Just like the title says, my cobalt likes to start accelerating by itself when I'm coasting. I first noticed this on the highway in stop-and-go traffic 2 months ago and it was hard to reproduce. Now it happens a lot more often.

Did some searching here and nothing came up. Anyways, heres the details, mods are in my sig (I havent changed my mods in forever, intake has been on since the first year I bought the car):

With clutch out and car in 2nd gear (and this happens ONLY in 2nd), if I begin coasting to a stop the car will slowly speed up on its own, but only at very low speeds (less than 20km/h). I turned my interceptor to TB% and sure enough, at coasting its around 15-17%. With foot off the gas, i can watch it creep up to about 23% on its own, die back to 15-17, and run back up to 23 over and over again.

Around the same time this started happening, my car has started making a slight lurch whenever I first press in the gas, and also when I let off. It's a slight shudder, but its ******* annoying, multiple passengers have commented on feeling it.

It's going in for some other warranty later this week, but I figured I'd see what everyone here thinks first seeing as I can't find any info on this problem through the search button.

Oh, I also clean my TB, supercharger inlet, and MAF sensor whenever I clean my intake filter (usually twice a year, car doesnt get driven much). I Don't spray the TB directly, as I know that causes problems, I spray a clean rag and use it to wipe out the TB.

 

sounds like you should try a new TB

 

Do you think it's strange it only does it in 2nd gear?

 

maybe, have you tried it in third with you Interceptor on TB?

 

Could be a bad/faulty accelerator pedal! As it's drive by wire so if that's bad or giving incorrect input the T/B responds accordingly! When i had my 07 LSJ it had a similar issue but was fixed by just taking it apart and making sure it was in tight. It would only read 88% when floored according to the interceptor. once it was fixed it went to 99%!

 

Could be air velocity at that certain RPM? I would try a different TB also

 

I stated in the post it only does it in 2nd, ive tried it in every gear. I would think first would be really notcieable, but nope... nothing

 

I dont know anyone with a spare TB around here, I could try taking all apart and reinstalling it. I'll test the TB % at wot as well, see what it reads.

 

Could be dirty.

 

i think 2 things carboned up throttle plate to where the cars trying to correct the idle and miss adjusts. or it got a bad position sensor, which if i recall is part of the throttle body on our cars

 

normal

 

It cant be a dirty TB, I cleaned it not even 2 months ago. The problem started a bit before I cleaned it and the maf/intake etc, and i was hoping cleaning them would fix it.

 

Did you check the intake filter for holes, cuts and so on? My old accord used to do this and a new intake filter fixed it.

 

I'll lend you mine if you come to Ottawa

 

I just dealt with an escape at work with the same issue. It was the idle air control being stuck. Does the car rev up in n aswell or just only 2nd?

 

I had this problem too before.. Somebody said its the anti-stall in cobalts. If i was to start in first and get off the clutch and gas, it wont stall. It keeps giving fuel so it don't stall... I think this is what you're talking about atleast..

Edit- nevermind, im an idiot.

 

Any codes at all?

 

Mine does the same thing and after some testing I discovered it starts to accelerate once its drops below 1k RPMs. And then stops once the car gets to about 1100. So im guessing it is some sort of antistall mechanic

 

Well I took it in today for a heatshield rattle that i cant find/fix by myself (car has a full bumper to bumper warranty anyways), right away they nailed it down as a faulty TB. The shop foreman there says hes seen it a few times.. dont know if hes saying that to make me feel reassured or what haha.. Theyve got a new one coming overnight so ****, even if thats not the problem I get a shiny new one for free :p I'll let you guys know how it goes.

Theres no codes at all, it only does it in 2nd gear, idles normally even when cold. And no theres no tears in the intake filter.

Manopka - thats exactly what it does.. but I've never heard of any such mechanic on this car. It definitely has not done it these last 5 years I've had the car.

 

oh. well I got mine like 6 months ago and it's done it since I got it. let me know if that solves it.

 

Might as well give you guys an update. They kept my car for 2 days, warranteed my both lower CABs and rear calipers while I waited. Throttle body still hasnt come in yet....

Turns out the rattle was a heat shield over the resonator, and to boot the resonator itself is shot. I got it down and looked inside to find this :





I got a hold of Corsa and they warranteed me a new one, just got it today, it'll go in this weekend. Apparently they discontinued making the exhaust for the cobalt.. I got one of the last resonators they had for our car.. hopefully this one lasts more than a year and a half for the amount I dropped on it =\


.

 

The other day I did some testing and found weird results. If I have the clutch pressed in and I slowly release it all the way it wont stall. The car automatically gives it gas so that it wont die. (Only slowly though and if you go too fast it stalls) Idk if this is normal cuz the only other car I've owned was a 2004 Ford Ranger manual. But yea. Just thought that was interesting.

 

Normal. Car drives itself in 1st.

 

this, mine did this when I was factory supercharged. I fixed it with a turbo kit and engine build.

 

Sounds a lot like deceleration fuel cut off dis-engaging. When the throttle is all the way closed and the car is in gear the computer cuts fuel to save gas. The computer starts feeding gas again around 1200 RPM to prevent the car from stalling. When the car adds fuel again you get a lot less braking off the engine, it will feel like a lurch forwards. Also the car will add throttle to bring the idle up if it's below target. Target while the car is moving is something like 1100-1200. Push the clutch in while coasting and watch your RPM's, they don't go all the way down to 800...they sit just above 1k.