I'm having an issue with my wife's car (08 G8 GXP 3.6l) and I'm really stumped. I drove it the other day and it shook like crazy and was also getting a bit hot she also had a P0430 engine code. I popped the hood and it was down on coolant. So I checked it all over an replaced the ball joints (that was the shake) and noticed the passenger side engine mount had some coolant in it so checked over everything and replaced the water pump. She drove it for about 2 hours yesterday and it didn't have any real issues, drove smooth and engine temp never went above 100-110. However I checked her coolant and its down about half an inch. She also has a bit of a shake when you first start it. Once its warm its fine. My question is does this sound like bad intake gaskets? If she's still slowly losing coolant and it stutters on idle those are usually symptoms of bad intakes. I have checked everything, no white smoke, no new visible coolant leaks just down a bit on coolant and sputter idle. Her gas mileage also kind of sucks averaging 26 mpg at highway speed. Also the P0430 code is a cat code but could it be throwing that code cause the intake gasket is leaking and sucking in air throwing off the computer.