engine tapping, low mpg

Good idea on the mechanic. I would suspect the noise is from the valve train. A mechanic's stethoscope might help track it down.

Here are a couple links on the P1151 code:
P1151 2008 Toyota Prius Coolant Heat Storage Tank
P1151 What if I don't fix this? | PriusChat

 

You have this posted in 2 different forums.

I suspect your hybrid battery is failing.

Your engine is tapping because it has a big load on it as its now powering the car with very little help from the hybrid battery. Engine was not really designed to solely power the car. When the hybrid battery fails people still demand the same "pep" and power it used to have so they push the car harder to produce that. The engine complains....very loudly. Instead of taking it as easy as you can because the electric motors are no longer contributing to the power.

If your battery tanks to purple bars in a drive thru and takes forever to get 2 blue bars in it it's getting very tired. First hint is mileage tanks.
Because the engine is basically doing all the work. I assume its the original battery?

Failed Hybrid battery is the #1 post in the G2 forum right behind the mystery's of the 12 volt battery.

 

This statement doesn't pass my sniff tester. With a good battery, the ICE runs at high load (torque) at as low an RPM as possible already, with the battery adding additional power only after the ICE has already reached it's maximum output. A bad battery might cause higher RPMs (due to a futile attempt to charge it) and you won't have the extra boost at full power, but it can't increase the engine load beyond the normal maximum.

Also, a properly functioning engine working at full load should not be making tapping noises! If the noise really is coming from under the valve cover, it could be slop in the valve train which means the corresponding valves aren't opening fully thus you get tapping and reduced engine power.

 

If theirs no dash light alerting you too a malfunction in the engine then its strictly mechanical and the engine monitoring systems do not notice any out of bounds condition.

The valve train is pretty tough mechanically and not alot of posts about issues with it. Lots of complaints about valve noise because the valve train is so noisy as it doesn't use hydraulic lifters. The engine valve noise always annoyed me since i bought it new so I use a higher quality engine oil that has a ZDDP package that has reduced the noise considerably. Its 5-30 that at operating temperature is essentially a 40 weight oil.

Which leads me to ask how have you treated the engine? Do you push it hard all the time? And how have you been on the oil? Hope your not running 0-20 weight.

At 190K miles its probably eating oil and have you ever ran the engine so low on oil it turned on the red exclamation point on the dash?

If the car lives on dino oil you may want to inspect the VVT oil filter for sludge. Especially if it gets the oil changed at the dealer.

I wouldn't waste my time and money at the dealer. They will want to do a valve lifter (bucket) inspection and will rape you. The $$ quote they will give you to repair it will shock you. Its a big job the camshafts have to come out to get to the buckets. Why don't you make a video of it. Put the car in inspection mode and make a video.

And after you go through all of that you are still looking at a 12 year old hybrid battery.

 

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LOL.

When a G2 hybrid battery is depleted to say even 1 or 2 purple bars or no bars at all it is so anemic you almost have to floor it to get going in traffic. And when you do the engine screams notably I can hear the valve train complaining the most.

The hybrid battery contributes power across the entire power band. It does not come on when the engine reaches its maximum output. The battery is constantly supplying power and being constantly charged. You can clearly see that on the MFD battery arrows.

The car is pretty worthless without a healthy battery.

 

I think your post above about the valve train issues is spot on, and will stop nit-picking your use of the word "load"

At two bars, the G3 ICE also revs a lot, and my scan gauge shows that it's putting a significant amount of power back into the battery (20-ish amps, ~4 kW). Add in the smaller engine and solid lifters in the G2, and I can see where you get a lot noises that might be described as tapping. Once the battery is back up to 4 bars, the scan gauge generally shows <10 amps in or out unless you floor it or are in stealth mode.