I have a 1990 300E 2.6 and I have recently gotten it running great. I replaced almost everything under the sun, air flow meter assembly, distributor cap and rotor, spark plugs and wires, idle micro switch by the throttle linkage, coolant temp sensor, o2 sensor, cold start valve, injectors and seals, air filter, check valve, fuel pump, idle control valve, and fixed multiple vacuum leaks. While idling, it runs at a perfect 45-50% lambda, but when I accelerate up to 2500-3k rpms, it starts to drop slowly down to 10% and holds there. I have tried 2 different ECUs, 2 different EHA's, plugged off the cold start valve, loosened 2 injector lines at once while running, and tried a different distributor. After doing all of that, it still idles perfect but runs rich at 10% while at 2500 rpms. I did get codes 5 and 11 today, which I believe are O2 sensor (5) and air injection system (11) which i have the air injection unplugged because it is noisey. I have already replaced the O2 sensor with an OEM part, which by the way, everything I have replaced was all from mercedes OEM. Please help as I am stuck. Thanks a lot.

bump

If the ECU is outputting a constant percentage then it is throwing out a fault.

According to the Mercedes service manual 10%:

"Sensor air volume meter wrong pole connection or defective, speed approx 2000 rpm"

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I'm pretty new to this but the things that jump out at me from the FSM is

1) Clear the fault codes after testing
2) The procedure for setting the mixture is pretty involved - see chapter 07.3-110 para 24 for example

FSM is available for free and legal download via www.startekinfo.com
Click on B workshop resources
Click on CD/DVD
Click on W201(easiest to reach M103 data this way as the W124 manual sometimes has internet glitches)

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

x2 for the airflow potentiometer.
Separate the plug just enough to get probes in.
Engine idling: Terminals 1>3 = 4.5 - 5.5 V; Terminals 1>2 = 0.5 - 1.5 V
Engine OFF, Key ON: Terms 1>2, depress airflow sensor plate, voltage increases continuously to 4.5 - 5.5 V

If these values are not obtained, the pot may be defective, or the plug is reversed.