RPM Fluctuation/bouncing While Driving at Steady Speed - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 16 (permalink) Old 06-26-2016, 07:56 AM Thread Starter
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RPM Fluctuation/bouncing While Driving at Steady Speed

Hello everybody, I've been dealing with much the same problem as others altho I still have't gotten the phenomenon addressed,or fixed. The bounce of the tachometer needle happens when speed is constant under little to no torque, anywhere between 30-58 km/h, the RPM ranges 1300-1800-ish {if I press or release the accelerator slightly the RPMs stay steady, and the bouncing tach needle stays steady}. And after the TC engages around the 60 km/h, like it's designed to, the tach will drop somewhat to where it should be. Again the bounce of the tach needle occurs if I duplicate the little to no torque, steady speed. Generally between 62- 79 km/h. My 08 cobalt is 2.2 naturealy aspirated automatic factory stock engine. No mods under the hood, no performance upgrades in exhaust, a really plain jane. I've replaced air filter numerous times, cleaned throttle body, mass airflow done, and I had the transmission scanned by a transmission shop. The diagnosis from the scanner shows the torque convertor works as it's designed to. It engages around 60 km/h-2000 RPM. When I drove and he watched the sanner, of course I was able to duplicate this bouncing tach needle. His reaction was one of disbelief, stating he's never seen anything like it. The torque converter locks pre maturely, but doesn't stay locked. After the TC does lock, up like it's designed to, it doesn't stay locked. This is where the "bouncing tach needle" lies. Now after all this, he gives me the phone # and name of a guy who can still access the the GM mechanics only web site. I contact him and he says he can download an update that may rectify this TC problem. I haven't gotten the update yet, I'm a little skeptical, he says it may not be a fix. I do not know if there are multiple downloads for this problem, or if it's a one horse show. So if anyone has any insight that may encourage me going ahead with the update,please let me know. ( I've got mechanic friends who have said updates for anything car related could possibly be a cure with defects, and this is where I'm puzzled. I live on Canada's east coast, kind of a small rural city, so I'm asking for some advice from all of you.) The download guy wants to check the car's problem himself, which adds to the total cost of something that may not be of use to me, and when I speak on the phone to anyone of GM's customer satisfaction people, they always state the local dealer has to test scan it before anything,I say while the cat was under warranty they had it twice for this problem twice and twice they did the TCM thing to no avail,so they won't try it again. Lemmeno whatever ideas you have, as I will appreciate.
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post #2 of 16 (permalink) Old 06-26-2016, 08:42 AM
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Mine does this too. Seems like the torque convertor is between a point where it can't make up its mind if it wants to lock or not. Don't worry too much about it. Mine's been doing it since I got it with 33,000 miles and I'm about to hit 150,000 miles soon.

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post #3 of 16 (permalink) Old 06-26-2016, 08:36 PM Thread Starter
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Cool bouncing tach

I know the car will never leave me stranded or otherwise, it's such an annoyance. I do have a possible fix in the form of an update to the cobalt's main computer. If it wasn't for the fact that may not resolve the issue and until this guy becomes available to see this situation for himself...I won't worry too much about it. And I appreciate ya chiming in with yer 2 cents, it's good to know the problem is acknowledged by others.
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post #4 of 16 (permalink) Old 07-19-2016, 08:55 AM
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I'd be surprised if all auto's in balts didn't do this. Mine does it, it's a constant battle with the tranny one I think I have it figured out at times. It's like it wants very badly to go into overdrive and getting it to come out takes a swift kick in the butt in the form of stepping on the pedal. Very difficult to just cruise slowly in 35 to 45 mph zones.. I wish mine was a straight drive....!
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post #5 of 16 (permalink) Old 07-19-2016, 09:17 AM
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Can't say mine does any kind of RPM bouncing. It shifts into 4th at 40 mph under light throttle, and RPM stays consistent afterwards.

