yearly is a good idea, at least once every two years. You will save in gas milage and have better power. I change the plugs and cap and rotor at least once every two years for best performance and gas milage. Change the plig wires at least every 5 years. Just my opinion. Buy the inexpensive super Bosch plugs and they are super easy to change. DOnt spend lots of money on the platinum or +4 or irridium plugs. The w201 190E likes the proper factory plug and nothing fancy. They run great and only cost about $2 each

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red