My 1988 740 turbo wagon is getting crappy fuel mileage. I can hardly get 400kms on a tank, I think it should be better. What can I check? Oxygen sensor? Can that be accessed from the engine bay? I check all the hoses going in and out of the turbo, all is fine except the AMM to Turbo hose which is currently patched and being replaced this saturday. Any other ideas?