Make sure you have all NEW filters, air, oil, and the air pump filter in the air cleaner. Also new Distributor Cap, rotor, plug wires and plugs. Look at your sticker on the front rad support, that is the things they check by sight. If they don't check the air cleaner(for Filter being there) take it out for the test. You might want to replace the O2 sensor also. Then do the warm up thing(30 miles to get it nice and hot).
Buy a new gas cap. Take it to a reputable Terst only station if you can. All they can do is test, no repairs.
Yes it does cost a bit but every two years they probably need changed anyway.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.