Weird engine rpm drop when coasting

Before I take it in and have the dealer guess on what the issue is I
thought I'd post and see if anyone has past knowledge of this issue.
2008 Aveo5 AT, 5k miles. When I coast with my foot off the throttle I
get a regular pulsing drop in RPM's. Normally from 50mph to around
35mph as the car slows the rpm will drop from 1600 to 900 or so and
then back up 1600. I first I thought it was downshifting as the car
slowed by I know I don't have 10 gears. It feels like someone is
touching the brake every second or two. I have tried this coasting
in neutral and of course I don't feel the slow down like when in gear
but I do the the rpm gauge bouncing up and down.
I have not have any codes or warning lights. I do have a scangauge
attached.
Any ideas ?
Steven

I have the same issue with mine but it seems to only do it when it's cold out and I only notice it about once a week. For me, it's not worthy of a dealer visit, I try to avoid them at all costs.

its funny i may have felt that also in my 04 with auto trans. almost like something "grabbing" while coasting. the aveo seems none worse for the wear and after 85000 miles, i accept it for what it is.

i am assuming this is an automatic?

i think what you are feeling may be the torque convertor. I don't own an automatic but torque convertors are rated to grab at a certain rpm range. i imagine when you pass under that rpm range the torque convertor lets go, the freed up drag of the transmission allows the car to rev up again. until the computer catches up and continues to cut fuel.

again this is all speculation but it used to happen in an old vw bus i had, and also in a friends older nova.

does this happen more than once on the same coasting?

Mine is manual and does the same thing too,i did ask around and found that the reason it is doing that is "the automatic engine warm up procedure"

i guess i will have to go out and try this with my car.

i have never experienced this with my car (manual trans.). I do notice that on a cold start that it sometimes shifts hard. like if i start the car and immediatley drive with only a couple of seconds to warm up, the shift from first to second will be hard and jerky. but i have never had any weird engine drops or revs when the car is engine braking. id get it checked out as long as its under warranty.

It's an automatic, seems to happen after the car is fully warmed up, only when my foot is completely off the gas. If I even give it just a touch of gas it idles/runs fine, if no foot on the gas at all the rpm's drop down slowly like it should, be every other second or though it will just drop like 500 rpm and feel like I'm hitting the break, then rpm back to where it was and continues to slowly come down as the car slows. So between 50mph and 35mph it will sometimes do this 10 times.
Steven

Mine does this too...It is definitely designed to do this but for what reason I don't know. If I had to guess I would say that it is some type of fuel cutoff system to add fuel economy while coasting. Fuel is cut, rpms drop, computer senses stall may be imminent and gives enough fuel to return to high idle. If the coasting period is lengthy it repeats over and over again until accelerator is pressed again.

Quote:

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Originally Posted by sedmonds

It's an automatic, seems to happen after the car is fully warmed up, only when my foot is completely off the gas. If I even give it just a touch of gas it idles/runs fine, if no foot on the gas at all the rpm's drop down slowly like it should, be every other second or though it will just drop like 500 rpm and feel like I'm hitting the break, then rpm back to where it was and continues to slowly come down as the car slows. So between 50mph and 35mph it will sometimes do this 10 times.
Steven

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I gess you better take it to the dealership... mine does it only when the engine is cold

It's the EGR valve working. I see this happening when I let go of the throttle on the vacuum gauge. When the EGR valve opens up, the engine lets exhaust gases in and reduces vacuum in the intake manifold. This allows the engine to turn freely and increases rpm even though no fuel is added. It's a way to save fuel and to meet emissions laws.

At least this is what I think. EGR only works when the engine is at operating temp.

Mine has been doing this as well. Auto trans and it does it at operating temp. The car as stalled twice because of this and has come close to stalling many other times. At one point the idle was around 1500 rpm until i stepped on the gas to go, very strange computer. Also get a check engine light that comes and goes, but that may be a bad gas cap. I'll have the dealer check it out next visit.

I've had it happen to me on a different car with a manual tranny. It would only happen when I shifted into neutral and coasted. But as soon as the car came to a stop, it would run at a constant RPM again, as if the speed of the car affected it.