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2007 Chevrolet Cobalt LT - M62 Supercharger, Ported Throttle Body, 42 lb/hr Injectors, Intense 2.9" Pulley, Cobra Heat Exchanger, SS/TC Exhaust, TTR Engine and Trans Mounts, Trifecta Tune, ZZP Header and Downpipe, '08+ FE5 Struts/Shocks, SSC Springs, FE5 Control Arms, SS/SC Front Swaybar, Powell Hardcore Rear Swaybar, 18" MSR 045s, Dunlop ZIIs, Nightshaded Tails/Side Markers, Shorty Antenna, Black Bowties, Black Grilles, Black Headlights, Dual JL 12" w1v2 Subs, A/D/S Front/Rear Speakers, G85 Recaros, Leather Rear Seats, 160 MPH Cluster.
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post #6 of 16 (permalink) Old 07-19-2016, 05:55 PM
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Can't say mine does any kind of RPM bouncing. It shifts into 4th at 40 mph under light throttle, and RPM stays consistent afterwards.
So then if you wanted to increase your speed a little does the TC come out of lockup and pull or does it seem like being stuck in high gear, hardly any power to accelerate?
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post #7 of 16 (permalink) Old 07-19-2016, 08:59 PM
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Depends on the scenario - if I'm slowly accelerating on a long on ramp and then roll into a higher throttle position, it'll take a second of two for it to unlock. If I peg the throttle a bit harder, it'll come right out and/or downshift.

I do have about double the stock power, so even in 4th-gear lockup it doesn't have much of an issue, it more just sounds a bit goofy.

1986 Chevrolet Camaro IROC-Z - Wrecked/Sold
2007 Chevrolet Cobalt LT - M62 Supercharger, Ported Throttle Body, 42 lb/hr Injectors, Intense 2.9" Pulley, Cobra Heat Exchanger, SS/TC Exhaust, TTR Engine and Trans Mounts, Trifecta Tune, ZZP Header and Downpipe, '08+ FE5 Struts/Shocks, SSC Springs, FE5 Control Arms, SS/SC Front Swaybar, Powell Hardcore Rear Swaybar, 18" MSR 045s, Dunlop ZIIs, Nightshaded Tails/Side Markers, Shorty Antenna, Black Bowties, Black Grilles, Black Headlights, Dual JL 12" w1v2 Subs, A/D/S Front/Rear Speakers, G85 Recaros, Leather Rear Seats, 160 MPH Cluster.
1998 Chevrolet Cavalier - Cold Air Intake, Dual MTX 12" Thunder6000 Subs, Pioneer Speakers, Camaro Front Seats
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post #8 of 16 (permalink) Old 07-19-2016, 09:45 PM
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That's about how mine is. Except if just driving on some level road and I want to go, say 3 to 5 mph faster it'll sort of hunt in rpm's like OP says. Or I can stab the throttle a little and rpm's will jump up and quickly settle back down unless I stay in the throttle in which case I'm by then speeding..
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post #9 of 16 (permalink) Old 07-20-2016, 07:52 PM Thread Starter
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Dudes, these interactions and how the posts are added to is the very reason I wanted to join yourcobalt in the first place. There are amazing real people here with real problems/real patience/real solutions, and you are all really appreciated. I'm thanking all who have chimed in and showed some camaraderie, past, present and future.

I did read or hear something about re-setting the learned shift points by way of a software download available from GM,except accessing that data may be the difficulty{unsure if that is accurate as it was told to me without any substantiated proof}. I'm going to get that download for my cobalt before August, I will post up any improvements or let downs that update offers.

Thank you all.
Peace

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post #10 of 16 (permalink) Old 07-21-2016, 06:57 PM
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Dudes, these interactions and how the posts are added to is the very reason I wanted to join yourcobalt in the first place. There are amazing real people here with real problems/real patience/real solutions, and you are all really appreciated. I'm thanking all who have chimed in and showed some camaraderie, past, present and future.

I did read or hear something about re-setting the learned shift points by way of a software download available from GM,except accessing that data may be the difficulty{unsure if that is accurate as it was told to me without any substantiated proof}. I'm going to get that download for my cobalt before August, I will post up any improvements or let downs that update offers.

Thank you all.
Peace
I read about the same thing when I got my cobalt back in February '15. Definitely let us know what you find out. It drives me crazy especially with my new MagnaFlow exhaust, makes it sound like an airplane flying around checking out the countryside....
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