Sensors - Which ones does ecu use to change idle speed

workaccount

Hi,
When your car is cold the idle is higher and when warm it's lower.

What sensors does the ecu use to control the idle speed?

Thanks.

guil

it has to do with the idle control valve in the throttle body and the the thermostat

crosstownk

Lots. Temperature sensor, air mass meter, intake air temperature sensor, intake manifold load sensor, etc, etc....

But to answer your question - it's most likely the engine temperature sensor. But it depends on lots of other conditions as measured by the sensors listed above.

VH

crosstownk wrote: »

Lots. Temperature sensor, air mass meter, intake air temperature sensor, intake manifold load sensor, etc, etc....

+1

Some also have fast idle thermo valves; a wax filled diaphragm valve thru which coolant flows. The wax expands when the coolant is hot closing the valve shutting off air flow and thus lowering idle speed.

guil

thats what a normal thermostat is isnt it

VH

No. The thermostat controls the flow of coolant to the radiator.

cena

i have the same happening to my car as well. my's a 02 punto. could you up more petrol with this happening. my seems to drunk alot. i.m 50mpg. is that good.

E92

Forgive my ignorance, but why does an engine with fuel injection idle at a higher speed when it's cold than when it's warm?

cena

i got new injectors put in on new years eve. thats when my started

Captain Slow IRL

E92 wrote: »

Forgive my ignorance, but why does an engine with fuel injection idle at a higher speed when it's cold than when it's warm?

The main reason is to get the engine up to operating temperature as quickly as possible - most wear is done to engine components while it's cold; metal contracts when cold and the engine components are designed to work when fully expanded at operating temp.

E92

So what you're saying is that a higher engine speed warms up an engine faster? I knew all the other stuff e.g. engine wear etc you mentioned though!

cena

so what your saying is that your doing more harm if the idle revs are higher when cold than when warm

VH

E92 wrote:

Forgive my ignorance, but why does an engine with fuel injection idle at a higher speed when it's cold than when it's warm?

1, a cold engine won't idle particularly well, and will stall easily
2, a warm engine is hugely more fuel efficient
3, idling at 2.5k rpm will warm things up a lot quicker

mickdw

Hate that high idle crap. Have had three audi s now and they only have a marginally higher idle when cold, likewise with alfa. Had a rented avensis for a week and it idled at 2300 rpm when cold, what a load of ****e. Couldnt help the clutch having an idle like that either.

E92

VH wrote: »

1, a cold engine won't idle particularly well, and will stall easily

That's what fuel injected engines have ECUs for, to control and prevent all of this. I could understand carb engines idling at a high rpm because if yoiu didn't have the choke out they stalled straight away, but fuel injected engines are supposed to work out the correct fuel/air mix etc and make the engine use petrol as efficiently as possible.

VH wrote: »

3, idling at 2.5k rpm will warm things up a lot quicker

Hmmm that is interesting. Without wishing to sound like a smartass, I thought the one thing you never did when a car was warming up is give it too many revs. I always thought you go gently with the throttle and more importantly keep the revs down until it warms up.

PaulKK

E92 wrote: »

Hmmm that is interesting. Without wishing to sound like a smartass, I thought the one thing you never did when a car was warming up is give it too many revs. I always thought you go gently with the throttle and more importantly keep the revs down until it warms up.

Indeed, your not supposed to let the engine idle to warm up either, instead drive away smoothly without using excessive revs. Revving an engine high when cold can cause wear as the oil will not be as viscous as when its warm and so wont get up around the head as easily.

VH

E92 wrote: »

That's what fuel injected engines have ECUs for, to control and prevent all of this.

Ya, and how do you think they prevent it? Petrol will ignite more easily in a hot chamber than it will in a cold.

E92 wrote:

Hmmm that is interesting. Without wishing to sound like a smartass, I thought the one thing you never did when a car was warming up is give it too many revs. I always thought you go gently with the throttle and more importantly keep the revs down until it warms up.

You are absolutely correct, and 2.5k RPM is not considered high revs.