Mercedes 126 Repair – Mixture Control

Achieving a proper fuel mixture is vital to the performance and economy of your Mercedes 126. This article looks at the mixture-control procedure for the early V-8 engines with K-Jetronic fuel injection. Many other European cars of that era follow essentially the same method.

For cars with lambda control, it is not possible to set the basic fuel mixture correctly without a properly functioning oxygen sensor. Please refer to the companion article Mercedes 126 Repair: Tracking Down Vacuum Leaks for more on that subject. As with the vacuum-leak detection procedure, we need our car’s oxygen sensor to be thoroughly warmed up before setting the base mixture. Naturally, any vacuum leaks need to be rectified before the mixture can be set.

A Little Theory

The voltage generated by the oxygen sensor is interpreted by the lambda control unit, which responds with a voltage of its own sent to the lambda valve (frequency valve) next to the fuel distributor (FD). The frequency valve is essentially a pulse-style fuel injector — the only such device on a car with continuous injection. It is a controlled “bleed-off” of fuel back to the fuel tank. The amount of fuel bled off by the valve affects the pressure drop across the differential pressure valves within the FD. More fuel bled off increases that pressure drop within the FD and increases fuel flow to the injectors, richening the mixture; less fuel bled off reduces the pressure drop and leans the mixture out. Fuel is bled off when the frequency valve is open, and this occurs when the voltage across the valve is zero. The valve is closed when battery voltage is applied. The voltage from the lambda controller pulses rapidly and can only be fully observed on an oscilloscope. (This is known as a duty cycle.) Fortunately, however, we do not need a scope to see what the frequency valve — and by extension, the lambda controller — are doing.

A Delicate Practice

The diagnostic socket inside the left fender allows us to observe lambda control in action. When we know what the controller is trying to do, we know whether our base mixture is too rich or too lean and can adjust accordingly. To be specific, the voltage across pins 2 and 3 of the diagnostic socket shows us an aggregate voltage across the frequency valve. The lower the voltage we see on our voltmeter, the more the lambda controller is trying to richen the mixture; i.e. compensate for a lean condition. The higher the voltage, the more it is trying to lean the mixture out.

So what should the voltage be at the diagnostic socket if the mixture is perfect? There will always be some fluctuation when the oxygen sensor is hot and the car is in “closed-loop” control; i.e. constantly responding to the engine’s behavior. But those fluctuations will be in a narrow band around 12.2 – 12.3 volts. (The exact center point can be found by observing the voltage at the socket when the oxygen sensor is disconnected.) Longer voltage “journeys” into the 11-volt range indicate a lean condition; up into the 13-volt range bespeaks a rich condition.

Our response to this intelligence is to make very fine adjustments to the mixture control screw, accessed with a 3mm allen key inserted down through the tower directly in front of the fuel distributor. Turning clockwise richens the mixture; anti-clockwise leans. The screw is extremely sensitive; one should only move the screw a fraction of a turn at a time. After each adjustment, remove the allen key (so it doesn’t get stuck when the air flow meter plate deflects) and rev the engine. Then recheck the voltage at the socket. If the voltage stops fluctuating entirely, the oxygen sensor has cooled off and you’ve gone back to “open-loop” operation, wherein the lambda controller sends out a default 50/50 duty cycle. Hold the engine at 2,000 rpm for a while to warm the sensor back up.