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I have a Chevrolet 2008 Cobalt (LS) that I've had for about 7 years.

No sooner do I have this thing payed off, when it starts giving me the following problem (I've never had any trouble with it before).

We've tried everything we can think of, but to no avail.

The symptoms:

  • Engine light comes on. During regular driving, especially after idling, while trying to speed up, the engine light flashes. This usually doesn't happen (the flashing) once I get going on the highway. What's more, whenever the engine light is flashing, I can tell that the car loses energy (I say energy, but what I really mean is that the car loses acceleration).
  • A simple computer reads "Cylinder 4 misfire."
  • When idling (most of the time), the engine sounds pretty shaky, like it's on the verge of dying or not getting enough gas, etc.
  • It shifts from 1st to 2nd pretty roughly, but not always, and when it does, the engine light is usually flashing.
  • While accelerating to higher speeds on the highway and the engine is flashing (thus the acceleration is poor, as if it were only getting fuel from 3 of the 4 cylinders [<-- just a guess/just what it feels like]), eventually I reach a good speed and then it's like the 4th cylinder "kicks in" (or otherwise regains more accelerating power) and I feel the normal torque of the car and the engine light quits flashing.
  • Now that it has gotten worse, it dies a lot while idling, at stop signs, at stop lights, and even sometimes when turning a corner and I'm not giving it gas.
  • The above symptom ceases when I have the air-conditioner turned off (that is, the car has a much better time "not dying").

Here's what all we've tried:

  • Changed the cylinder 4 fuel injector (twice in case we got a bad part).
  • Changed the cylinder 4 coil pack (twice in case we got a bad part).
  • Changed all spark plugs.
  • Did a compression test on cylinder 4 (read a little low, 125, I think, but I was told "not enough to cause this."
  • Tested the fuel pump. It tested just fine.
  • Changed cam-sensor.

And that's all the symptoms and things we've tried that I can think of. I'm no car guru, but even our technician is having trouble figuring this one out.

I'm at a loss, as I've never heard of this problem before. Does anyone have any ideas what this could be?

UPDATE:

Okay, since this question was posted, I've had the pulley and belt system replaced (it was aged a bit) and all 4 cylinders tested (they tested all about the same at approx. 125 psi, each). The problem still persists, though it seems a little better.

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  • Did you happen to also change the cylinder 4 plug lead? Sometimes when the coilpak goes bad, it fouls the spark plug and damageses the plug lead. It should be pretty cheap to get your hands on a lead, so maybe it's worth checking out before we start checking the really expensive things.
    – Captain Kenpachi
    Commented Aug 3, 2015 at 14:07
  • Check the power and ground to the #4 coil pack. You can find a reference on this image as to what to check. You don't need to check low reference or the ignition control signal. Also, how did you check your fuel pump? Did you check it while under load? Check your tail pipe. Is the inside very suety or does it look okay?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 3, 2015 at 14:17
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    @JuannStrauss - These are coil-on, so when the coil was replaced, it all should be replaced from the coil up to the spark plug.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 3, 2015 at 14:18
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    What were the compression test figures for the other cylinders?
    – Zaid
    Commented Aug 3, 2015 at 14:23
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    Given that everything has already been replaced and compression tests have been done. I'd suggest checking engine vacuum then maybe doing a leak down test if engine vacuum appears abnormal. Also you can check cam crank correlation with a digital storage oscilloscope, kill three birds with one stone. As to which engine you have just supply the 8th number/letter of the vin.
    – Ben
    Commented Feb 6, 2016 at 20:18

2 Answers 2

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There appears to be a similar case on the Cobalt forum, where compression test figures are reported to be around 220 psi, roughly double what you're reporting.

If this is the case for your vehicle, it would seem that the problem is related to a lack of compression and not fuel, air or spark.

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  • What part needs to be replaced if this is the case? The engine itself?
    – VoidKing
    Commented Aug 3, 2015 at 14:30
  • @VoidKing : If you follow the thread I linked to, the fix was to replace the head (top half of the engine). Not the best news, I'm afraid, but hold your horses until the numbers are confirmed by your mechanic
    – Zaid
    Commented Aug 3, 2015 at 14:34
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    I was a little surprised the OP only reported on the single cylinder. Normally you are looking for the difference between cylinders as well as the overall for any given cylinder. Good find on the forum link. Also, I agree with the top end being the issue for one cylinder being low like that (in this case anyway).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 3, 2015 at 14:36
  • @Paulster2 Ha, ha, yeah, I told you, I'm no car guy. Thanks for you help!
    – VoidKing
    Commented Aug 3, 2015 at 14:56
  • @Paulster2 Okay, since this question was posted, I've had the pulley and belt system replaced (it was aged a bit) and all 4 cylinders tested (they tested all about the same at approx. 125 psi, each). The problem still persists, though it seems a little better.
    – VoidKing
    Commented Aug 6, 2015 at 14:23
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have you checked the Throttle Position Sensor, MAS Air Flow Sensor, have you cleaned your Throttle Body Unit ?, have you checked & cleaned your EGR Valve,

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