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Discussion Starter #1
2008 Cobolt LS
2.2 Eco Tech
Manual Trans
205,000mi

A bit of poking around and it seems like others have posted about the same issue of having engine stuttering and intermittent tachometer malfunction/loss.

https://www.yourcobalt.com/forums/problems-service/64186-tachometer-occasionally-does-not-work-engine-stutters-over-bumps.html
https://www.yourcobalt.com/forums/problems-service/49226-06-stutter-hiccup-cruise-speeds-only-no-codes.html
https://www.obd-codes.com/forums/viewtopic.php?t=14620
https://www.yourcobalt.com/forums/chevy-cobalt-owners-lounge/100674-wierd-stutter-jerk-jump.html

They either never fixed their problem or, unfortunately, did not follow up on what the issue was.

Anyhow, I am thinking this is an early sign of a crank position sensor failure and am going to go that route. My other thought is that the ECU module is building up stray charge and the grounds need to be checked.

Thanks for any help.
 

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I read quite few posts about folks disconnecting battery, unplugging the ECU, ensuring the contacts were clean, and then replugging it. Also ensure you have a good battery and it is securely connected. That seems to solve some electrical gremlins.
 

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I did find a loose negative connector on the battery post but even after tightening that down it still exhibited the same issues. I was thinking of adding another ground strap from the ECU body to the frame but am still leaning to the crank position sensor starting to show its' age.

I'll report back how it works out.

Cheers
 

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Do you have an OBD reader? There may not be any check engine light but there could be a pending code that could help troubleshoot. I bought a 30 dollar one on amazon and it has been worth every penny.

Also, there is a wiring harness on these that may be routed poorly. It can rub on the engine block near the oil filter housing and cause a short which wreaks havoc on the signals use to run the engine and tranny. Not sure if the 08 had the issue but my 2010 did, I used some foam and new wire wrap to isolate the wiring and my issues went away.
 

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Discussion Starter #5 (Edited)
Thanks Montagc!

Well the issue showed up again today but this time the engine check light popped up and I was able to have the local auto supply give me the code. P0336 Crank shaft position sensor. I'll check out the harness when I pull the sensor but just to be sure would like to check the sensor too.

Does anyone know the target resistance values or continuity checks for the various sensor pins?

I hope this helps someone diagnose similar issues in the future.

I also found this useful post which seems to suggest the same issue with the wiring harness to the sensor.
https://www.yourcobalt.com/forums/problems-service/30065-2008-2-2lt-has-stange-problem.html
 

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Discussion Starter #7
Good news, replaced the crank position sensor with an AC Delco part. It wasn't too bad a job, a little bit of hand dexterity certainly helps getting it back in. I did a resistance check of the 3 pins on the sensor and the old sensor certainly showed much higher resistance vs. the new part. The short wire harness was checked for continuity and line cross talk and passed. I also looked for any chaffing or wear of the various wiring harness sections around the engine and did not find anything as most of it has been protected by wire loom. Cleared the code and it is running very smooth now. This process took about an hour.

I had enough time to change the serpentine belt, clean the throttle body, replace rear shocks and check rear brakes. Not a bad Saturday morning.

I did discover that cleaning the throttle body requires the drive servo to relearn the throttle valve idle position. At first it idled quite high after cleaning the throat around the plate, ~1550 rpm. After I pulled the battery connection and a few drives the ECU has been gathering more info to adjust the idle. Impressed that this vehicle has these systems.
 
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