PeachParts Mercedes-Benz Forum - View Single Post - Question: Timing-advance Vs. Mixture

dpetryk · #6 05-07-2008, 10:13 AM

Timing does not affect the air fuel ratio. I assume you are talking about ignition timing. The air fuel ratio is determined by the mechanics of the injection and fuel delivery system. Changing the ignition timing will have no effect on the injection system.

IF you are trying the get the engine into a closed loop mode the fuel delivery system must be set so that it is "close" to the correct ratio under most operating conditions. Then the oxygen sensor is used to "trim" the air fuel ratio to keep the engine at Lambda. If the fuel delivery system is too far away from lambda then there is not enough authority from the air / fuel trim device (usually the EHA for the older cars) to adjust the ratio the keep the engine at lambda. The engine will run either too rich or lean.

The adjustment procedure is to get the basic mechanical settings of the system as close to perfect as possible and then the lambda computer will be able to keep it in range as the engine operating conditions vary.

Ignition timing is just not a parameter that varies the air fuel ratio. It is reasonable to expect that the air fuel ratio needs to be adjusted as the engine moves through the various load and speed conditions, but it is difficult to say exactly how. There are so many variables that affect direction that the "trim" needs to move to hold the engine in lambda. That is why they invented the "closed loop" system to account for all the variables involved.

I guess the simple answer you may be looking for is to determine is when you adjust the timing, does the engine go lean or rich. You can tell by looking at the voltage on the oxygen sensor to know for sure. Then adjust the mixture as needed to get it where you want it. But know that it will only be good when the exact conditions exist such as - humidity, RPM, engine load, manifold pressure, engine temp, barometric pressure, air density etc.