1972 300SEL Fuel delivery F.I. system ouch - PeachParts Mercedes-Benz Forum

silverbenz · #1 03-25-2008, 03:18 AM

I love this car, but the fuel system; Fuel dampner, fuel pressure regulator,
that vacuum controlled fuel thing at the firewall, and that little wheel you
can turn with your fingers up by the RH headlight?!
How do I get all these children to play well together? (4.5L V-8)
Symptoms;
Starts easy cold, then runs worse as she warms up, especially at speeds 10-30 mph.
Got stuck overnight, no fuel delivery at all under the hood. I hit the fuel pump the next with a wrench, then it started and has been Ok for 3 wks.
I'm getting 12 mpg.
Ignition equipment, fuel injectors are fairly new and not suspect.

Any tips would be appreciated.
Viele Danke!
Baron von Krieg

Housecrew · #2 03-25-2008, 08:23 AM

first spend your car a new fuel filter. then set the fuel pressure to 2 bar.
if that does not help check your injectors. there are small filters in the
injectors which can be dirty. i found a lot of rubber from bad fuel lines in mine.
then you can also pull a spark plug out to see if the car runs too rich or to lean.

12mpg is not good at all...

cth350 · #3 03-25-2008, 04:42 PM

12mpg isn't that bad, but it's not great.

How to get the parts to play nicely together? The answer is simple, fix it one step at a time and don't cut corners on the diagnostics.

- First and foremost, your ignition must be "to spec", your battery properly charged and your fuel tank at least three quarters full.

- Second, make sure there are no vacuum leaks.

- Third, get the fuel pressure at the fuel rail to 32psi and make sure the pressure holds there after the system is turned off.

- Follow the diagnostic tree in the service manual. There are two versions of it. One for those people lucky enough to have the fancy diagnostic tester and a second version for using just a multi-meter. This will make sure individual sensors are working correctly.

- With a CO meter in the tail pipe, get your idle to 800rpm and the CO% to 0.5%

As for changing the MPH, you can adjust the MAP device to get you incredible performance or fuel economy, but not both. There are other threads with guidelines about how to adjust it.

-CTH

silverbenz · #4 03-27-2008, 08:09 PM

I bought a nice fuel pressure test kit for FI cars on ebay, am still waiting for it. Where's the best place to tap into the line for fuel press test?
I replaced fuel filter, no probs.
I noticed that my fuel pump has screws that hold the top on; Is this
a rebuidable unit? I notice that replacements are $500 more or less.
What's up with that? Or can I just shove a high perf. aftermarket electric fuel pump in there ($50) adjust the fuel pressure, and off I go?
Thanks for your tips gentleman, it is much appreciated. This is my first Benz.

Baron von Krieg

Tom Drew · #5 03-28-2008, 10:02 PM

The service manual says to connect the fuel pressure gauge into the "ring line", which is the flexible line that connects the left & right side fuel rails at the front of the engine.

cth350 · #6 03-29-2008, 09:35 PM

Simplest place to connect the fuel pressure guage is by disconnecting the cold start valve. This also removes a source of leaks, so if everything suddenly works better, you will have some idea as to why.

Any 70s or 80s fuel pump will do to replace your efi one. I look for them at pick&pulls or ebay on a regular basis. Having a few spares handy is always helpful.

-CTH

silverbenz · #7 05-10-2008, 04:48 PM

Please don't laugh too hard at my lack of knowledge, this is my first benz.
I've got the rail fuel pressure at 2 bars with a 3/4 tank of gas. Fuel filter is new. Now please, correct my assumptions, as my technical literature makes no menion of either.
The MAP is the device by the firewall, adjusted my a hex wrench.
The ECU is that covered module by the right head lite (ECU is Engine control unit?)
The car is running good, but I'd like to give up some power for some fuel economy.
I don't have a CO2 sniffer. Any tips on how to adjust the MAP and ECU
to maximize fuel economy?

thank you,
Baron