My 2001 Mazda Millenia s just got the valve cover gasket, spark plugs and wires replaced. The engine now misfires after it warms up. I took it back and the shop replaced one of the plugs....said it was fixed but it still misfires even worse. I never had the problem until I took it in to have the gasket replaced. Any ideas?

SOURCE: bad valve cover gasket, misfire and tapping lifter.

what do you mean with 'bad valve cover' ?I asume it is leaking oil.it means that the rubber seal is not sealing anymore,it has gone hard.tighten the cover bolts sometimes seals it for a while.to get to the rubber seals the inlet manifold has to be unbolted and moved,a fairly involved procedure but easy to do if you have a manual and standard handtools.The misfiring is likely stray arching of the coils .remove the top plastic cover of the motor andlook when it is very dark [like lightsof in your garage] and look for sparking.If so ,replace sparkplugs or/and leads.[remove inlet manifold also]Tapping lifter,works with oil,try engine oil flush and/or change oil brande.if problem persist replace lifter,lifter undervalve cover

SOURCE: How to replace valve cover gaskets on a 2000 mazda

just look at the valve covers some are held in place with 6 bolts some only 3 do one at a time and keep working area clean and free of dirt n grease. clean valve cover good and have clean hands when handling gasket, and there your done just replace and bolt down , check bolts again for tightness in a couple weeks.

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SOURCE: Hi, 2001 mazda millenia S,

Hi,

The best would be the operating manual which had come along with the car. In case you donot have it then there would be couple of links which can help you. As you understand its very difficult to get all under one website I am posting all the links which would give you the pictures and steps and procedures :

http://www.redlinemotive.com/replacement/mazda/valvecovergasket.asp

http://www.youtube.com/results?search_query=2001+mazda+millenia+S%2C+valve+cover+gasket%2C+spark+plugs+and+wires%2C+step+by+step+and+pictures%2C+replacements&aq=f

Thank You

SOURCE: Hi, 2001 mazda millenia S,

Workshop manual for your KJ-ZEM Miller Cycle engine
http://rapidshare.com/files/305248450/kj_engine_workshop_manual.zip
You will also find other useful documents here
http://www.milleniatech.net/