[slowbalt08]

Car is a 2008 Chevy Cobaly LS 2.2l auto. I have a weird misfire that doesn't seem to affect the car at idle or around town driving I've done a few hundred miles at slower speeds and never noticed any issues or even slight misfire.

If I get on the highway and do above 60MPH sometimes I will get a flashing check engine light and a noticeable loss of power 1/2 throttle will cause me to keep speed but it could do this for about a mile. If I give it 3/4-full throttle it will downshift and start moving like normal again and the issue will go away temporarily. Some drives it will do it every 2-3 miles other times I could go 50miles without a issue. The only code I am getting is P0300 and when I check the misfires in my OBD2 reader it shows cylinders 2 and 3 as being affected but no code for those specific cylinders misfiring.

This issue started about 3k miles ago and since then I have replaced spark plugs, coil packs, and fuel injectors all with AC delco parts. I cleaned my MAF and throttle body and they seem to look okay. At a loss for what else I could check. It seems like a sensor is getting a inconsistent reading but no idea why my speed or other variables would affect it from work correctly sometimes.

 

[1966tbird]

how many miles?
save a bunch of time and get a compression check. Rent/borrow a compression gauge setup from your local auto parts store. Log the numbers both for dry cylinders and then again with a cap full of oil in each cylinder. Without this, your spitting into the wind waiting time, energy, money, and ….

 

[RH45]

There’s a long list of things that can set off the dreaded PO300 code.
Good luck with your quest

 

[slowbalt08]

So did a compression test over the weekend and it seems like it is good. Internet results vary for what is spec for the 2.2 but I had the following results and they seemed pretty consistent. Really not sure what else I can check. It doesn't seem like it is MAF or throttle body related since only 2 cylinders show misfires in the scanner. I am not sure how cylinder 2 and 3 are related to both have the misfire, but 1 and 4 don't show it. Is there any modules or relays that specifically control those 2 cylinders ignition?

Cylinder 1 175psi
Cylinder 2 184
Cylinder 3 170
Cylinder 4 184

 

[1966tbird]

You can continue to check by swapping plugs…..then swapping coils and determine if the error follows the plug or coil. Finally, you can swap injectors. Best get a set of fresh injector orings before you pull one.

also, verify no air leaks / vacuum leaks past the MAF, such as the vent from rocker cover to air plenum and on top of the carb.