I am just wondering if this comes into the category of Useless DIY Repairs, but I am thinking of changing the HT leads/wires from distributor to spark plugs as part of an ongoing effort to cure a rough idle on my 1989 230TE (W124). (I have already changed the rotor arm, cap and spark plugs). Is there any way of testing these for function before replacing them as they are too expensive to junk unnecessarily? The car has the older style TSZ ignition system and it is an M102 engine with moon mileage but good compression.

Thanks for all your inspiration and help.