What Dictates A/F Ratio?

[Merlin]

In a hypothetical over fuelling situation what dictates A/F Ratio?

O2 sensor? MAP sensor? Any others?


I know @Ammo had a strange fuelling problem. Can anyone remember how he fixed it?

[Lude-dude]

I would have thought tuning? or those vafc things

prelude's tend to over fuel, from what I can gather

[BMCC]

Don't know about dictating but I would have thought MAP, O2 as well as the ECT & IAT & BARO

I think I would also want to check the ignition timing and ignition system if you were looking at emissions info etc

[Confused]

On most cars with a single, narrow-band O2 sensor, this will only be used at idle or very light throttle.

The O2 sensor will change between outputting 0v and ~1v at around the stoichiometric point (an AFR of 14.7:1), with no real graduation between the two.

So, the ECU will compensate the fuelling/ignition slightly to keep this voltage fluctuating between 0 and 1, which is why some cars will seem to "hunt" slightly at idle and on light throttle (on the Galant V6 n/a manual you can feel this very slightly if you're driving)

When you add more throttle, it basically does not use the O2 sensor, and runs purely on the map coded into the ECU, usually based on throttle position, airflow (if a MAF is in use) or manifold pressure (in case of a MAP sensor) and engine RPM. There are minor adjustments to this based on Intake Air Temperature (colder air is denser, so requires more fuel), and Barometric sensor (higher altitudes = thinner air = less fuel)


A leaky injector could cause the engine to overfuel (more fuel is going into a cylinder than the ECU thinks), for example.


Piggy-back devices like the VAFC will fool the main ECU into thinking there's more or less air at a given point, which will cause the ECU to introduce more or less fuel.

I'm using a piggy-back ECU called a MAP-ECU on the Anglia to remove the MAF and run from a MAP sensor - I can adjust the overall fuelling (or fuelling at certain load point) by changing how much air the engine ECU thinks is being consumed by the engine.

[NafemanNathan]

Are you trying to cure a problem, or just tweek it? If a problem, then possibly a dodgey thermo sensor. My third had flames coming out the back and the engine was flooding until I replaced the thermo sensor.

[Merlin]

Thanks. I am talking over fuelling to the point of poor MPG, pops, bangs, flames, and an horrendous stink of fuel. But does not give any error codes and idles fine

Barometric pressure sensor? Where is that?#

Thermo sensor, I'll have to have a look if there is a test in the manual.

[Confused]

Ah yes, a coolant temperature sensor could cause overfuelling.

If the ECU thinks the engine is cold, it'll put more fuel in too.

I had that issue with the Anglia - on the 6A13TT engine there's 2 temperature sensors - one for the ECU, another for the dash.

[Merlin]

Nice the manual says the thermo sensor should give a resistance of between 20-400 ohms. Will check that.

The ECU uses a different temp sensor to the gauge, so the gauge could be displaying correctly whilst the ECU is seeing something different? If push comes to shove I at least have a spare temp sensor on my a2.

[BMCC]

I think the BARO is part of the ECU so not a sensor as such. Looking at your symptoms I would look at the Engine Coolant Temp Sensor. Difficult to prove if it is this as I had a problem with the exhaust popping and poor MPG on my Pulsar, but when I checked the ECT resistance it looked ok. Eventually got fed up as I needed to change the gauge sender as well, looked in the manual and it was side by side with the ECT so thought I might as well change that to given how much of a git it was going to be to change the gauge sensor (i.e I had to remove the ECT to get easier access). No pops and MPG is better as long as I don't spool the turbo too much

[Merlin]

Cool the thermo / coolant temperature sensor looks like a good candidate then. I take it a faulty thermo sensor wont throw a code?