forzaf1
Member
Noticed recently that on the 1.2 if I knock it out of gear and coast, the revs drop to around 800. When the car comes to a stop they then rise to approximately 1000.
Any ideas?
Any ideas?
Technical Idle when coasting is lower than if stationary
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Technical Idle when coasting is lower than if stationary
Why are you coasting out of gear?
It shouldn't idle at 1000 stopped should be about 650-800 So if it's consistently 1000 then something's not right?
Interesting. As the car comes to rest, at what point exactly are you seeing an rpm increase?
Don't feed the trolls. Advanced drivers will know why this can sometimes be useful, will understand the risks, and will do so only when appropriate. Others should avoid it - there be dragons past this point. In inexperienced hands, this could lead to loss of control of your vehicle and a nasty incident, so best not discussed on a public forum.
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forzaf1
Member
Literally as the car is coming to a stop, like 3mph. It flicks to 1000rpm. In gear or not.
Not matter what speed you knock it out of gear, it idles around 800 but as soon as you literally get to walking pace, gear in or not, it rises to 1000.
Not matter what speed you knock it out of gear, it idles around 800 but as soon as you literally get to walking pace, gear in or not, it rises to 1000.
The ECU should be asking the engine to idle at 850rpm, so if your "around" is slightly above 800rpm, rather than slightly below it when the car is warm and idling normally then it's fine.
If you're holding it in gear (clutch in) once you're stationary, the ECU might be noticing the load from the clutch and could be compensating with a bit of additional fuel, to prevent the engine from stalling. Ideally it would always settle at 850rpm.. but the beast probably has some dirt/gum/wear/basic brains that makes it difficult to modulate that closely.
If your revs increase when the car is stationary but in neutral, then that would be a bit weird.. since there's no reason the ECU should be bothered that the car is not moving. I can only imagine it's the opposite of the "load" problem.
The car hurtling downhill under gravity probably reduces the idle load on the engine, so the ECU could be reducing fuel until the anti-stall intervenes to keep it from dying. In that case, I'd guess the normal idle rpm for your car is not "850" but closer to the 1000 you see when it's stationary.
I would clean out your throttle body/idle control valve and the MAP sensor so that they give the best reading they can.. then stick some kerosene through the beast (250ml per a full tank of the best 99-octane fuel you can get hold of) to remove any gum/varnish.
Then the beast ought to be performing closer to consistent.. although it'll probably never be "text book"... too many moving parts and sensors for that. It doesn't sound like there's much wrong though.
Ralf S.
If you're unfortunate enough to experience sudden total engine failure on a smart motorway, it could mean the difference between making the refuge area or having to stop on the carriageway.
This actually makes sense. The S/S system is designed not to kill the engine if you're in neutral with the clutch released if you're travelling any faster than that, for obvious safety reasons.
If coasting out of gear in a S/S equipped car, that's the point at which I'd expect the engine to stop - about walking pace. Does yours have S/S, and if so, does it normally work, and do you have it enabled?
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forzaf1
Member
OK, try this again with S/S enabled and see what happens. At the point where you've been seeing an rpm increase, the engine should stop (assuming S/S is working correctly).
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And do they teach that on any advance driver course? Is that the only reason?
What happens if your sudden and unexpected engine failure, stops the engine when you put the clutch in and you lose your servo assistance for the brakes? Or power steering? Or electrics, at least clutch up you can keep some power and therefore control.
portland_bill would be the resident expert
If it ain't broke, just leave it, for now.
If you lose all engine power at speed, sometimes the only way to reach a safe haven is to coast the car in neutral. Otherwise your life depends entirely on the drivers around you reacting correctly and not hitting you, and you can't be much more out of control of a car than that.
You'll retain PAS & electrics for as long as the ignition remains switched on. The brake servo holds enough vacuum for 3-4 full brake applications with normal pedal pressure; after that, the brakes will still work, but the pedal pressures will be significantly higher and you need to be prepared for that. If you've experienced all this before, in a safe environment, you'll know what to expect and can easily handle it safely. If you haven't, best leave well alone.
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forzaf1
Member
Tried it tonight, made no difference.
Attached some pictures that my passenger took.
Attachments
Thanks for posting this. Does the engine actually stop when the car comes to rest?
forzaf1
Member
With S/S works just fine. Goes from 1000 to 0 then back to 1000 when clutch depressed.
Is yours Euro5 or Euro6? As we all know, the Euro 6 cars behave differently when pulling away (or when they think they may be about to pull away
). There have also been a few software updates over the years, so not all 1.2 500's may behave in an identical fashion.I think what is happening here is that the ECU may be programmed to increase rpm to 1000 if it expects moving off may be imminent, and that this programming is for some reason taking precedence over the programming which keeps the engine running at 700 rpm if it thinks the car is still moving.
The algorithms which control engine running on modern cars are complex, and usually poorly documented (if they are even documented at all). The ECU likely isn't programmed to respond optimally in the specific situation where you coast to a stop in neutral with S/S disabled.
Everything you've posted thus far suggests to me your car is working as it was designed to do. I wouldn't stress over it, but it's certainly interesting and gives an additional insight into how the engine management system is programmed to operate.
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A lot of cars are now programmed to increase the revs when downchanging, to help match speed and revs. This could be what is happening here.
Advanced driving training does not usually contain any advice about advanced breakdowns, other than positioning as far left on the hard shoulder as possible and getting out of the nearside. Engine seizures are rare, usually preceded by the oil warning light illuminating, but unless this is as bright as the sun, anyone pushing their car to the point of failure would probably not notice.
If the engine does seize, holding down the clutch and coasting to the hard shoulder is the best option, and the only time when coasting is acceptable.
When the clutch is disengaged, the car sits lighter on its suspension. Steering is less accurate, and braking can be extended. (Lots of physics going on there, did all the theory a long time ago, can't be bothered to dig out the notes, and they'd bore most anyway, but that is what happens, reason why is not important.)
Find an empty car park (supermarket late Sunday?). Roll along at 15 mph, then turn sharply. 1. with engine pulling gently. 2. with clutch engaged, but off the throttle. 3. with clutch disengaged. Feel the differences. Always tighter with engine pulling gently. Clutch disengaged will feel sloppy. Can be quite disturbing at higher speeds, so don't unless you have plenty of space and clean underwear available.
Disengaging the clutch for the last few feet to a stop is not coasting.
Some electric power steering ceases to assist as soon as the engine stops, others give you just a few seconds, some stay live until the key is turned off, or the battery gives out. As said above, brakes should have 3-5 presses available after engine stops. If diving for cover, you probably won't be using the brakes until arrival at the hard shoulder, so one press is all you need.
