yes, I was correct. I replaced both exhaust man gaskets. Bank 1 where the leak was so very bad, was a flypro gasket and it had completely failed. The center core of it disintegrated. The original GM gasket on bank 2 was still in medium condition after 124k miles, but it was leaking due to a burn through. GM gaskets are $30 each vs. $16 for the flypro pair. I'm using grade 8 studs with a stainless steel washer, lock washer and short brass nut on each stud. The lock washer by itself is to small, it catches on the edge of the hole in the exhaust manifold. 2nd brass nut to hold on the spark plug shield cover on the stud. Double tall brass nuts on the rest of the non-shield studs. Fired up today, cool over night, re-torque, then finish assembly.
The mis-fires on cyl. 4 and 6 are totally gone, but still seeing a very small misfire on cyl. 1 and 2. I'm thinking that perhaps the upstream 02 sensors are failing. They appear to be giving a strong voltage, but they are 17 years old with 125k miles. I've decided to replace them. Engine seems to idle smooth once it warms up, better than ever. But still thinking of those misfires.
Did I mention I checked the compression on all cylinders? all were at about 140 with all plugs out except 2 that were at 130. When I added oil they came up to 134, no surprise. I can't figure out how I got 148psi in 4 an 6 earlier. I did a leak down test on the low ones and found them at 20 to 24%. Norm new is about 10%. No cross cylinder leakage, So valves are probably weak, big surprise for 125k.
