Hmm....have you had it checked out by a mechanic. It seems to me that if there really was a critical engine problem it wouldn't be able to drive around up to 55mph like that....

Does it smoke or anything? If its a critical engine problem an oil analysis would tell you, if it came back fine then its something else. Personally I'd probably check it out as much as I could before I decided to scrap it.

If the two year old fuel in the tank has a lot of water in it that could be causing a knock/bad firing. Try running the engine from a bottle of fresh diesel with some diesel injector treatment in it. Just disconnect the two lines from the metal ones coming from the tank, and stick them in a bottle of diesel. Make sure the filters are new/not plugged too.

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