I have a 1988 Mercedes 300 SEL automatic with 240,000 miles.
It starts with difficulty when cold (70 F) then runs drives and idles smoothly for about 1 to 3 minutes after this time it stalls as if it's too lean. It is then difficult to restart and when it restarts it doesn't keep running. Again it sounds like it's too lean.
When cold cranking with the gas pedal floored, it occasionally (about every 5 seconds) it fires on one cylinder too early and stops the cranking instantly. This also happens when warm starting but more often.
The oil dipstick smells like gasoline.

I checked the overvoltage relay and it is good.

Is it possible the problem is the fuel pressure? I don't see any schrader valves in the fuel line to measure the pressure. Is there an easy way to check this without creating my own pressure measuring tee? This car has what looks like a fuel distributor as well as electrically controlled fuel nozzles at each intake port.

Any suggestions?

The oil dipstick smells like gasoline.
That maybe a problem? I believe that sometimes when your fuel pump is going out.. fuel gets in your oil!?!
As for the runs good cold & not warm... could be the module in the distributor under the rotor button?

Have the temp sensor checked out! Chance's are it is bad!! let me know if this heklp's you thank's lance Quick

E-mail me at ironworker9669@hotmail.com If you wish

I looked into this problem a little further, It looks like the engine runs on the cold start valve until the timer times out then this valve probably closes shutting off the fuel from this source. There seems to be no fuel squirting out of the fuel nozzles when turning over after it shuts down. I pulled a nozzle out of the intake and turned the engine over and saw no fuel delivery. There seems to be plenty of fuel pressure at the inlet to the fuel distributor. I also saw a large crack in the ignition distributor which probably accounts for the low energy sparks Yellow and they don't arc more than about 1/8 inch at the spark plugs. The coil seems to be putting out a good energy spark when cranking. Looking at the coil wire it has a blue arc at least 1/4 inch long. I'm going to change the distributor cap look into the fuel distributor issue further.

I have the same car with the same symptoms, minus the weak spark , any thing new that you have tried?

Hello.
The main problem I found was the electrical connectors to the Air flow sensor and the throttle position switch were corrorded so there was no signal going to the computer that there was any air flow. The computer apparently runs the fuel mixture a bit richer after cold start but then about a minute or two later goes to the normal operation based on the air flow sensor.

I found the best way to clean out the white corrosion was with some "ZAP" and a Qtip. You can find "ZAP" at you local Walmart or maybe Target. It's an acid based cleaner so after you see the corrosion is gone you need to wash out each contact, pin and socket, with water. I used a spray bottle set to "stream" with distilled water. Afterwards I blew it dry with compressed air and wiped each socket with a small dab of "Lubriplate" grease delivered by another qtip with the cotton tip snipped off.

After this cleaning you can test the air flow sensor with a volt-ohmeter.

With the ohmeter test between the outer two male pins on the air flow sensor. It should read about 5000 ohms. Then check between the top and the center pins. Watch the meter while slowly moving the air flow flap in the top of the air horn. It should slowly increase in resistance smoothly through it's travel. If the resistance reading doesn't change smootly and linearly you may still be ok based on the next test.

You need to also check the voltage reading between the top two pins with the ignition on but not running. You will need to get something that will connect the top two pins to the + and - leads of your meter while the connector is plugged together to the cable harness. You may need to get some small meter tips with hooks on the ends. These will fit with the connector about half way on. Polarity doesn't matter on a digitial meter.

Once the meter is connected, turn the ignition on without starting the car. you should read less than 0.5 volts. Now push the air flow flap slowly and watch your meter. The voltage change should be smooth all the way up to about 4.5 volts. If it's not smooth, you air flow sensor is bad. According to Mercedes parts the sensor is part of the entire air horn assembly and cannot be purchased separately. It must be calibrated to the air flap.

Using the same method you should verify the voltage applied to the air flow sensor by measuring the voltage between the outer pins. It should be about 5 volts. IF you don't get 5 volts here, you need to check out either you wiring between this connector and the computer. My computer is located inside the passenger compartment behind the passenger side kick panel.

As a side note. I also had an intermittent coil that would arc through a small crack in the case to the mounting bracket.

Good Luck

thanks, I will check the ness sensors, I also have a couple of other benz for parts so I can swap out any bad parts. If any one needs parts for 300 series or 190 series I have about anything including motors and driveline parts . I need to get them from under my sheds so feel free to ask I may make you one helluva deal on parts .

email me here: rivermanfarms@yahoo.com

thanks again I'll keep you up to date
Nate

someone sent me a mail but the message didn't come through , could you please try again ? thanks Nate