Coasting in neutral, good idea or bad?

Wig

Belnahua wrote: »

Probably most of all the post mid 1990's cars on the roads have it. It was fitted to my '86 escort and my '92 Rover, and that was a base model. So I'd guess that all modern cars on the road have it, probably at the point when catalytic converters become mandatory.

I don't see why. A normally functioning engine will keep firing and burning fuel as normal.

You just missed me saying that it is listed as an optional feature on Corsas did you? I've never seen it mentioned on all my previous cars.... and I've had a fair few.

Belnahua

Wig wrote: »

I don't see why. A normally functioning engine will keep firing and burning fuel as normal.

Only from the point that electronic engine management systems would've been needed to operate with a cat; and car without a cat, could still have the old manual timing system.

Wig wrote: »

You just missed me saying that it is listed as an optional feature on Corsas did you? I've never seen it mentioned on all my previous cars.... and I've had a fair few.

This is odd when you think about it, as it's not a option as such is it. It's just a mapping in the ECU that says, when certain conditions are met, don't operate the injectors. It doesn't take any extra equipment or devices; so why wouldn't a manufacturer have it as standard?

tomstickland

Every engine I've known since about the early 90s has made use of overrun fuel cut off. The manufacturers want to obtain the best results in the mpg testing.

If you think about it from the point of view of the engine mapping requirements, the engine is not being asked to produce any output, so the minimum amount of fuel will be required. If no fuel at all will work, why would any engineer decide to inject fuel? If they did, it would be a very small amount.

pault123

Wig wrote: »

Where the * indicates that it is an optional item not fitted to all models.

Would this indicate the automatic models working differently with the overrun?

Or an ommision left in the manual?

My old Nova used to have overrun and that was 87! I really can't see why a modern Corsa wouldn't have it.

cyclonebri1

As Tom says, all cars with fuel injection so that is almost all since the late 80's have a fuel injection system. They all shut the fueling off at a predetermined ECU controlled figure off around 900 to 1100 rpm when the car is on the move. The exceptions are the last caburettor fueled cars but these died out around that time.

Try crawling up a hill in 1st gear and releasing the throttle pedal, the ECU will fuel enough to keep the car going at the lower rpm figure. This also keeps the engine tickover speed even when additional loads such as lights, blower etc are on.

To confirm if you run downhill at an engine speed above that preset figure, in any gear, with the throttle closed, you use zero fuel.

If you chose the correct gear to achieve a safe speed you are perfectly in control.

If you knock it out of gear you will use an amount of fuel and technically you haven't the same control as being in the correct gear with a closed throttle.

Assuming you travel at a safe speed which could be achieved in gear I would expect in gear closed throttle to be the best saving.

I think these consideration are only valid in urban conditions as on the open road you are likely to need a small throttle value under most conditions.

The saving would be very small and I don't think measureable on a standard fuelling computer, as most of these max out at 99.9mpg. That is obtained both on the overun and coasting out of gear when I've tried it.

Taking off the big yellow boots is a better saving;) and daft as it sounds it's true, shoes with sensitive soles give much more feedback as to how you are actually operating the throttle pedal.

Inactive

cyclonebri1 wrote: »

Taking off the big yellow boots is a better saving;) and daft as it sounds it's true, shoes with sensitive soles give much more feedback as to how you are actually operating the throttle pedal.

Best advice in the whole thread...;) :T

loofer

I was going to start a new thread on this but I guess there's quite a few knowledgeable folk here.

My car (and I'm guessing this could be common with other of diesel cars) can move along in 1st gear without me even touching the accelerator. The rev meter will read about 1000rpm whilst moving. I would only do this in extremely slow moving traffic and would lift the clutch pedal slowly to prevent stalling.

Could this be causing any damage to the clutch/flywheel/engine or other parts/components??

Pew_Pew_Pew_Lasers!

No. Its called torque, and is quite normal.

Wig

I'd say it was the idle control motor or the throttle control motor keeping the engine alive and no, it would cause no damage.

Wig

cyclonebri1 wrote: »

Try crawling up a hill in 1st gear and releasing the throttle pedal, the ECU will fuel enough to keep the car going at the lower rpm figure. This also keeps the engine tickover speed even when additional loads such as lights, blower etc are on..

This is the throttle control or idle control keeping it going. Nothing to do with overrun.

Apparently the way to test for overrun is to drive at 30mph come off the foot pedals and allow the car to slow down, when it goes below 1100 - 900 rpm it will suddenly spring into life as the fuel goes in.