I drive a 1.2l petrol fiat panda and i have a few questions about the fuel consumption when decelerating.

So when i am driving in 3rd gear at about 3000 rpm and let my foot of the gas, the car starts to brake and the fuel consumption reads 0l/100 km, this is because the fuel cut off is in effect. The thing that bothers me is that as soon as the engine gets to about 1500 rpm the fuel consumption starts to rise and it acts as do the car is in idle, if the rpm fells to about 900 (idle) then the fuel consumption rises even more and the car is trying it's best not to stall.

Shouldn't the fuel consumption read 0l/100km until the engine gets to 900 rpm? Because i don't see the logic in that the engine starts to use fuel at about 1500 rpm.

If anyone has a 1.2 l panda can you please test this scenario for me?

We have a ford focus 1.8 TDCi and when I engine brake, the fuell consumption reads 0 until the engine gets to idle.

It'll start to fuel again to stop the engine stalling, it'll also start to pump fuel in when it detects the clutch being depressed, say on a down change, as it won't sustain a suitable rpm when the clutch is disengaged (no over run).

This is programmed differently in a petrol than a diesel.
In effect it anticipates speed, torque etc and cuts back in the fuel ready for when you clog the pedal again, too low rpm and it'll cough and splutter as the torque and power aren't there.

As a diesel produces far more torque, it can reintroduce fuel much later, modern common rail can measure and inject tiny amounts to save a stall in these conditions.
They also don't shut off the air inake on the over run, so they'll pump air quite happily until it detects a low rpm or you dip the clutch, where a petrol does shut off air and has to balance the air and fuel accordingly to save knocking and excess heat.

This pumping air is a problem for emissions, lean combustion leads to high cylinder temps, because of this high temp production of Nitrgen Oxide is higher.
EGR's (exhaust recirculation valves) are often empolyed to send in exhaust gas (which low in oxygen) back into the air intake to cool the temps and lower the production of Nitrogen Oxide.

I've had petrols and diesels with EGR's, I consider them the biggest headache of the modern engine, they just don't work effectively for very long.
The 1.2's don't use them (a least the older ones, not looked at our's yet) and seem to be efficent and clean enough how they are currently set up.

krugoval said:

The thing that bothers me is that as soon as the engine gets to about 1500 rpm the fuel consumption starts to rise and it acts as do the car is in idle, if the rpm fells to about 900 (idle) then the fuel consumption rises even more and the car is trying it's best not to stall.

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That's exactly what the engine is programmed to do; at around 1500rpm it gradually starts feeding in fuel to keep the engine running smoothly.

If you let the car decelerate from about 40 mph in 5th with feet off the pedals on level ground, you can feel the point at which the ECU starts feeding in fuel.

Yes but the thing that you are talking about starts at 900 rpm, you can feel the engine is starting to get fuel.

That seems extremely strange.

Your engine fuel supply should be completely cut off until such time as you reach Tickover, at which point the engine will add fuel to stop the engine stalling.

I have a twin air so unable to test the theory, but i would suggest there is something wrong with your ECU.

^my point exactly, that is why i would kindly ask someone with a 1.2 panda to test this out for me before i send my car back to the dealers.

There's nothing wrong with the car. My twin air will start to add fuel at around 1500 rpm if you don't do so yourself by pressing the gas pedal. It's normal.

deeyup said:

There's nothing wrong with the car. It's normal.

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:yeahthat:

The 1.2 starts feeding fuel at about 1500rpm; there is absolutely nothing wrong with the ECU or the car.

krugoval said:

Yes but the thing that you are talking about starts at 900 rpm, you can feel the engine is starting to get fuel.

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No, it starts at 1500rpm; it's very subtle but totally reproducible.

If it kept the fuel shut off completely until 900rpm, it would feel like you were driving a kangaroo.

Thank you all for your replys and solving my doubts

I tested it a couple of times yesterday and it works exactly as jrkitching described.