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I have a 2017 Equinox 2.4l Ecotec , started running rough the driveway did a compression test cylinder 4 was low , after tearing down discovered a piece was missing off of a valve
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OK here are a bunch of questions for ya.... Did you find the piece? Or did it go out into the converter ? How is the piston and cylinder wall ? Is that oil covering the valves ? How many miles on it ?
 

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Is the 2.4 an interference engine?! I’m asking because I don’t know...but that pic ain’t good!
 

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Also check for vacuum leaks near that cylinder as a lean condition can burn a exhaust valve causing a slow failure.
 

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Kind of odd to see all the oil on that chamber while the one beside it is normal. As asked above, how's the piston and bore look.
 

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Kind of odd to see all the oil on that chamber while the one beside it is normal. As asked above, how's the piston and bore look.
Without compression and not firing that cylinder will be wet and will not look like one next to it that is firing.
 

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Interference engine is an engine designed to where if the timing belt or chain breaks, or even stretches too far, the pistons will come in contact with the valves usually causing severe damage. Ferrari’s are probably the most famous interference design because they need engine out service to replace timing belts every 20K miles. Obviously, not all engines are interference engines, but if they are, you need to replace the timing belt or chain when the manufacturer says to or you may come to regret letting that bit of service going..,
 
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In comparison to non-interference engines, interference engines “breathe” better because the valves can open earlier, close later and open wider. Interference engines can also achieve higher compression ratios. These designs extract more power, use less fuel and generate fewer emissions.

...but the trade off is severe damage if you don’t keep the timing belt or chain replaced when required...
 
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