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Old 19th March 2011, 11:20   #76
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Default re: RPM shoots up when changing gears

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Blurred engines? Progressive lenses recommended. ...
I have, but... infra-red emission causes problems in focusing.

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... You're quite the linguist ...
Bangalore can make a linguist out of anyone.

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... And in drive-by-wire accelerator pedals? ...
Errr ... (thinking of a graceful retracing of steps after putting foot firmly in mouth ) Sorry, completely forgot the flap positioner in the throttle body. Yes, you are right, the ECU definitely can control the opening in DBW.

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... How and why so? ...
The problem of actuator failure leading to engine failure.

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... Carburettor throttles are 100% leakproof when new. ...
My experience is only with Bullet carbs (e.g. Mikuni, etc.) - there is a curve at the bottom of the airflow-control cylinder, the height of which is controlled with the idle air screw. If completely blocking, how do you expect the engine to idle?

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... Non. Inferences being drawn, Sir, where none exist - it is certainly not my logic. ...
Mais non, pardon monsieur, no definite inferences drawn. It was a question, not a statement, on the possible inferences that can be drawn.

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... Very interesting & intuitive observation. ...
Have spent 25 years deploying freshers in my projects. Though academic content has always lagged at least 10 years in Engg. even when I studied Engg. (typical in Electrical, Electronics, Instrumentation, CS, IS etc.; to a slightly lesser extent in Mech, not Civil or Architecture), there has been a gradual decline in understanding in students that Engg. education teaches one how to teach oneself beyond the curriculum, and to make oneself current is ones' own responsibility - not that of edu inst or companies.

Today, students think they will be trained on technology when they join a job, and companies can only afford to train them on processes and quality systems. The result is 0 or very limited productivity in the first 2 years. Only those that understand knowledge and skill is their own responsibility (about 20-30%) excel in the first year itself.
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Old 19th March 2011, 14:39   #77
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Default re: RPM shoots up when changing gears

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I have, but... infra-red emission causes problems in focusing.
Bangalore can make a linguist out of anyone.
Errr ... (thinking of a graceful retracing of steps after putting foot firmly in mouth )
ROFL... . Now I need to meet you too!

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...there is a curve at the bottom of the airflow-control cylinder, the height of which is controlled with the idle air screw. If completely blocking, how do you expect the engine to idle?
Difficult for me to explain in words, but opening the throttle valve just so, uncovers small ports on the side wall of the venturi tube, from where the idle circuit allows the vacuum in the manifold to suck up a mix of air and fuel into the cylinders. The air supply is just enough to ignite the miniscule amount of fuel going in.

When the throttle valve 'hinges' wore out, and leaked, idling smoothness was seriously affected. I remember lathe work being done on carbs to get the leakproof properties back, and the acid test of a good job done was to fill up the venturi tube with kerosene, and wait and watch a few hours to see if the kero dribbled out past the throttle valve or not.

An interesting read: Popular Mechanics - Google Books.
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Old 19th March 2011, 16:55   #78
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Default re: RPM shoots up when changing gears

Perhaps we are on parallel tracks (or you on carbs for multi-cylinder carbs, me on single cylinder). In 1-cyl carbs, there are 2 controls: a screw that controls the idle fuel, and another that controls idle air (idle speed, effectively). A good mechanic could really set it low.

My Bullet mechanic way of tuning idle: "The dhup-dhup sound should come from outside the silencer, not inside it. At this point, the mileage will be maximum" (and all that in Bangalore Urdu).
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Old 20th March 2011, 01:53   #79
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Default re: RPM shoots up when changing gears

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...there are 2 controls: a screw that controls the idle fuel, and another that controls idle air (idle speed, effectively).
Depending on the design of different types of carbs, yes - the idle air control screw is what usually cracks the butterfly/slide in the carb open by a few tenths of a mm, to allow the idle port to be exposed to the manifold vacuum. The idle fuel adjustment screw increases/reduces the amount of atomised petrol + air being allowed to go through the idle port.

Tuning by silencer beat was an art - experienced mechs could tell about carb and ignition timing issues just by listening to the beat, feeling the flow of the exhaust on their palms, and sniffing the exhaust gases.
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Perhaps we are on parallel tracks...
Tracks merged.
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Old 20th March 2011, 11:49   #80
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Default re: RPM shoots up when changing gears

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... the idle port to be exposed to the manifold vacuum. ...
So it is air that carries the vapor to the intake? That's what I was referring to as the small amount of air. This prevents vacuum build-up in the cylinder. The difference between what the cylinder can take in and the throttling is finite vacuum which reaches an equilibrium. A gradual air leak, as shown in the diagram in the PM article, causes the idle mix ratio to become off-kilter resulting in rough idling. Have I understood it correctly?

