Acceleration without the pedal - what's going on mechanical?
Discussion
Often for the first few minutes after I set off and I'm doing 5-15 mph, the car accelerates on its own by 3-5 mph after I release the clutch pedal e.g. 5-10mph, engage 2nd gear fully, release pedal, car accelerates on its own to 10-15mph. I think it happens more when the car is cold, but I haven't tracked that data.
It's not really a problem because I can control it, I am just curious as to what is happening mechanically.
It's not really a problem because I can control it, I am just curious as to what is happening mechanically.
I can speculate.. The speed is directly related to engine speed and gear. What's the engine idle speed when cold? It could be you change gear and the revs are increased to prevent stalling. When the engine is warm, the idle speed/stall speed of the engine will fall.
In a similar way I used to find the slow speed stopping a little unnerving. If you brake the car gently at low speed especially when cold, it seems to want to keep moving before the gearbox disengages ( I've found this with both my asm gearbox and the db9 I borrowed). Effectively it feels like you need to give it a slightly harder push on the brake at the end although in no way is it a hard push,
In a similar way I used to find the slow speed stopping a little unnerving. If you brake the car gently at low speed especially when cold, it seems to want to keep moving before the gearbox disengages ( I've found this with both my asm gearbox and the db9 I borrowed). Effectively it feels like you need to give it a slightly harder push on the brake at the end although in no way is it a hard push,
Correct on the above.
When cold, the engine management ISC circuit, (Idle Speed Control) will increase the engine rpm to prevent stalling during the fuel enrichment stage (Choke) by opening a small throttle, (idle speed control valve) or by increasing the throttle valve angle on cars with fly-by-wire electronic throttle bodies. This circuit is used on most modern cars to compensate both for electrical and mechanical load on the engine to prevent stalling as the load on the engine varies.
High loads such as heated seats/screens, power steering and Air Con would otherwise cause the engine to stall on idle.
8Tech.
When cold, the engine management ISC circuit, (Idle Speed Control) will increase the engine rpm to prevent stalling during the fuel enrichment stage (Choke) by opening a small throttle, (idle speed control valve) or by increasing the throttle valve angle on cars with fly-by-wire electronic throttle bodies. This circuit is used on most modern cars to compensate both for electrical and mechanical load on the engine to prevent stalling as the load on the engine varies.
High loads such as heated seats/screens, power steering and Air Con would otherwise cause the engine to stall on idle.
8Tech.
Edited by 8Tech on Wednesday 31st December 10:04
The idle RPM is also a little higher when the engine is cold due to emissions control. IIRC, this is to allow the cat to warm up slightly quicker from start.
It is probably these higher revs that is creating the effect that you describe. It is also a factor in higher than expected reversing speeds (which leads to people slipping their clutches when in reverse. This, in turn, leads to SCS*).
It is probably these higher revs that is creating the effect that you describe. It is also a factor in higher than expected reversing speeds (which leads to people slipping their clutches when in reverse. This, in turn, leads to SCS*).
- Stinky Clutch Syndrome!
When I first got my Vantage I used to have to reverse out each day from cold and soon became accustomed to it. A few months later I was on holiday touring in Norway and after meeting oncoming traffic I had to reverse up some steep inclines on single track roads ...and managed without clutch slipping or using the throttle. It was weird reversing backwards up hill with just clutch control and trusting the car not to stall.
Anyway, once you get the knack its great, and Im still on my original clutch in my manual 2006 car at 70k miles ...(fingers crossed)
Anyway, once you get the knack its great, and Im still on my original clutch in my manual 2006 car at 70k miles ...(fingers crossed)
Couple of things to look at:
It could be the throttle flaps sticking slightly (there is cleaning guide in the FAQ's - its dead easy)
It could be a hub speed sensor / pick up on the blink (used by ABS / Traction system IIRC) when it fails / gets a misread, it can accelerate the car (happened on mine in early ownership). It will need plugging into an AM diagnostic machine at a dealer / indy to tell you which one it is though.
Hope this helps.
It could be the throttle flaps sticking slightly (there is cleaning guide in the FAQ's - its dead easy)
It could be a hub speed sensor / pick up on the blink (used by ABS / Traction system IIRC) when it fails / gets a misread, it can accelerate the car (happened on mine in early ownership). It will need plugging into an AM diagnostic machine at a dealer / indy to tell you which one it is though.
Hope this helps.
mikey k said:
V8 Animal said:
Its the Choke
They don't have chokeThey have a cold start map to heat the CAT up quickly which gives higher revs
All my cars do it
The sticking throttle butterfly would not give higher revs?
