BatManv8v

Hi all

Some advice please.
I am currently chasing a perceived lack of power at WOT on my 2007 S8. VCDS doesn’t show any faults.
I have collected some logs, specifically MB 020 - Ignition Angle Retard, and have included the data below in an attachment. I am only showing cylinders 1-4 on ECU1. I notice that I am getting retardation starting just after I hit peak torque.
A few questions:

• The retardation peaks at -7.5. Is this excessive? I assume that if it was outside the built in tolerances a fault would be thrown.
• Why do the values differ between cylinders?

My thoughts on the possible causes:

• Underfuelling
• MAF under-reading on airflow
• Knock sensors playing up
• O2 Sensors

I had my injectors removed and tested in January of this year - a few of them were leaking and were causing codes to be thrown. I have had a carbon clean done twice in the last 2 years.
Any advice would be welcome.

Thanks in advance.

volvopentaman

If you have cleaned injectors, we can probably rule them out.

So we have on that engine a carbon buildup issue that can cause overfueling (too little air into the cylinders due to carbon buildup in the intake runners).
Second issue is intake manifold flaps.

Third issue is bad compression, leaking cylinders, valves or rings.

So first do a compression test (first without oil, IF cylinders are low on compression, under 12bar test by putting a teaspoon of oil in through ONE spark plug hole at a time and re-do that cylinder compression test. If you get a significant difference you have leaking rings, if not, you have leaking valves..

Second check for intake carbon buildup and clean. Even a Small amount of carbon is bad. This both restricts airflow and makes the valves leak.

You could also do italian tuneup carefully.

I doubt you have faulty sensors, as it would trigger a fault code if the lambda sensor or MAF readings would differ more than 10% from what is expected.

The FUELING system Works separately on both banks like this:
1. MAF reads airflow and feeds fuel accordingly
2. Front lambda reads AirFuelRatio. If it differs more than 10% a lean or rich code is thrown at idla and/or normal load. NOT on WOT as far as I know.
3. Rear lambda monitors catalyzer AND front lambda function. If the rear lambda gets wrong fuel mixture it will throw a P0420

AND to note the fueling only checks the BANK. So if one cylinder is rich or lean it only compensates for the WHOLE bank, not for one cylinder.

BUT compensation in both spark advance and ALSO FUEL IS DONE by inductive knock sensing ON EACH IGNITION COIL. So if the ECU senses knock by under or over fueling (due to bad airflow or bad compression) IT WILL alter the ignition AND FUEL for that cylinder.

So you CAN and probably have some cylinders over and underfueling on each bank.

So as I sad check compression first and intake runner cleanliness. They are key on THIS engine!

BatManv8v

Thanks for the advice.

We did a compression test when we had the injectors sorted out, but we can do one again.

I will take the manifold off in the next week or two and check the carbon buildup.