Replace fuel filters first, they are the cheapest and easiest. Then inspect the ignition system -- cap, rotor, and wires for corrosion, worn contacts, and evidence of current leakage ("spiderweb" or "lightening" marks on wires and anything the get close to), and pull the plugs and check for condition. It's entirely possible that you melted oil ash and either masked the electrodes or shorted them.

If everything is fine there, check the vacuum -- if the economy gauge is aways high instead of sitting at the bottom peg at idle, you need to loosen the catalytic converter from the header pipe. If the car then performs normally (although VERY loudly!), the cat is plugged and needs to be replaced.

Actually firing will have to be confirmed by either ignition analyzer or diagnostic computer.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!