With good clutch control and without dipping into the security of anti-stall, you should be able to manoeuvre the car around on tickover, it's just that a cold start cycle will mean the engine speed is higher.
Impasse said:
It's an electronic version of a choke. The revs are raised and the mixture is richer for a cold engine - just like a choke. All perfectly normal.
With good clutch control and without dipping into the security of anti-stall, you should be able to manoeuvre the car around on tickover, it's just that a cold start cycle will mean the engine speed is higher.
It's for emissions (like Mikey & I have been saying):With good clutch control and without dipping into the security of anti-stall, you should be able to manoeuvre the car around on tickover, it's just that a cold start cycle will mean the engine speed is higher.
BamfordMike said:
The raised / fast idle is normal
Besides one DBS around Rockingham, here is a link to the most boring video ever published...
https://www.youtube.com/watch?v=J0kvZ9yRecg&fe...
from cold start revs will be @ 1500 rpm for about 25 seconds.
it then takes a total of about 3 minutes (without moving) for idle speed to return to normal, about 750 rpm
The reason for the fast idle is to put heat energy into the exhaust gas / manifold after cold start to react with the secondary air supply injected into exhaust port which is needed to get the catalyst as hot as possible as quick as possible to convert harmful gas into less harmful gas and pass emissions. The reason why V12 does not behave the same way (exactly / for as long) is because the V12 has catalysts closer to the engine than V8 therefore doesn't need as many tricks to get the cats converting harmful gas into less harmful gas
yes, the downside of this trick is that the elevated idle speed is not a friend of the single plate clutch, but this scrabble to pass emissions commands the situation because is needed to sell the car.
From this (previous) thread about the higher revs on idle: http://www.pistonheads.com/gassing/topic.asp?h=0&a...Besides one DBS around Rockingham, here is a link to the most boring video ever published...
https://www.youtube.com/watch?v=J0kvZ9yRecg&fe...
from cold start revs will be @ 1500 rpm for about 25 seconds.
it then takes a total of about 3 minutes (without moving) for idle speed to return to normal, about 750 rpm
The reason for the fast idle is to put heat energy into the exhaust gas / manifold after cold start to react with the secondary air supply injected into exhaust port which is needed to get the catalyst as hot as possible as quick as possible to convert harmful gas into less harmful gas and pass emissions. The reason why V12 does not behave the same way (exactly / for as long) is because the V12 has catalysts closer to the engine than V8 therefore doesn't need as many tricks to get the cats converting harmful gas into less harmful gas
yes, the downside of this trick is that the elevated idle speed is not a friend of the single plate clutch, but this scrabble to pass emissions commands the situation because is needed to sell the car.
Trick I was shown on the Aston Performance Driving course in a V12V was to place the car in 1st, foot off clutch, no accelerator pedal.
As speed picks up, slip into 2nd, 3rd ....6th...all without right foot input.
Cant remember the top speed exactly but on a long straight level track im sure it was eventually up in the mid-40s in 6th??
Perhaps someone else can clarify my claim?
As speed picks up, slip into 2nd, 3rd ....6th...all without right foot input.
Cant remember the top speed exactly but on a long straight level track im sure it was eventually up in the mid-40s in 6th??
Perhaps someone else can clarify my claim?
BravoV8V said:
It's for emissions (like Mikey & I have been saying):
Yep, all very normal and is neither Aston Martin specific or a particularly new phenomenon.But to help explain its actions (rather than function) to someone of the Old School of driving and of cars in general, calling the effect a "Choke" is perfectly acceptable and requires no further explanation.
Guycord said:
Trick I was shown on the Aston Performance Driving course in a V12V was to place the car in 1st, foot off clutch, no accelerator pedal.
As speed picks up, slip into 2nd, 3rd ....6th...all without right foot input.
Cant remember the top speed exactly but on a long straight level track im sure it was eventually up in the mid-40s in 6th??
Perhaps someone else can clarify my claim?
As speed picks up, slip into 2nd, 3rd ....6th...all without right foot input.
Cant remember the top speed exactly but on a long straight level track im sure it was eventually up in the mid-40s in 6th??
Perhaps someone else can clarify my claim?
Yes, starting that run on the 1 Mile Straight, using 6th gear in the way you describe, (V12VSR) gives a speed of about 155mph at the 3/4 mile marker.
Doing the same run, but this time changing each gear at 7,000rpm, only increases the terminal speed by 10mph.
Gassing Station | Aston Martin | Top of Page | What's New | My Stuff