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Hello all I have some issues with my cobalt. to start it was running rough and throwing a code of misfire cylinder 4 so replaced plug coil still having issues so replaces injector, same problem so went ahead and replaced all plugs injectors and coils and fuel filter, after the fuel filter it seemed a lot better of a couple of days but then issues return by issue i mean rough idle but goes away when taking off but after 2k rpm feels like a cylinder completely drops off but will come back after 3k rpm and here recently the tac gauge will jump all over the place and the motor shutters but once the tac gauge falls to 0 and stays it runs fun but gets horrible fuel economy took it to a shop and they said cylinder 4 compression was at 20 and needed a new motor but got the feeling they just didn't want to mess with it
 

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Try disconnecting and reconnecting the ECM and BCM connectors, you seem to be losing Low speed GMLan. Did you clean your throttle body? Any C codes with your trouble codes? Is your timing chain tensioner still tight?
 

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Try disconnecting and reconnecting the ECM and BCM connectors, you seem to be losing Low speed GMLan. Did you clean your throttle body? Any C codes with your trouble codes? Is your timing chain tensioner still tight?
I will try this and cleaning the tb was going to be my next step seems to be mentioned alot on here, I am not sure about the codes it was awhile back when I had them checked. as far as the tensioner I have not even looked at it
 

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took it to a shop and they said cylinder 4 compression was at 20 and needed a new motor but got the feeling they just didn't want to mess with it
Not sure why the shop would tell you the compression was at 20 (psi), unless they are dishonest. If they are right and it is at 20 then they may be correct regarding the engine. But low compression could also indicate needing a valve valve job, etc

a couple of things you can do to verify it is most likely the valves...

put some oil (small amount) in the spark plug hole of the cylinder and do a compression test on it. if the compression increases up with the oil in there, it is most likely the rings as opposed to the valves. if it does not, there is a chance it is the valves
 

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Cylinder Misfire Service Traction Answer

Read my POST just a few down from yours.
Titled Cylinder Misfire Service Traction Answer

You have leaking valves, head gasket or rings. Taking into account that it runs rougher when cold, I'd say its your valves. Head to Sears or auto parts and get a cylinder compression checker. That will tell you right away. In any even, read my post in its entirety and save yourself time.
 
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