davegtt

is this OK? Normal? not sure how its supposed to be but its an electronic gauge showing lean stoich???? and rich and when the foot is firmly planted it lights up constantly saying rich (only just in the rich bracket compared to the far end of the gauge)

is this right? OK? normal?

I know rich will give u bore washing eventually but is the car supposed to run slighty rich or what?

and what does stoich mean?

cheers

Mad Scientist

Thats normal mate. Under WOT it should be to the lower side of the rich section on the gauge. When you come off the throttle it will fall down off the scale as the fuel is cut. Under normal driving (cruise) it should do its best knightrider impression and buzz up and down. Stoich stands for Stoichiometric ratio (sp). The ECU tries to keep the mixture at an average (i.e. the middle range) as it can't do this, the gauge shows the mixture going from lean to rich and back constanly as the ECU puts fuel in to try and reach this ideal median......which is the Stoichiometric ratio. Thats my understanding of it anyway...I think i'm just about right!!!!

Mad Scientist

In a chemical reaction between two substances, a given quantity of the first reagent "uses up" an equally exact quantity of the second. The ratio between these two quantities is known as "stoichiometric ratio".
In the air-fuel reaction, the stoichiometric ratio is, for instance, 14.7, which means that 14.7 kg of air are necessary to burn (or totally combine with) 1 kg of petrol. By contrast, 14 are required to burn 1 kg of gas-oil and 34 to burn the same quantity of nitrogen.
It is the fuel supply system (carburettor or electronic injection) which supplies the right doses to the engine to ensure that the air-fuel ratio is always very close to the ideal (stoichiometric) ratio.
In order to decrease petrol engine consumption, "lean" mixtures can be used, containing a higher amount of air than that indicated by the stoichiometric ratio. While to increase power, "rich" mixtures can be used.
To decrease polluting emissions and guarantee the catalytic muffler efficiency, the deviation of any mixture from the stoichiometric ratio should never exceed 10%.
Certain "lean burn" engines (used in cars) can reach air-fuel ratios of 30/1!


Robbed this off a Ducati web site. Thats what I was trying to say!

The_Judge

Stoich is short for Stoichiometric. It is "the ideal combustion process during which a fuel is burned completely".

When the fuelling is closed-loop, the lambda sensor is used by the ECU to detect the amount of oxygen in the exhaust gas. It then either adds or removes fuel accordingly on the next combustion cycle. This should result in your AFR gauge flitting about all over the place when idling, or part throttle cruising.

When the fuelling is open-loop, the MAF sensor and fuel map is used by the ECU to determine how much fuel is required. This should result in your AFR gauge giving a more static reading, somewhere in the rich zone, when you're foot is to the floor. However, even though your fuelling is being determined by your MAF and fuel map, your AFR gauge is still displaying what your lambda sensor is reading. If the sensor is a standard narrow band, it cannot be relied upon for accurate WOT (wide open throttle) data. If you had a more expensive wide band sensor, the reading would be much more accurate...

So, back to your question. Is your AFR gauge an Autometer, with 4 lean, 10 stoich, and 6 rich segments? If so, each segment represents 0.05 volts. So the first rich segment represents 0.75v, the next 0.80v, etc, up to 1.00v. An accurate WOT AFR reading should be around 0.90v (or the 4th rich light), but depending on the age/condition of your lambda sensor, could read as low as 0.80v. Only a wide band would tell you for sure.

You're correct regarding bore wash for excessively rich mixtures (over a period of time), but an excessively lean mixture could lead to det. This where a Knocklink becomes far more necessary than an AFR gauge. For instance, my Autometer AFR gauge displays 0.8-0.85v under WOT, which is too lean. But my Knocklink is quiet, and so I'm reasonably confident my engine isn't detting...

The_Judge

Took so bloody long to type that lot, Mad Scientist beat me to it...

Mad Scientist