Lots of places to start. Make sure you reconnected all the hoses. Also re you throwing a check engine code yet? Who recommended bosch plugs? The consensus is that ngk is the preferred plug. A lot of post valve cover issues are related to a bad seal and a vacuum leak. Valve covers are not hard, but they can be tricky. You disconnected a lot of stuff to do the VC so make sure everything is where it should be including the grounds for the cool pack. Also make sure the o2 sensor wires didnt get crossed.

The good news is that if it hasn't, it will eventually throw a code and that should clue you in on the problem.

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