Author Topic: Deceleration fuel cutoff  (Read 223 times)

January 21, 2019, 01:59:53 PM
Read 223 times

Mathieu

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I haven't seen this discussed before:

I've learnt that Subaru Foresters (at least SG's and some SH's) don't cut off fuel in a deceleration situation.
What I'm talking about is any time the car is coasting and you have your foot off accelerator. This can happen when you slow down before coming to a stop or when rolling down a hill under engine braking.
Most fuel injected cars with ECU's will detect RPM above a certain threshold and zero throttle input, at which point it cuts off all fuel.

I've noticed (using torque app in my 2006 XT) that while coasting down a hill in gear, RPM at say 3000 and foot off the accelerator, the car is still consuming 3.5L/100km. (most cars will read 0)...

That's a lot of extra fuel to be burning and I would like to know if there is a technical reason for this to happen? My friends 2010 SH Forester X does the same thing.
Only potential reason I can think of is some fuel is always burnt in order to keep the cats hot and functioning efficiently.

Second question is can anything be done about this? I don't know how the ECU's are mapped/tuned but can write a simple algorithm with the following inputs:

If: throttle = 0
 and;
RPM > 2000

 then fuel injector duty cycle = 0

 else; fuel injector duty cycle = value from lookup table

I suspect there is a fairly big chunk of potential fuel saving here, thoughts?

January 21, 2019, 05:32:24 PM
Reply #1

berndp

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I'm not sure how these ECU's work, nor all the info you see on Torque....

However, I am 100% (minus 1) certain the SG's do not have a fuel flow meter, so any fuel consumption indicated is purely due to some algorithm in the ECU/Torque....

If it was still injecting fuel, how would you be able to decelerate? If the spark was stopped, you would get unburnt fuel into your exhaust, which you would most likely smell. If you would then accelerate again, there would be quite a nice bang as the exhaust "flame" met with the unburnt fuel.... (Army Bedford springs to mind... ;D ;D ;D ;D)

I do think the fuel get's cut off..

But, I may be wrong
Subaru Forester 2005 2.5 Xsel
Always looking for a tar free "road"
Brackenfell, W'Cape

January 22, 2019, 09:16:26 AM
Reply #2

Mathieu

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Interesting, I think the fuel consumption data on torque is fairly accurate since it lines up with the numbers I calculate. The deceleration is there, but can be supplemented with the brakes when necessary  ;D

Something to note, the car has been tuned, and maybe the tune overrides a factory fuel cutoff function?

January 22, 2019, 09:24:34 AM
Reply #3

berndp

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"Something to note, the car has been tuned,... "
that may be just it.
You'll have to find out from the tuner..
Subaru Forester 2005 2.5 Xsel
Always looking for a tar free "road"
Brackenfell, W'Cape