I'd bet your poor fuel economy is due to the car being driven too gently all these years. I had the same mileage numbers when I bought my '98 at 115K miles. Now, 35K miles later I am seeing 29 around town and low to mid 30's on the highway and it runs smoother and quieter than ever. I'd say just drive it hard for a while, run some diesel purge through it and see what happens. I don't think the beahvior you described from the transmission is abnormal. They do run around 2500-2600 RPM at 70MPH and do rev a little to speed up. Frankly, I am not even sure if these cars have a lockup TC.

As for your broken glow plug, it usually entails pulling the head, as others have said, though, depending on which one it is you may be able to drill it out. If the climate where you are in NC is temperate in the winter you may get away with 5 good plugs and be OK (it will always start rough on 5 until that 6th cylinder starts firing) but it is unlikely to do long term damage to run it that way - it only takes 10 seconds or so of running to build up enough heat to start compression-ignition. If you do pull the head, then, obviously replace them all and use anti-seize when putting them in to prevent this from happening again.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz