay around 16 psi?  Does this indicate possible restriction on the return line or some filter problem?

Also I replaced the main fuel filter and also took old one apart and i did not see any rust particles. (I didn't expect to see any as I had cleaned out the tank back in 2016)


Regards

  Pinder

[WRe]

Hi,
the delivery pressure of the fuel pump should be between 0.8 and 1.1 bar (11.6-16 psi) measured before the starting valve (CSV).
If I notice a rough idle, it's usually the spark plugs (1971 280SL with 123).
...WRe

[Benz Dr.]

You will usually get a small voltage increase as you throttle up from idle speed and increased voltage will make your fuel pump run a bit faster which may result in extra fuel pressure. I'd call that normal running.

[Pinder]

Ok thanks  Dan and WRe.  Ive ordered a set of non resistor plugs and will see if that makes things better. I also have a new set of fuel injectors that just arrived. will try those and see if it idles smoother.

[Pinder]

In case anyone else wants to measure fuel pressure at the fuel filter canister I used the fitting included in the vacuum and fuel pressure tester from harbor freight.  But I suspect you can also use an air pressure hose fitting. See picture. I used a gauge I had at home that I think is for pool filter pumps . Rated to 60psi.

[Kevkeller]

Thanks Pinder. I’ve been meaning to ask what type of thread it is.

[rwmastel]

Fuel pump pressure I measured at the fuel filter canister (using the plug port on top) and ...

Pinder,

For the more visually educated, you mean at the large bolt head as indicated in this thread?
https://www.sl113.org/forums/index.php?topic=35867.msg261925#msg261925

[Pinder]

That's correct its the large bolt. If you remove it you can connect a fitting and hose to a pressure gauge. It worked for me and appears to be a close match to regular high pressure quick disconnect fittings.

[bracurrie]

I've been following this thread wondering why Mercedes wants the fuel system pressure and flow to be measured at the cold start valve versus at the fuel canister? Pressure should be the same, I think. But the flow might be different because the fuel pump supplies way more fuel than is needed to operate the engine so at the CSV you would observe if the fuel return from the MFI pump is working properly.

[Pinder]

Thats a good point.  I think I will do a pressure test at the CSV. See if I observe any differences.

[jim 56]

My fuel pressure does go up as I increase the throttle about 2-3 psi. I have a gauge installed in the fitting on the fuel filter. While on the subject of fuel pump has anybody had any expierence with an aftermarket fuel pump? If so which one
Thanks Jim

[rwmastel]

Search with "aftermarket fuel pump" with the quotes and you'll get several threads.