2000 Mercury Grand Marquis – engine not firing completely

Dear Doctor: My 2000 Mercury Grand Marquis was in a flash flood and a wave of water passed over the hood. After riding a short distance (2 blocks) my engine started to knock. I was 10 blocks from home, so I drove home and put the car in the driveway. My mechanic came over and towed the car to his shop. After checking it out, he found a piston was bent and he replaced it.  Even since, the car runs as though the engine is missing when it’s idling at traffic lights. If I put the engine in neutral the roughness stops. Hope you can help. I love the car! Ray

Dear Ray: The roughness at idle speed indicates the engine is not firing completely on all cylinders. A weak cylinder vacuum leak or other worn component can cause a rough idle. Have the technician perform a cylinder balance test. Using a professional scan tool (in mode 6), the technician should be able to look deep into the computer for information. A weak sealing valve will also cause a rough idle.

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