*Knocking/rattling sound after cold startup*

To see where you're at with the head gasket, I'd have a leak-down test done ASAP, and maybe boroscope inspect the cylinders.

At 200K miles, your EGR components, and the EGR passages through the intake manifold, are likely near-solidly blocked with carbon. 100K miles is about the longest you want to go without cleaning.

At this point, with the knocking and misfire codes, the head gasket is likely failing though: do the tests first, see where your at. Cleaning EGR is good, but won't reverse the damage due to longterm neglect of the EGR condition.

How's the engine coolant level in the reservoir?

See links in my signature for head gasket repair and EGR cleaning.

 

The car has been doing this for 50,000 miles and no concerns? "It seems to be a common issue with the Prius, but doesn’t appear to be causing anyone any serious issues."

Actually you are likely to be extremely lucky because the vast majority of people with this classic head gasket symptom progress to an expensive repair or an entire replacement engine much sooner. Some end up blowing a rod through the side of the engine because of hydrolock. I have to assume you do a lot of driving per day, which would hide the problem longer.

Never had to add any coolant?

 

☝️ This has happened to me twice now. Moving the car from the street to the driveway...15sec runtime, will cause a knock on the next start. Never happens otherwise.

 

Get rid of it.........! Blown Head Gasket...... just the beginning of the mental and financial pain.

 

Thanks for mentioning this, I hadn't put this together, but I think I might've been experiencing similar for this reason. I just had the egr replaced anyway bc these threads have me worrying about it half the time, there was a considerable gap in my car's service history and I just haven't had the time to clean it myself the last three months that I've wanted to. We shuffle the cars in the driveway regularly, so I'll let it run a little extra next time

 

You have roughly 15 seconds after “making the car ready” before the engine kicks in. Sometimes that’s sufficient to do the driveway shuffle, without engine start up. Too bad it’s not more, say 60 seconds.

and yeah, if you mistime it and engine does kick in, let it run till shut down. Frustrating though.

I have one more trick: if I know I’ll need to do a shuffle, I’ll plug in the block heater on ours for at least an hour. It spoofs the car into thinking it’s semi warmed up, allows EV mode.

 

It appears now that I actually have a bad head gasket. My shaking has gotten worse and happens not only at a cold start now, but when accelerating from any stop or slow speeds. Recently, the temperature light has been coming on momentarily along with the check engine light. They both clear when driving at higher speeds for long distances, but my coolant level keeps going down rapidly and appears to be going into the oil as evidenced from the dip stick. I’m grateful that it lasted me 213,000 miles, but I think it’s time to trade it in. I had expected the high voltage batteries to go first, but apparently the Gen 3 batteries last much longer than the Gen 2s did. I think it was always a head gasket leak due to normal expansion and contraction over time. It started rattling and shaking only on cold starts at just over 200k miles, and continued to get worse as suggested by @rjparker in one of his earlier threads. Thanks to all those who have contributed to this discussion, especially Mr. Parker and Mr. Leisk.

 

Don't have any solutions, but my 2011 Prius does this extreme scary shaking on first start-up after leaving it for an extended time. And then it smooths right out. Always scary though. Only thing is have noticed a ping now (94,000 miles only) until the car warms up in general driving. Not sure if it could be an ear issue or what....