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Discussion Starter #1
I have a 2017 Cruz LS w/ 60,400 miles. It started idling rough but driving fine. Took it in and was told there was a random misfire. Replaced all sparks plugs and had a fuel system cleaning. A week later it's back, it comes and goes in waves. Took it to auto zone, and there was no codes. Turned the AC on and cylinder 1 had a misfire. Replaced cylinder 1 coil pack but it's still idling a little rough. Car runs good and has stopped sputtering while driving at low rpms (for now). What else could it be if I'm not getting codes??? I dont want to keep throwing parts at it.
 

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They run funny when a piston breaks, often #1, but you're about 400 miles late to want to hear that.
Were any of the old plugs damaged?
 

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Discussion Starter #3
Not that I know of. I had a shop replace them. It sure runs good if it has a broke piston. You are the first person I've seen say anything about a broke piston.
 

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Discussion Starter #7
So you think the piston rings may be cracked verse the entire piston being shot to ****? I would think if I had a damaged piston the car wouldnt run this good. I can get on the gas and she doesnt miss a lick. It's mostly at low rpms I can feel the sputter.
 

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If you had a bad cylinder. You'd know it.

Less power, Lowered fuel economy. You'd feel a dead cylinder throughout the entire rpm range. With a rough idle.

The motor might feel good. But the power won't be there.

I bought a grandam from an auction. I thought it would be a simple ignition fix. New plug and wire didn't fix it. Afterwards I figured out excessive blowby from oil cap. Which would indicate dead cylinder. Compression test showed 0.

Pull off your oil cap and look for excessive blowby smoke. But in case there's no shield in the cap hole. Don't run it too long as you'll splatter oil everywhere.

Blowby comes from compression leaking past the piston rings. Or cracked piston if the case may be.
 

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Discussion Starter #9
Just had a check engine light come on. Bad Knock sensor. Could that be causing all my problems? A bad knock sensor can cause misfires, vibration, and poor acceleration.
 

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Bad knock will pull timing, but shouldn't cause outright misses. A bunch of misses will drive your knock sensor bonkers and possible throw a code for over / erratic activity though.

Pull your oil cap at idle and see if its pushing out excessive amounts of air.
 

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Discussion Starter #11
If a lot of air comes out what would that mean? I had my wife drop it at the dealership today. I've already replaced the plugs, had a fuel system cleaning, replaced one coil pack and a knock sensor and it still isnt 100%. But my wife drove 35 mins to the dealership and said it was running fine. It's really weird, it come and goes.
 

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A lot of air would indicate a blow by issue. The compression isn't staying in the cylinder but entering the crankcase. It vents out of the oil cap when its running. A little is normal, but if its chugging pretty good, well...
 

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Most people here have heard of the problems with fuel getting behind the rings, going kaboom, and breaking off the edge of the piston crown. The cause now appears to be additives in some motor oil reducing the octane rating of the gas. Running Dexos 1 GEN2 oil and 93 octane gas is the best prevention.



 

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LSPI isn't ignition of fuel in the ring or ring land, its a very high pressure event where the whole cylinder, piston, and rings are shocked. Eventually this breaks the ring, ring lands, or both.
 
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