I have a 2003 Rs impreza with 228kms on the clock it has always been serviced every 5000kms with expensive full syn oil and always used 95 fuel. It has headers and cat back exhaust system with Bosch platnoum fusion 4 prong spark plugs. Ever since I put the Bosch plugs in it has had hesitation on start up when engine is cool. Could it be the plugs or maybe something else?
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