Knock sensors: How do I know if my engine is knocking?

[ssm1991]

Been trying to 'tune' and maintain my engine in preparation for FI.

Went to a dyno a few weeks back and found out my car has been running lean at higher rpms on load. My tuner and most references suggest 12.5-13.2 AFR (0.85-0.90 lambda) as the ideal values when accelerating. Mine was going from 0.90 at low rpms to 1.0 on higher rpms. We heard intermittent knocking during the dyno with the hood open. Of course I hadn't noticed this before when driving.

So using my AutoEnginuity scan program, I've been trying to diagnose the source of the lean mixture. I want to get it fixed since I might blow up my engine going to FI.

Here is some OBD-II data I gathered:
1) Mixture Adaptation (Additive) - Bank 1: 0.56 ms (also called short term fuel trim STFT)
2) Mixture Adaptation (Additive) - Bank 2: 0.56 ms
3) Mixture Adaptation (Multiplicative) - Bank 1: 12.0% (also called long term fuel trim LTFT)
4) Mixture Adaptation (Multiplicative) - Bank 2: 12.0%

I understand 12.0% LTFT is a bit on the high side, but then still can't compensate enough for the lean mixture. I also never had a fuel-trim DTC (P017x) or any other ignition/fuel related DTCs.

My car runs perfectly on idle, so I don't suspect a vacuum leak as the cause of the lean mixture.

The chart below shows the voltage of the knock sensors during a spirited 4-min drive, as seen by the ECU and read live by the scan tool:
http://forum.e46fanatics.com/attachment.php?attachmentid=249109&stc=1&d=1195045157

Questions: Is my engine knocking? At what voltage is it considered dangerous?

Any thoughts also on my next steps to fix the lean mixture? Plugs are brand new and fuel filter/regulator was replaced 9 months ago. I also don't suspect the injectors since fuel-trims are the same for both banks.

[02black330ci]

First and for most, check your O2 sensors, make sure they are functioning correctly. One quick way to tell if you are knocking is to log your timing values. When your car knocks, timing will be retarded by the ECU and if all things are working right, fuel should be added to prevent from things blowing up. I haven't done much research on the LTFT's for beamers, but I know that 12% is out of place for sure. Like you said start with simple things, ie: O2 sensor, vacuum leaks, malfunctioning injecters, etc.

Also if you are going to be FI soon, why not just invest in a wideband O2 sensor? Now a days they are very affordable, replacement O2's are cheap and they will save your ENGINE!!!

Hope this helps.....

[ssm1991]

Thanks. The stock O2 sensors seem to be working fine. I checked the live graphs of the pre-cat O2 sensors through Autoenginuity and the output is consistent with lean mixtures on load -- on idle/cruising the voltages oscillate regularly, but at full throttle would stay low. Fuel trim (STFT) also works in that it moves the opposite of the O2 readings. What I don't get is why the ECU doesn't increase LTFT anymore despite clearly showing lean mixtures on load, or why it doesn't throw a P017x DTC if 12% LTFT was indeed a limit.

And yes, I'm looking for a wideband O2 sensor before going FI for peace of mind too. Looked at AiM, Innovate and others but might order the PLX Devices SM-AFR/DM100 combo (http://www.plxdevices.com/products/dm100/smafrcombo/) instead.

[ssm1991]

I'll check and graph the ignition timings later today. Thanks.

[02black330ci]

PLX Devices SM-AFR/DM100 combo (http://www.plxdevices.com/products/dm100/smafrcombo/) instead.

PLX is what I've always ran and had no issues. I haven't bought a gauge from them in a while or been up to date with their products, but WOW!!! That SM-AFR/DM100 combo is just insane :bow: