Why is MY EXPO SHAKING A LITTLE BIT And the exhaust smells a little like gas but on 1997 Ford Expedition

I recently had my engine replaced so when i got my expo back the mechanic told me that there was a little shake nothing to serious but still I didn't want to driveit,mechanic told me that it was the catalytic converter because on the freeway it was fine so it couldn't be the engine,and when it was stopped it was shaking.also the check engine light was flashing my dad told me that the reading on the codes was engine misfire and catalytic converter so could it be that please i need valuable info maybe it has happened to you thankx.

by in Nogales, AZ on March 23, 2012
2 answers
ANSWER by on March 24, 2012
It's not fine.Assuming that the mechanic put in a used engine? could be quite a few things.Does it have new plugs? it could have a bad ignition coil as well.Doubtful that it is the cat converter-that is reacting to problem,shouldn't be a problem on it's own.
ANSWER by on March 24, 2012
When the check engine light flashes that means the on board computer is detecting a misfire. That is very important that it be taken in and fixed asap. Continued driving with a misfire can damage catalytic converters. If this work is it its warranty period ....the shop that installed the engine (in my opinion) should be correcting the missfire condition. I highly suspect that is what you discribe as shaking.