How do you test if a spark plug wire is good or needs to be replaced? The wires in my 560sl are fairly new however the engine seems like it's running on 7 cylinders. Plan to check & replace plugs tomorrow but not sure how to determine if wires are good. Thanks.

if you don't have the equipment (do a search), as a shadetree mechanic, you can look in the engine compartment at night, the darker the better, and with the car running. Look for any sparks or lights in the engine compartment. It shouldn't be any.

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J.H.
'86 300E

Thanks. I'll try it tonight.

In addition to what J.H. mentioned, mist some water over the wires from a pump spray. It'll greatly enhance any sparking that is present.

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95 E320 Cabriolet, 169K

And clutch each wire with your fist near the spark plug end or distributor end (where the spark leaks generally occur). The bad one is the one that dramatically alters your hairstyle!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

if these tests don't show the plug wires as bad, the crack the fuel lines at the fuel dist while it's running. a weak injector will look like the fuel dist isn't supplying fuel to it. the rest will sort of shower the fuel upwards. if a weak injector shows up that way, then change them all with the holders and seals. it's only a matter of time on the rest. good luck, chuck.