Intermittent preignition knock / pinging

Hey all, I recently got a 2007 Prius from my dad that has 275k miles and a newer battery. So far I’ve loved this vehicle - my second hybrid since having a 2001 Insight many years ago!

Since getting it a few weeks ago, I’ve noticed that the engine has VERY bad preignition knock, but not all the time. It doesn’t occur when the engine is cold, only warm, and it tends to only occur during low- or mid-throttle, rarely when under full throttle. I saw this thread, but it was for Gen 3, and EGR isn’t a factor here.

The engine has always had fresh full synthetic oil (uses 1qt every 3k miles) changed every 5k, and had the PCV valve replaced a year ago. I’ve already run two full fresh tanks of top-tier grade 93 octane and a can of Seafoam, but the pinging remains and is just as bad as on regular 87 octane.

I live in a very hot and humid climate, so pinging isn’t new to me - every single vehicle I’ve owned has had it to at least a minor extent, including a 2019 Accent. But this is the absolute worst I’ve heard. I’ll post up a video if needed to confirm it’s what it sounds like to me. In any case, my dad is a bit hard of hearing, but he has confirmed he’s heard this noise with the engine during the time he’s owned it, which has been a few years now. I’m worried about doing permanent damage, but since it’s been this bad for years now and nothing has come of it so far, I’m wondering if it’s not a big deal? The internet has always made preignition knock out to be a swift and fatal thing, but that’s never been my experience. I’m just afraid here because it literally sounds like a bag of rocks and metal being violently shook.

 

Plugs were changed around 250k with NGK 5464 iridium at the time, wires were not changed. Records show the last owner replaced plugs through a dealership at 175k and 90k back in the day. Unsure of the coils.

That’s exactly what I’ve noticed as well - when the battery is lower and the engine has to do more work, the pinging is louder than the normal engine sound itself. The fuel is from Costco, but I have an Exxon local that gets frequented that I can try for the next couple tanks

 

No CEL codes at present or stored in history. I certainly think it's preignition simply because it sounds identical to what I've experienced with others cars during the summer heat on 87 octane, just amplified and occurring at much lower throttle. Here's the best clip I've managed to get on my dash cam of the sound:

 

Replaced just last year as I understand it. It’s what I thought of as well, but already asked if my dad changed it out while he had it and confirmed it got done sometime a year ago.

 

Alright, ran through three full tanks of Exxon 93 octane, and surprisingly it hasn’t made one iota of difference - knocks just as bad as it did on 87.

Considering everything previously mentioned except spark plug wires have been replaced, is there anywhere else I should look, or could the wires alone be a culprit? Is it safe to continue driving as-is? Again, it’s been doing this for years without getting any worse (or better), what throws me off is just how severe it is and the fact it’s intermittent - you can literally hear the profile change when you go above 75% to compensate for the knock, like it loses some relative power but completely clears up all knock until you drop down in throttle, and only occurs when the engine is fully warmed up on warmish days (didn’t do it during the couple of cold snaps we’ve had as spring has settled in)

 

Oh yeah, this was already mentioned! Nope, not replaced yet, so I’ll make that the next order of business

 

Coils replaced, ran two more tanks of premium gas, and still no change. Is it possible the knock sensor is just not working at all anymore?

 

Banking on high carbon buildup given the mileage. HV battery was replaced new (not refurb) just a couple years ago and pulls good and strong when charged, but I sometimes have to sit parked with the A/C running for a few mins, and taking off with two bars makes the pinging literally sound like rocks and metal shavings in a blender.

I’ll look into what it’ll take to get the carbon addressed - never had it done before and no idea the cost.

 

This was in fact the way! Reviving this simply to say that after going through twelve tanks of 93 octane from Exxon, the pinging has FINALLY abated. The only way you can get it to do it even a tiny bit now is if the battery is at 2 bars and you're trying to pull onto an interstate on an uphill onramp on a hot summer day at full throttle. Otherwise, it's as quiet as it should be. I did try some mid-grade 89 octane on one tank a few weeks back, but it immediately went back to terrible. Seems this Prius is just gonna be a high-octane machine from here on out.