Help with Misfires please

[jan_mast]

I have a 2008 Chevy Cobalt LT that is having misfires in the #1 cylinder again.

I had the same issue last year and ended up having to change out the valves because I tore the head off and noticed blow by at one of the valves. Put the head back on, re-timed her, and haven't had any issues until now. (Also changed out the plugs, the coil pack, and head gasket.

Now i have misfires again! In the same cylinder! I swapped spark plugs between cylinders (plus are less than 1 year old) and swapped coil packs. No joy, the misfires remain. I believe logically the next step would be a compression test right? Does anyone have the part number for the compression test rig?

Thanks!

 

[YelloEye]

Any generic compression tester will work. Have you swapped injectors around?

 

[jan_mast]

I have not swapped injectors. That seems like a great idea. I will have to do that!

 

[jan_mast]

So i swapped injectors on the fuel rail and yesterday it ran smoothly with no misfires. Today... Misfires again. I got the test gauge in the mail today so I guess I will run the compression test tomorrow unless anyone has any better ideas. Thanks!

 

[YelloEye]

Did the cyl it's misfiring on change?

 

[jan_mast]

Did the cyl it's misfiring on change?

Nope. Still misfires in the #1 cylinder.

I got the compression test set today and sure enough the compression in #1 cylinder was really low. It's was 85/185/175/175 psi.

I suppose I will replace the head bolts, gasket, valves and springs. I also plan on having someone look at the valve seats. I wonder if there is a test to see if the valves are dealing properly after I replace them.

 

[YelloEye]

Well if you're taking the head off you'll be able to see if the valve seats are toast. Just gotta rotate the cams and open them. Did you try adding some oil to the cyls and testing again? Would tell you if it's in the rings.

 

[jan_mast]

The service manual only called for a dry test, but since you mentioned it I will do a wet test before I pay for anymore parts. I guess the idea is that by adding the oil to that cylinder it would seal up like the piston rings should be doing. Therefore if the wet test improves compression then it's the rings, however if the compression remains the same then it's the valves. That sound right to you?

Where do you live? I feel like I owe you some beers. Haha

 

[jan_mast]

So I did the wet compression test on the #1 cylinder. Though it improved the compression to 130 psi, which is >70% of the highest cylinder @ 185 psi. However the manual states that no cylinder should be more than 21 psi lower than the highest cylinder. In this case it fails that criteria.

How would you interpret this reading? As worn rings because of the improved compression with the oil?

Or as failed valves because cylinder 1 is more than 21 psi lower than the highest cylinder?

Thanks!

 

[YelloEye]

I see that as a failed test as specified in the manual. So I think it still warrants a disassembly to checkout the valve seats and rings if it comes to that. My money is on valves or head gasket.

I live in WA btw.