I replaced the head gasket on my '97 V90. Now the motor only starts for a second or two, then suddenly starts stumbling and backfiring and dies quickly. The only diagnostic trouble codes are for mis-fires in multiples cylinders. All the basics are ok: timing is exactly lined up, no air leak in intake, all spark plugs are firing, fuel pressure at the rail is good, compression is 180-200psi in all cylinders except cylinder 5 which is 160psi. I cleaned the EGR and MAF meter.

Clues...
- When the motor is cranking, if I remove the oil cap there are puffs of smoke coming out, presumably exhaust. Maybe this is from cylinder 5 and the valve seals are worn?
- The new spark plugs are already fouled black after only a couple dozen times of cranking. They're covered in carbon, completely black. Does mean oil might be getting sucked into the intake?
- The voltage at the connector for the CMP cam position sensor doesn't match what the Chilton manual says, and Chilton says this indicates a problem with the circuit. My other 960 also doesn't match what Chilton says, although it also doesn't match this V90.
- The coolant temp sensor in the rear of the head is not connected. It appears it never was, because I don't remember disconnecting it, and I can't find a connector/wire for it anywhere in the engine bay. However, when I disconnect it on my other 960, the motor has trouble starting, which makes me think it's important for that to be connected.

Any ideas are much appreciated.