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!:p |
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 http://www.2carpros.com/how_does_it_...trol_motor.jpg Quote:
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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. |
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. regards Mech |
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. |
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 |
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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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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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. |
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) |
do not screw with the IAC
the VSS is a load signal
the ECM sees it and has been programmed to do certain things when the car is moving faster or slower or stopped if you tamper with the IAC the ECM will see that and it will respond as it has been programmed the ECM can control idle speed with ignition timing AND your faster idle while rolling may be a result of that as opposed to change in IAC command you may want to connect a scan tool a real scan tool not a wheel chock code reader or scangauge and drive around watching IAC command and IGN timing command with VSS with TPS or closed throttle switch just to learn more about your system , before you start fixing that which is not broken . |
Some auto's will idle at a higher rpm when moving to better match revs when going back into gear. If the idle were too low, you would feel a clunk as the tranny re-engaged.
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I know on some of the Fords (I know, not same vehicle), that the ECM is programmed to hold the rpms up when shifting (manual trans) . It will also increase the rpms while coasting in neutral.
For example, we were waiting in a long line of cars trying to get over the Hoover Damn. We were facing down hill, so I just put the car in neutral (wasn't going to turn it off, was way too hot) and just used the brakes to let us go forward. At about 15 mph the ecm had the rpms at almost 2000. If I stepped on the clutch they would come down. I asked a few people about reprogramming the ecm to get rid of that feature and to get rid of the RPM hang is has when we are shifting, but haven't gotten any farther into it. I just hold the clutch in when I'm doing my coasting in that vehicle. ~Mark |
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