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I have a 1997 civic dx coupe (auto) with about 150,000 miles.I swapped the engine and got in a "zc" with supposedly 30,000-50,000 miles on it. The first week the car ran smooth no problems, however, after a week the car became sluggish and I have to push the gas peddle all the way down just to get it running at traffic speeds. Does anyone know what could it be?

The car has new oil, spark plugs, tranny oil, cables, coil ignition, rear main seal, water pump, etc.. (from honda).

I also took it to a transmission specialist and said the tranny is fine, so I can rule that out.

Anyone know what could be the problem? Thanks