I've noticed, maybe a handful of times in the 5k miles I've owned the car, that the engine will rattle excessively for maybe 30 seconds or so on a drive. This only happens during high load/low speeds and I think after the engine has been off for at least about 10-15 minutes. It doesn't happen on start up, only after the car is warm, but hasn't been using the engine for a bit. I'm thinking this is fuel enrichment (really rough running) because the cat has cooled off, but I figured I'd post about it and get other member's insights.
It's right at the full demarcation on the dipstick, although it happened before my last oil change when the car ended up at about half that by the time I changed the oil at ~5k miles. IIRC I've noticed it only 3 or 4 times.
The last thing this car has on its mind is fuel enrichment. Really there is nothing fuel adjusted to warm the cat. That would be very hard on the cat. if the car detects the cat is cold it will start and run the engine like it does first thing in the morning. And you cannot ev enough during your day that the cart will get cool. So forget that. You have the beginnings of a rough running engine. That is caused by a few things all of them maintenance or mileage related neither of which you mentioned in your post: Dirty MAF Dirty throttle body (really common) Needs spark plugs Or water has gotten into a sparkplug well and shorting out ignitor. Pretty common. Usually caused by either a really hard rain washing over dam or the seal between the hood & dam has rotted away and water spilling over dam onto engine hits just perfectly on top of ignitor. Usually cyl #4 last one in the back. Have to pull plugs to check for rust in well and on sparkplug and a corroded ignitor connection where it connects to plug. Check condition of water dam seal and if its still glued down.
It's not related to ignition, and I've cleaned the MAF/TB. I think I've finally isolated it. Initially, I thought it was related to cold starts, but it only seems to happen to MG1 charging the battery when the car is at a stop or almost at a stop. It almost sounds like someone tied a can to the back of their pickup and it's rattling on the ground, except the noise is at a lower frequency.
~152k. Maintenance records show the ATF was swapped a little after 100k at the stealership IIRC. They could have underfilled it too, which would definitely be a pain. I'll check the fill level and grab some WS if need be, then change it next weekend. Just to put this in context, this is the last funny noise I noticed after the AC compressor, coolant pump, brake pump, inverter PWM, and it seems to be confined to high load/low speed engine operation with MG1 charging, so I'm still wondering if something is loose someplace in the engine bay.
