I got a stick and noticed something odd when coasting down some hills recently... if I drop the car in neutral and coast the tach still revs up and down.

If I am coasting around 40-60 it moves from about 1000 to 2200 rpms or so in about a 4 second cycle as if I am revving the engine in gear.

This gets less and less as the speed goes down but even just coasting down from normal speeds I notice the the tach moving a little.

I am confused how this could be and I haven't heard of it from anyone else. Is this normal?

 

It's happened to me a few times, but only when I hold the gas to up the idle a bit (to like 1k) because when my AC is on my engine dampner shakes the money out of my pockets. Well, not that bad, but it's annoying.

Afterward it felt like my motor was re-learning it's idle speed, because it stopped acting up after a couple of minutes.

If you didn't mess with the car like I did, i'm not real sure what could be the problem but i'm watching to see what else it could be.

 

happens to me too.

 

That seems pretty bad... I wonder why it would do that?

 

Mine does that, too. It's no big deal. I've had my car for three years with no problems, so don't worry about it.

 

interesting scenerio because i have a stick and mine doesn't do that.

i'd take it to the dealer if you're under warranty

 

The car is under a month old so it's definitely still under warranty...

 

it's somewhat normal... it's your ecu adjusting to the lower load on your motor versus the faster incoming air (compared to at idle) going into the intake and running past your mass air flow sensor... if it's really erratic, and happens for a long time as you're coasting, then take it in.

but you know you shouldn't be coasting for that long now...

 

do you have the ac on? Is it hot outside? The radiator fan, alternator, and other things kicking on and off will cause rpm changes.

 

oh yeah, a dirty MAF sensor can do that too...

 

that happen to my car when i first got it i asked the service department they say it jumps around when you in neutral to prepare you to shift to the next gear with out that much stress. they say thats how all the new stick cars are

 

..

 

no cel = okay.

 

I was going to state this. It makes sense. As you slow, the rpm required to match lowers. It bounces around as your speed moves from the optimal range of one gear to another.

 

Mine does this as well.. it's normal.

I'm thinking it's something to do with the ECU seeing that the car is moving at (insert speed) but there's no input coming from the gas pedal. ECU confusion maybe, or some kind of safeguard for an emergency stop (pulsing the revs for faster ABS engagement or the likes).

It's nothing to worry about, i'll assure you this.

Jay