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Old 10-06-2010, 10:55 PM   #1 (permalink)
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Why is my idle higher while coasting than when I'm stopped?

I just noticed today that when I'm coasting with the engine on my tach is showing 1100-1200 RPM, but when I come to a full stop the idle drops to 750-800 RPM. Is this normal? Could it be a gauge anomoly? Any way to get my coasting idle down to the stopped idle?

I'm planning on putting a fuel injector kill switch in anyway, but it's come up in another thread that I may want to keep the engine idling sometimes to retain engine heat during the winter. I'm not even sure how the car knows I'm coasting vs. stopped. Damn computers!

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Old 10-06-2010, 11:31 PM   #2 (permalink)
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endurance -

It's normal for me. On my car, if the ECU/PCM sees that I am traveling at highway speeds, it increases the idle RPM using the Idle-Air-Control (IAC) Valve. It also increases the idle RPM when the engine is cold :

How Does an Idle Air Control Valve Work? IAC

Quote:
An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The cars computer or ECM (electronic control module) receives information from various sensors and will output signals to adjust the IAC motor in or out to adjust engine idle speed by controlling engine idle air.
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Old 10-07-2010, 07:44 AM   #3 (permalink)
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You could go and find the IAC, wait till the car is idling as low as it ever does, and unplug it. Sometimes, however, the higher idle while moving is to keep things lubed up in the transmission (for autos, generally speaking)

Or, if the computer would get angry from not hearing from the IAC, you could just take it off and replace it with a bolt you can use for adjusting the idle speed. Either way, the engine will idle lower with high alternator loads or AC or P/S use, because it cannot compensate for the higher load by opening the throttle a bit more.
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Old 10-07-2010, 08:08 AM   #4 (permalink)
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Try turning all accessories off and see what the idle speed RPM reads.

In my VX it would drop to normal ilde speed when coasting but you had to have no accessory loads.

It took a few seconds.

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Old 10-07-2010, 09:57 AM   #5 (permalink)
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Interesting. I noticed this morning that once the engine reached operating temperature, most of the time it was dropping all the way down to 750rpm even at 65mph. I'm guessing it was the alternator, since nothing inside the car that I had control over was on last night.

I'll just keep an eye on it and see if I can see a pattern. I guess I could probably use my scangauge voltage and see if there's a coorelation.
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Old 10-07-2010, 09:59 AM   #6 (permalink)
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One of my car mods was to decrease the idle of the car which gave a fairly good improvement (about 20 % when idling)
I turned down the idle control screw (the mechanical idle control) which sets the lowest the idle will go to
Then i also put a switch in the cabin to stop the ECU increasing the idle using the IAC valve mentioned in past posts. This ended up being a switch which put the ECU into a "diagnostic/tunning" mode which is used to tune the idle. I found that it just caused the ECU to stop compensating for low idle
So now when i get to stop lights or when im in neutral coasting i will flip the switch and my idle will go down really low to about 600 rpm (probably lower than it should) but engine hasn't stalled yet and it also has not damaged itself from bad lubrication either although it is probably shortening the life a bit.

this might be worth doing if your finding your idle is particularly high in neutral
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Old 10-07-2010, 10:46 AM   #7 (permalink)
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Some cars increase idle when moving so that the engine doesn't stall under load, for example if you steer. My wife's old car used to slightly increase revs when reversing so you could use idle speed and the clutch to reverse slowly without stalling.

I suspect this isn't the case here though.
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Old 10-07-2010, 12:08 PM   #8 (permalink)
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Quote:
Originally Posted by saand View Post
One of my car mods was to decrease the idle of the car which gave a fairly good improvement (about 20 % when idling)
I turned down the idle control screw (the mechanical idle control) which sets the lowest the idle will go to
Then i also put a switch in the cabin to stop the ECU increasing the idle using the IAC valve mentioned in past posts. This ended up being a switch which put the ECU into a "diagnostic/tunning" mode which is used to tune the idle. I found that it just caused the ECU to stop compensating for low idle
So now when i get to stop lights or when im in neutral coasting i will flip the switch and my idle will go down really low to about 600 rpm (probably lower than it should) but engine hasn't stalled yet and it also has not damaged itself from bad lubrication either although it is probably shortening the life a bit.
I like the idea of being able to turn the IAC off. Definitely something to consider. While generally I think the computer probably knows better than me and Arragonis' suggestion brings up yet another appliance that could play a role, there are definitely times where I'm going to want to override the higher idle.

BTW, I had a 1989 MX-6 (with a 5 speed, but the same 2.2 liter, 12 valve engine). Probably my favorite car ever. Great car in the snow, 32-36mpg for normal driving (up to 38 on the highway), lots of room for passengers and the only unscheduled mechanical problems I ever had was exhaust parts (one muffler under warranty followed by three more, plus an exhaust manifold before I sold it with 180k (and he drove it to 232k and replaced the muffler one more time)). Slightly jealous you still have one... Also wondering why they ever stopped manufacturing the 1989 MX-6. If they still made it, I'd still be driving one.
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Old 10-07-2010, 12:54 PM   #9 (permalink)
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Quote:
Originally Posted by endurance View Post
Interesting. I noticed this morning that once the engine reached operating temperature, most of the time it was dropping all the way down to 750rpm even at 65mph.
I'd say the answer is right there : operating temperature.
As long as it's cold, the engine will run in fuel-enrichment mode.
As a result, idle rpm are a tad higher and FC is higher as well.

On my V50, idle engine load is 30% when cold, but only 18% when warm.
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Old 10-07-2010, 06:22 PM   #10 (permalink)
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I agree with euromodder, the ECU or a mechanical device will raise the idle when the car is cold. I have also "fixed" this in my car.
for anyone interested i have written up in the wiki for how to reduce the cold start idle
Cold Start idle reduction - EcoModder

and the warm engine (standard) idle
Car idle reduction - EcoModder

for me both mods caused noticeable reductions in fuel usage but my engine seemed to be designed to rev fairly high when cold.
I have also never stalled with these mods (except when i stupidly put the car into drive from neutral when i have forgotten to turn off the torque converter lock up)

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