P0306, Again

[alohamonte]

I've been chasing a P0306 for a couple of weeks. This is on my daily driver 98 OHV with 278,000 on it on original motor. The misfire is worse on street driving, meaning there are bursts of misfires. On the highway, the misfires and fewer and more steady. Like 50 miles on the highway will be 200 misfires. But 10 minutes on the street hitting lights and stop signs could be 500. It will also misfire on #5, but to a lesser extent and it has never CEL on #5.

I've replaced the plugs, wires, coil, all injectors, fuel filter, air filter, pcv, and upper intake gaskets. The o2 sensors were replaced two years ago and the fuel pump about 4 years ago. I have good fuel pressure and spark. Negative result on block test through the radiator. Fluids are clean, not mixing or burnt. No overheating issues.

Several mechanics have guessed at it but all seem reluctant to actually do diagnostics due to age. They just tell me to get a new car. The guesses so far have been bad valves, leaking head gasket, and vacuum leak.

Just curious what the board thinks.

 

[boominXplorer]

Pull all the spark plugs and check compression. I'd say your due for a new set of heads going just by the milage but never know till checking everything.