Well i have the 98 deville with the 4.0 northstar v8 and it had a misfire then we changed the spark plug and it fixed it about 3 days after that it runs really rough and misfires on and off worst then last time we thought it was the head gasket but the test came back negative any tips i forgot to mention that its showing misfire codes and as well 2 02 sensor codes thanks guys 🙂
Can you list the codes? Also, what is the mileage?
Realize I had to get some blow in different ways sometimes it will go with the ring around the Pistons going to the water sometimes it will blow other ways and the water test won't show it knowing those things I'd bet on the head gasket problem
Even if it passed the test, I would say the head gasket. Those motors loved to eat them.
