Originally Posted by y=mx+b

At 231k, I've got my first real signs of a HG leak today

Installed an UltraGauge about 500 miles ago to help keep an eye on trans temp and coolant temp and such. It randomly got hot in traffic about 2 months ago, and cleaning all the mud and dirt out of the radiator seemed to fix in. Then two weekends ago (~550 miles give or take) coolant and trans temps quickly spiked up to 230 as I was pulling into my house after a 220 mile trouble free trip. Thats what prompted purchasing the UG for monitoring.

Fast forward to yesterday, the UG alerts me of a pending (but not yet faulted) code P0304 for Cylinder 4 misfire. I didn't notice a miss, so it must have been quick on startup or something. Of course, 4 is one of the cylinders known for HG leak, so I'm a little skeptical

The 4runner then sat from around 1pm yesterday to 4pm today, since I was off today. Before starting, I took a peek at the coolant level and it looked fine. I popped the rad cap for fun, been sitting now for ~27hrs, and theres enough pressure to make a good hiss, and squirt a little coolant from under the cap. Definitely weird it help pressure for this long, makes me think of leak only when up to temp puts combustion gas in cooling system, then when cools down the leak closes up. So then on startup, it misfired pretty bad for about 2-3 seconds and then smoothed out. No CEL trip or fault codes, still just the pending CYL 4 miss.

So that sucks. I'm going to keep it and fix it eventually, but kinda in a pickle with school about to take up a lot of my time.