Re: 1999 grand marquis code P0308
The misfire could be a coil, spark plug, spark plug boot etc. When have the plugs been changed?
As far as the intake, you can't repair them. I still have the plastic one on mine. When Ford saw the problem, they extended the warranty to 7 years or 100,000 miles whichever came first. I bought mine with 132,000 miles from the county here. I now have 154,000. These cars are good for many miles. The GM vehicles in our fleet had intake gaskets leak and they'd go into the oil on some. The engines would be toast after that. No recall there.