Illustration for article titled Quick question about spark plug heat range and gap

Its a long-ish story, but to keep it short, I took my car into the dealer for some issues I’ve had with the engine stumbling under power or WOT. They blamed the spark plugs and said I need to check with my mechanic who changed them to make sure that they’re OEM or NGK plugs of a certain part number.


He calls me back and tells me that his parts supplier sent him the wrong ones (Denso plugs for a 2016 Impreza as it turns out), and he didn’t double check when he got them delivered because his parts supplier insisted they were OEM. Hes going to take care of it and put the right ones in today.

Anyway, according to the part number they have a heat range of 22 (equivalent of 7 for NGK plugs) and a gap of either .04 or .044. My car requires a heat range of 8 (colder than 7) and a gap of .02-.022.


It seems like this is a recipe for preignition however I never got any CELs. I’ve only driven ~6k miles on the wrong plugs. I’m not that aggressive of a driver, I rarely go WOT and it took me a couple months to notice there was a problem in the first place (I went WOT from a near standstill to jump into a gap in traffic and had no power until my car dumped a bunch of black smoke, it hasn’t done this since). It drove almost normally lower in the rev range.

Based on the difference in specs and my driving habits/mileage should I be worried about piston erosion? Should I insist on a compression test?

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