Zero fuel input on trailing throttle

rmunns · Post by **rmunns** » 26 Mar 2011, 19:52

I have heard that modern cars use no fuel when they are on trailing throttle. So now as I approach traffic lights etc. I change down, allowing full engine braking, rerasoning that this should use no fuel at all.

Is this true for my 90Hdi?

If so, then how does it work and how do you check that it is working ok?

DickieG · Post by **DickieG** » 26 Mar 2011, 20:05

Fuel cut off on a trailing throttle has been around for years, especially on petrol engines, for example it was fitted to my 1983 XR3i. It works by the ECU taking input from the throttle position sensor (idle position) and engine turning over at a higher speed = turn the fuel off. When the revs drop to around 1200 rpm ish the fuel is turned back on to prevent stalling, you can feel this if you allow the car to slow in say first or second gear to around 1200 rpm whereupon the car rate of slowing reduces until the idle speed is reached and the car continues at idle revs/speed.

An HDi 90 would have it fitted.

Other than feel for it as in the above test the sure way of observing it would be to have a Lexia wired up and observe the fuel flow rate.

spider · Post by **spider** » 26 Mar 2011, 20:52

Indeed, it has been around for many years.

No point having any fuel injected if engine speed is well above idle (as pointed out greater than say 1200 to 1500 rpm) and the throttle is closed.

There are fuel savings to be had by doing this, albiet maybe small ones, but it is better than a carb'ed engine which will always draw some fuel

Stempy · Post by **Stempy** » 26 Mar 2011, 20:56

spider wrote:Indeed, it has been around for many years.

No point having any fuel injected if engine speed is well above idle (as pointed out greater than say 1200 to 1500 rpm) and the throttle is closed.

Other than that nice popping sound from the exhaust

spider · Post by **spider** » 26 Mar 2011, 21:09

Stempy wrote:
spider wrote:Indeed, it has been around for many years.

No point having any fuel injected if engine speed is well above idle (as pointed out greater than say 1200 to 1500 rpm) and the throttle is closed.

Other than that nice popping sound from the exhaust

Well, that's true. Although I seem to recall carb'ed engines would do this with a (tiny) inlet manifold leak too.

Post by **Peter.N.** » 26 Mar 2011, 21:16

Diesels have done this more or less by default for years, in the old mechanical pumps fuel delivery was determined by fuel pressure in the pump, all the while the engine was turning faster than than fuel delivery allowed the throttle would be virtually shut until the revs dropped below tickover speed, a very small ammount of fuel may have been deliverd but it was only very small.

Peter

Xantidote · Post by **Xantidote** » 27 Mar 2011, 12:08

I've noticed sometihing akin to this on my TD as revs drop, which I've always assumed to be the anti-stall cutting in.

Post by **Peter.N.** » 27 Mar 2011, 13:51

I think that's what the antistall is there for Martin, to prevent the revs dropping to low under 'throttle back' conditions when there is little or no fuel being delivered.

Peter

Xantidote · Post by **Xantidote** » 27 Mar 2011, 14:49

Thanks Peter,

Guessed it was, but better to hear it from someone else!

Post by **Peter.N.** » 27 Mar 2011, 15:33

Well - I'm assuming that's how it works, if its not adjusted correctly the engine will stall because the throttle closes to quickly before the fuel can start flowing again, the anti stall slows it down on the last bit.

Peter

Post by **Paul-R** » 27 Mar 2011, 18:56

In our old Montego TD I always coasted down hills out of gear. Whilst the amount of fuel injected might have been miniscule on a trailing throttle there would have been more injections of it per minute if left in gear.

Since we got the Xsara and C5 I now always go down hills in gear. And I can confirm that on the C5 the instantaneous MPG readout goes up to 999.9 for a while and then indicates a blank (or is it a dash?) if left long enough.

Post by **Peter.N.** » 27 Mar 2011, 19:16

My 406 does the same.

citroenxm · Post by **citroenxm** » 27 Mar 2011, 20:30

It ABSOLUTLEY amazes me that so many people do not know about "No fuel used" on over run or trailing throttle... the momentum of the car keep the engine going ALSO, if you have just climb a steep hill on the opposite side when you do this the engine COOLS DOWN quicker...

This is because NO fuel is being burnt, so not fuel burnt no hot explosions in the cylinders, so no excess heat being produced...

Ive known about this since I passed my test on 1992...

Paul

Post by **Peter.N.** » 27 Mar 2011, 21:44

That's only on modern cars Paul, try 1955

Peter

citroenxm · Post by **citroenxm** » 27 Mar 2011, 21:47

Well its always been obvious to me Peter..!!! Always...!

Paul