Question: Timing-advance Vs. Mixture

[peter190e]

Thanks for a great forum-most informative!

My question is this:

After advancing the timing, would I need to make the air/fuel mixture leaner or richer in order to stay in-range for the O2 to achieve closed-loop operation?

Thanks

[dpetryk]

The ratio of air to fuel is not affected by timing.

[mbdoc]

1986 was the last 190E 2.3 that timing could be adjusted,
What year & model car??

[peter190e]

Removed the pre-cats and cat(but not the O2 sensor) from a euro 2.5 16v. There are two trimming resistors in euro models, one for the injection and one for the ezl ignition. With the original trimmers, and without the cat, closed-loop operation is achieved (as before the cat was removed). Replacing the trimmers with the ones specified for non-cat cars, duty-cycle remains constant at 45%, and does not sway as it should if it was running closed-loop. These trimmers alter the timing. I assume the mixture is to blame.

Hence my question arises: would the mixture have to be enriched or leaned in order to be in-range for closed-loop operation to occur after the timing has been advanced. I assume it would need to be made leaner (by altering the plunger position in the fuel distributor, because turning only the mixture adjustment screw did not help ), but I am not sure.

Thanks

[mbdoc]

Heat is the only thing needed to bring the engine into closed loop.

[dpetryk]

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.

[pidgy999]

Disregarding the 02 question, since I don't know the answer...
I'd vote for running it a tad richer to possibly compensate
for the higher combustion temperatures caused by the advanced
timing.