Idle problems

[illinoiSpark 3]

I've noticed that my RPM's don't always drop rapidly between shifts or at a stop, has anyone else had this problem?

[Greg_E 211]

Yes, it is normal and has to do with emissions. Long ago some cars had a mechanical or vacuum control "dash-pot" that would hold the throttle open for an extra bit. Apparently when the throttle closes under high vacuum the emissions shoot high. In my manual t-mission I've also noticed that if I'm coasting with the clutch in that the RPM stays a little above 1000, I can only think this is to soften any shift. As soon as I stop rolling the rpm drops back down to normal idle.

You may also notice that when coasting down a hill in gear that the car will sort of lunge after a few moments, it is apparently programmed to not coast for too long and introduces a small amount of fuel for a few seconds and then back to coasting. Not sure why it does this, but if you look at the instantaneous fuel milage coasting down a steep hill (in gear) you will see it go from 99mpg down to about 70mpg when you feel it lunge, then back up to 99 after a few seconds.

[LukeC 3]

Thanks for the info, Greg.

[joehornback 5]

Yes, it is normal and has to do with emissions. Long ago some cars had a mechanical or vacuum control "dash-pot" that would hold the throttle open for an extra bit. Apparently when the throttle closes under high vacuum the emissions shoot high. In my manual t-mission I've also noticed that if I'm coasting with the clutch in that the RPM stays a little above 1000, I can only think this is to soften any shift. As soon as I stop rolling the rpm drops back down to normal idle.

You may also notice that when coasting down a hill in gear that the car will sort of lunge after a few moments, it is apparently programmed to not coast for too long and introduces a small amount of fuel for a few seconds and then back to coasting. Not sure why it does this, but if you look at the instantaneous fuel milage coasting down a steep hill (in gear) you will see it go from 99mpg down to about 70mpg when you feel it lunge, then back up to 99 after a few seconds.

Mine does this on a slight downward incline while in 4th gear, I'm usually coasting around 40 MPH and notice a small influx of fuel when my foot is nowhere near the gas pedal. I thought it was malfunctioning until it did it the second day I hit that same spot too. This spot is on my trip home every day and it always does it without fail. Very interesting....

[Kevin Pavey 4]

Yes, I have noticed a couple of weird things, When coasting it seems to slow down and jerk even with my foot off the gas. When I stop at a light after going to 55mph or so, the engine reves to 2,500or so for a couple of seconds. The the idle drops down to 600. Once it stalled.

[ScoutII 0]

Yes, I have noticed a couple of weird things, When coasting it seems to slow down and jerk even with my foot off the gas. When I stop at a light after going to 55mph or so, the engine reves to 2,500or so for a couple of seconds. The the idle drops down to 600. Once it stalled.

I have a 2014 Spark LS Manual Trans with 2000 plus miles and have noticed similar engine rev and once it died. Chev dealer checked for bulletins, but I would think GM would be cautious about letting that kind of problem start any rumors.

[hogue2112 0]

I've had the high idle issue after shifting out of 5th into neutral while stopping for a red light. My rpms have gone up to 4-5k before because of it! Anyone have any info?

[Devinmafluf 0]

yeah; my spark does this too. i have a manual transmission and it's happened a few times. the RPMs go u to about 3K after shifting between 3rd to 5th into neutral. it never does it in first or second. it has even stalled out a few times. i read up on other places and it may be a faulty MAF sensor, a vacuum leak or something along those lines. i don't have any other issue except for the RPM surge.

[Retired old Gearhead 201]

It's another one of those hard to explain emission controls. VERY basically: Xtra rpms burn off burn off gasses and fuel when they are suddenly not needed to move the car...sorta.

New cars from 2000 onward are all about fuel conservation and emissions. The amount of sensors and electro-controllers in newer cars are insane as it only takes one bad sensor/module to cause hard to diagnose issues. I know of a guy who was nervous his old 2000 Cavalier would not pass the emission test & went into my mechanic's shop only to find out the emissions on his old Cavalier were 1/10th the allowable emissions and my Mechanic says almost no cars fail that test.