Master Caution, Overheating, No Cabin Heat, P1116

Hello, I've just joined the forum. My friend has an '08 Prius, and it's had problems since he bought it earlier this year. After driving for about 20-30 minutes, the master caution and overheat warning will come on intermittently. Cabin heat will also stop and blow cold air. He took it to his mechanic, but the guy was unable to figure out the issue, he also had 0 experience with hybrids. The OBDII threw P1116, and we replaced the coolant heat storage tank temperature sensor on our own, though that did nothing. I'm assuming the reader he has is not designed for the car. We did our best to bleed the coolant system, but while going through the process, coolant begins to overflow through our funnel in the radiator cap, and the cabin heat turns off until we restart the car, then everything continues normally. We've tried bleeding it a couple of times, and it seems to get better each time, but I'm not optimistic. He's probably going to take it to the dealer tomorrow. Does anybody know what's going on? Could it just be air in the system, or is something else going on?

 

We tried to diagnose a possible head gasket problem earlier. We do see bubbles in the coolant reservoir coming from the engine inlet, along with exhaust that seems misty. The RPM also seemed to bob slightly. Aside from that, there's no white smoke, and no discolored coolant. We'll try to figure out if the thermostat is the issue, though head gasket seems possible.

 

I have a bit of an update on the situation. We replaced the thermostat and it seems to work as it should. We've tested it, and it seems that once the coolant temperature reaches around 180-190, hot coolant begins to flow out of the engine.

We also bled the system once more, both the CHS tank and heater core. We are fairly certain that the tank was bled fully, but we're unsure about the heater core. We drove around for a little while today and things were working great. We decided to put on the heat for a while to see if that worked, and things started to go wrong.

Once we turned on the heat, we were getting hot air for a while, and the coolant temp briefly dropped from it's range of around 180F-190F to 170F-180F. After a while, it began heating up to the level it was at before. Eventually, the heater began to blow cold unless we were accelerating. While accelerating, the heater started to blow at max heat again, until we let off the gas. At this point, coolant temp would fluctuate from around 180F-200F. We decided to turn off the heater, and parked for a little bit to listen for air. I believe I heard one or two small pops, though I wasn't positive. We idled in a parking lot before returning home, and the temp briefly reached 215F before returning to around 200F.

When we began to drive home, the coolant temp rapidly spiked to around 250F, giving us the master caution and engine overheat warnings. The temperature would then fluctuate quite rapidly from around 230F-270F depending on our acceleration until we parked it back home. The heater never worked during this time.

When we shut the car off, we believe we heard some more bubbling, and the engine coolant reservoir seemed to be near overflowing, with bubbles appear on the passenger side of the reservoir. It also felt like the engine was bucking as we were returning home. There were no check engine lights, and no codes stored on the system that would indicate misfires. The bucking of the engine was the only possible symptom of a head gasket issue that we noticed. The coolant is still the proper color, and the exhaust looks and feels normal.

There have still been no codes since we cleared the original one in this thread's title.

So now, I have a few more questions.
1. Is it possible that the system was just not bled properly, and that bubbles in the heater core caused the problem?
2. Is it also possible for excessive temperature to cause engine bucking through some other mechanism than a head gasket fault?
3. If everything is working properly, what should the coolant temperature look like on the OBDII?

 

Well, I have a bit of an update. So we tried to replace the thermostat, that wasn't the issue. He took it to a few mechanics who tried their own fixes. In the end I believe the 3-way valve and a water pump were replaced, but that didn't fix anything. The head gasket was checked, but was undamaged. We finally found out that there was a crack in the engine block, so the car is likely dead. I'm not sure how nobody noticed that.