Fuel Trims - Are Rear O2 Sensors Used?

[R3VM3UP]

Does anyone know for sure whether the rear o2 sensors are used in any way for fuel trims or anything outside of catalyst monitoring?

[Tbonem3]

No, only catalyst monitoring

[R3VM3UP]

Do you have any more details on this, or a source for this information? Some modern systems use the cat out o2 sensors to adjust the primary fuel trims from the upstream sensors. I wanted to verify that the MSS54 is not doing anything like that.

[thegenius46m]

Do you have any more details on this, or a source for this information? Some modern systems use the cat out o2 sensors to adjust the primary fuel trims from the upstream sensors. I wanted to verify that the MSS54 is not doing anything like that.

MSS54 is pretty dated. Rear 02s do virtually nothing but cat monitoring. The dme controls all the fueling based on sensor input conditions and tables.

[sapote]

Do you have any more details on this, or a source for this information? Some modern systems use the cat out o2 sensors to adjust the primary fuel trims from the upstream sensors. I wanted to verify that the MSS54 is not doing anything like that.

I don't think it's safe to use post cat sensors for trimming the fuel -- what if the bad cats don't burn the HC clean then the post sensors tell the DME to trim down the fuel, causing very lean and damage the valves?

[Tomba]

On MSS54 there is little information on trim-regulation with post lambdaprobes. There is a monitoring used to detect if the pre-lambdasondes are not aging and responding to slow with the help of the post lambdasondes. Imagine if you go from lean to rich mixture (non working catalyst) and the post lambdasonde would detect the rich mixture earlier than the precontrol lambdasonde. It would mean they respond to slow and an error code can be triggered.

On MSS60 (and newer) the post lambda sondes are used more to control the desired lambda. In case a fuel cut occurs (coasting with no throttle pressed) it takes some time purge the catalyst from its oxygen once injection is started again. Once you have a richer mixture for a short time it helps to get the catalyst working again faster. This is done in MSS60 ECU's. Based on the calculated lambda value from the post lambdasonde a correction is made to the target lambda.

On MSS54 I haven't read this. Might not be necessary with Euro3. The MSS70 ECU in Z4 might have it it to comply Euro4 exhaust emissions...