I'd check chain stretch for sure and your injectors. You can build a pop tester yourself out of a grease gun and some easily obtained fittings to see the opening pressure and spray pattern. Its really easy to make. PM me if you're interested. If your chain is stretched the pump will be injecting the fuel late and not combusting the fuel as completely as it would be if it were set where it should be. Plus the cam timing would be late. I think this was the problem on my wrecked '85. Its motor is in my garage now being rebuilt and the chain had about seven degrees of stretch on it. It always got about 18-20 miles to the gallon and I tried everything under the sun to fix it. My '84 gets between 23-24, sometimes up to 26 on interstate trips and it only has about two degrees of stretch.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo