My daily-driver E30 with the six-cylinder engine was getting bad gas mileage, and the exhaust was smoking more … not blue or bad-smelling smoke, but still smoke, or something that looked like it.
My tech waited until the car was cold, and then removed the six spark plugs and kept track of which ones went where.
By inspecting each one, she could diagnose which cylinders had which issues. One of the spark plugs’ gaps was just about closed, and one other spark plug was of a type inconsistent with the rest. All but one were Bosch Platinum plugs which are great when used in the BMW M30 engine … and not this one, i.e., not the M20 engine. As I recall the story, Bosch Platinum was developed for the BMW M30 engine and really work well only on that, but popular demand from less-informed buyers who think the name sounds fancy wanted it for other engine too, and Bosch obliged.
My tech forced the gaps wide enough to get sandpaper in there, and she cleaned the electrodes and then changed the gaps size of the plugs to once again be to spec — good approach, to not rush out and buy new plugs all the time. And she replaced the odd-one-out with a type consistent with the rest.
She inserted them finger-tight and then one more gentle quarter-turn to tighten. She’s wisely wary of stripping the threads in the aluminum heads such as could happen with over-tightened spark plugs. And by making sure the engine was cold, she prevented the plug from going in cold and then expanding and being stuck.
I started the car and it ran better than it has in a long time, yay!