I know there’s other threads similar to this one but none specific to my scenario.

I have a 09 cobalt ss/tc LDK swap that threw a cylinder 3 ms while driving on the freeway. When it threw the miss, grayish white smoke came from exhaust, no blue or black. Running 50/50 E85 and 91 octane fuel.

ran a comp test: #1: 130, #2: 130, #3: 0! #4 145

then did a leakdown. Cylinder 3 had 10% leak, all others less than that. So this was less than helpful.

Under the hood, the engine sounds fine. Down by the exhaust pipe, sounds like ****. Sputtery sounding.

im at a loss. I’m thinking maybe a valvetrain problem, but I’m not 100% sure. I’m doubting cracked piston or rings now.

anybody have any other thoughts or diagnostic steps to take I didn’t think of (other than a shop)?

 

If cylinder 3 Is 0, then obviously u have a problem there. Pull the head off and have a look. Or use a boroscope in the plug hole

 

If the compression is 0, but leak down is 10% the engine shouldn't sound fine. If the intake valves are not opening that could provide a 0 compression and 10% leak down, but that doesn't make sense if the engine runs fine.

I'd re test the compression for cylinder 3, and try a wet compression test as well (add a bit of oil to the cylinder).

 

And I'd pull the valve cover when you do the compression test to watch the valvetrain for that cylinder.

Pulling the valve cover on its own might give you the answer you seek.

 

I am betting it spit some rockers out.

 

Which wouldn't be unheard of... although with LSJ's, that usually happens on the exhaust side and is usually the fault of the lifters.