Greetings folks. My '04 MM started acting up for the first time ever the other day during a 20-ish mile highway run in very heavy rain, and I'm trying to narrow down the possible cause. It's got 195k clicks on it. Upgrades are a JLT cold air, BBK throttle body, Steeda underdrives (minus alt pulley), Lidio tune, and a JLT oil catch can.
When accelerating moderately at highway speed (say from 110 km/h to 120 km/h) I started to experience a "bucking" sensation. As I said, this was in heavy rain. Not overly violent, and at a steady cruise it doesn't do it at all. On cruise control, it will do it a bit if you hold the accel button down. But if you just tap the accel button once, it doesn't do it. I don't think it's a transmission shudder, which I've experienced in my '94 CV. But you can feel it clearly in the throttle. The car itself doesn't seem to be affected - you just feel it in the throttle. I've read the term "cutting out", but I don't really know what that feels like.
I've been researching the site, and it seems like the cops and plugs might be the most likely cause. I changed the plugs 4 or 5 years ago, but I believe the cops may be original. I've had the car since 45k clicks, and don't recall changing the cops. I've also read a few posts about the TPS, IAC, crank and cam sensors. It never stalls or even comes close, so I don't THINK it's the IAC. There is a bit of roughness to the idle, but nothing major.
So I'm looking for any input at all as to whether I'm on the right track here thinking it's the cops and/or plugs. I suspect the heavy rain allowed water into an area that has degraded with time and mileage. I have been working from home due to covid, so my daily 45 mile commute to work has not happened all summer. Although I drive it often, I haven't been commuting with it much. Perhaps that is a factor as well? Also, I have driven it in dry weather since the big rain and it is still "bucking" at highway speeds. Also, it doesn't seem to affect heavy throttle, although the power may be down a bit.
Any assistance would be greatly appreciated.