3.8l NA L36 engine with stock 4t65e transaxle. I’d rather not tear off my engine heads without knowing what all could be wrong. I did a CO2 test and it almost immediately turned yellow. it’s been 2 months and i’m still driving it with no problems except the usual all cylinder misfire that pretty much locks up the transmission for a split second (i believe its an ignition or tuning problem because it happens all at once then comes back to life like nothing happened). Is there anywhere that could leak exhaust fumes in the cooling system other than the head gaskets.

no smoke, no rough idle, no coolant in the oil, no oil in the coolant, no two pistons firing at the same time, no loss in compression, no oil cap residue, no loss in throttle response,

does backfire after letting off the gas in full send (hole in resonator), does sputter holding speed down the highway (believed to be the misfire problem), engine does have movement (old motor mount), has a squeal from upper intake manifold, can hear rods, pistons, and crankshaft from inside cab but i’ve been able to in every chevy i’ve ridden in.

if you can think of any other entry points lmk

 

The two main areas that would leak exhaust gas into the coolant would be the head gasket or a crack In the head. You can pull the plugs and compare them to locate where the leak might be. I don't think the EGR valve would introduce gas into the coolant , but maybe others have ideas. Good luck.