Last edited by DerAlte : 20th March 2011 at 11:53.
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Old 20th March 2011, 15:50   #81
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Default re: RPM shoots up when changing gears

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So it is air that carries the vapor to the intake?
Have I understood it correctly?
Yes Sire.
That clears the air
about the air.
What fanfare!
about this affair...
Fuel and air
supply discussed threadbare,
Not found anywhere!
Got no breath to spare!
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Old 19th February 2012, 19:08   #82
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Default Ford Fiesta rev low response

Friends,

Sometime I have a peculiar problem in my Ford Fiesta 1.4 Duratorq.

When i rev up in 2 gear and 3 gear and push my clutch to change to higher gear, the rpm does not come down immediately and takes time to come down. It is awkward when I push the clutch and the engine is still at higher RPM.

I am sure I have articulated my problem well.

shanks
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Old 25th September 2012, 14:36   #83
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Default Re: Ford Fiesta rev low response

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Friends,

Sometime I have a peculiar problem in my Ford Fiesta 1.4 Duratorq.

When i rev up in 2 gear and 3 gear and push my clutch to change to higher gear, the rpm does not come down immediately and takes time to come down. It is awkward when I push the clutch and the engine is still at higher RPM.

I am sure I have articulated my problem well.

shanks
This is called "rev hang" and is common to all cars with drive by wire accelerator pedals. It is a feature to rev match enabling smoother gear shifts.

In my Ertiga, this feature works upto 3rd gear when revved hard. I found this feature useful while driving up the ghats of Ooty and Yercaud. Taking those steep inclines, shifting from 2nd to 3rd with "rev hang" was a breeze! I'm not sure if it takes its toll on the clutch though as there is slight clutch slipping due to the higher RPM.
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Old 25th September 2012, 15:24   #84
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Default re: RPM shoots up when changing gears

It is a 'drive by wire' feature and you need not be worried about that. The usefulness of this feature has been well described by swiftdiesel above---i was not aware of it so far.
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Old 25th September 2012, 18:06   #85
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Default re: RPM shoots up when changing gears

Just wanted to document that I had experienced this on the Chevrolet Beat petrol. It is only now I have stumbled upon the right thread, but the layman in me clueless by all the geek talk

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One another thing I noticed is that the frequent use of the clutch lowers the FE. While depressing the clutch for gear change, I have experienced a momentary over rev by the engine before it settles to the idle RPM. On older cars, once we depress the clutch, the engine would suddenly die down to its idle. Is the ECU settings the culprit? But why is it so?
Quote:
It is a feature to rev match enabling smoother gear shifts.
But I had noticed this absent on the Baleno or the i10. And they were never less smooth to change gears.
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Old 14th December 2012, 10:41   #86
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Default Renault Duster: Automatic engine racing

I was driving in town today through the usual route in third gear and just as I was about to overtake a slowing bus the engine started racing on its own.

I had depressed the clutch and got the leg off the accelerator when this happened. I parked the vehicle and switched it off and when I restarted it the engine was still revving automatically. I switched it off for about 15 seconds and the restarted again. No problem. Till I reached back there has been no repetition of the event.

Do any Renault Duster owners have a similar problem? If so how was it solved. I am due for 1st service on 18th. What should I report if I have no similar experience during the intervening period?

Rajesh
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Old 14th December 2012, 10:48   #87
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Default Re: Renault Duster: Automatic engine racing

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I parked the vehicle and switched it off and when I restarted it the engine was still revving automatically.
Does Duster has 'drive by wire' or it is mechanical cable? Were there nay lights flashing?
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Old 14th December 2012, 10:58   #88
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Default Re: Renault Duster: Automatic engine racing

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I was driving in town today through the usual route in third gear and just as I was about to overtake a slowing bus the engine started racing on its own.
Rajesh, before you start getting tense, let us start with the basics. Since the incident did not happen again, are you 100% sure that it was not the floor mat fouling with the accelerator pedal?

It could also be that the pedal operation is not smooth through it complete range due to some dust/ a piece of leaf/plastic/grass etc stuck to its hinge.

Remove the mat and check the pedal mount/hinge for obstructions. Let us take up other things thereafter.
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Old 14th December 2012, 10:59   #89
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Default Re: Renault Duster: Automatic engine racing

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Does Duster has 'drive by wire' or it is mechanical cable? Were there nay lights flashing?
Guna, You have me stumped about the 'fly by wire' but there certainly were no lights flashing on the instrument cluster. I know they have the OBD port in the glove box but I'm not sure what you mean by 'fly by wire'

I've had various niggles including a major water leakage issue that is due to get sorted out next week. Could something be getting short circuited somewhere?
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Old 14th December 2012, 11:06   #90
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Default Re: Renault Duster: Automatic engine racing

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Guna, You have me stumped about the 'fly by wire'
In 'drive by wire' the accelerator pedal and the throttle mechanism are not linked by a cable but rather through an electronic system. Many modern cars have it these days.
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