Mixture Adjustment in K-Jet & LH - 200/700/900 Models

Improper fuel mixture is the easiest way to lose MPG.

Not only will too rich a mixture waste fuel but too lean a mixture produces less power per engine cycle and ends up consuming more fuel per mile. For early model cars using the K-jet (mechanical fuel injection) system, mixture adjustment is quite simple. A 3mm allen wrench is necessary to access the adjusting screw. Turning this screw to the right richens the mixture while turning it left leans it. The adjustment is located on the main fuel distributor body and is fairly easy to access for both turbo and non-turbo cars. A good service manual like the Bentley bible goes a long way when doing adjustments like this.

To adjust the mixture correctly a few basics must be in order. First, a working O2 (oxygen) sensor must be present. The oxygen sensor does exactly what its name implies. It measures the unused oxygen in the exhaust stream. A low voltage reading (0.1 – 0.4 volts) indicates a lean mixture while a high reading (0.6 – 1.0 volts) indicates a rich mixture. Be careful when using this terminology, as it is very easy to forget the oxygen sensor only measure oxygen not fuel.Therefore a rich mixture really means a low oxygen content in the exhaust, not necessarily more fuel. Oxygen sensors that operate from 0-1 volt are called ‘narrow band’ oxygen sensors.

Another consideration that is commonly overlooked is the exhaust condition. Every time one of the four cylinders opens the exhaust valve a pulse of positive pressure pushes the exhaust down the pipe and back to the tailpipe. In between these pulses a small vacuum (negative pressure) develops that can suck in ambient air if the exhaust system is not 100% sealed. This in turn affects the oxygen sensor output and can indicate a lean mixture. In turn the ECU will add fuel to compensate, even though the engine may not need it.

The job of the ECU is to constantly balance the fuel mixture right at the perfect mix (called stoichiometric). This means the oxygen sensor should constantly swing from rich to lean about once per second at idle and about 2 times per second at cruise. For K-jet vehicles this adjustment of fuel mixture by the ECU is done via the frequency valve. The frequency valve is like a conventional fuel injector for its time. This valve simply adds fuel based on the ECU’s interpretation of the oxygen sensor. Because of this style of mixture control smaller intake vacuum leaks do not necessarily cause big trouble as it does with later model LH (electronic fuel injection) systems.

Consult your service manual as to how to use a multimeter to measure the frequency valve’s duty cycle. By adjusting the 3mm allen screw in the fuel distributor you can “tune” the frequency valve into its normal operating range making sure that the engine is getting the right amount of fuel.

For LH fuel injection fuel mixture is more tightly monitored by the ECU and includes additional parts such as the airflow (MAF – Mass Air Flow) sensor. The computer constantly looks at incoming airflow, RPM, engine coolant temp, etc.. to determine optimal fuel mixture based on a preprogrammed map. Most of the time the system can adapt to changes in operation such as engine wear, small vacuum leaks, etc. However fuel mixture can sometimes run out of range through wear of the MAF, engine coolant temp sensor, or other engine sensors. When this occurs a manual adjustment of the fuel mixture may be necessary. To determine if your model has an adjustment locate the MAF sensor and look for a small circular metal plug about one half inch in diameter.You’ll have to remove this plug to gain access to the slotted screw underneath it that adjusts the mixture. To remove the plug a small blade screw driver can be used to pry it off the MAF housing body, be careful as replacement MAF sensors can be expensive. Once the plug has been removed you can adjust the MAF mixture screw so that the oxygen sensor swings from lean to rich about once per second while idling.

Sometimes viewing the oxygen sensor output graphically can be easier than trying to interpret the raw numbers from a multimeter. A handy tool for monitoring oxygen sensor output is an AFR meter (Air Fuel Ratio). These types of devices can range from the simple to the complex and price is usually reflective of features and quality. For narrow band oxygen sensors there are some different choices available.

One type of AFR meter is the standard gauge type that fits in a normal 52mm or 2 1/8 inch opening. This gauge has LED’s that indicate the voltage with each LED referring to one tenth of a volt. This makes viewing the oxygen sensor output very easy and allows the driver to get a glimpse of what is happening with the fuel mixture as the car is being driven. Other types of AFR meters include their own oxygen sensors typically called wide band. The term wide band refers to the sensors ability to measure air fuel ratios over a wider range or wider band. These types of oxygen sensors are usually only necessary for high performance vehicles especially turbo models. Certain late model cars are now coming equipped with wide band oxygen sensors from the manufacturer.