mystery 2015 Prius with 16,000 miles

Just bought a 2015 Prius with 16,000 miles. No zeros missing there. Got the carfax and the numbers look right. Car was sold new Nov 2015, driven for about 18 months and accident in front end at 15,400 miles. Repaired and sold about 3 months later by known shop in Fresno, Ca. New (clueless) owner drove it about 1000 miles, then left country for two years. When came back experienced knocking engine noise at idle and low rpm and 'triangle of death' from P0A80 code. Sold it to me as a 'mechanics special' for very good price. Even if I have to replace engine and battery I will come out ahead. First bought new 12v battery to make sure it was not causing codes. Then checked hybrid battery and found #11 pair of modules were down by over a volt, so I will replace those later/recalibrate or get new battery.

Now to the engine knocking:
-consistent P0304 code and obvious misfire. Occasional P0303 code.
-P261B code - water pump.
-my Priuschat reading says that possible issues for P0304: bad coil pack, bad spark plug, plugged EGR, bad fuel injectors, bad cylinder head gasket.
-changed #2 and #4 coil, no difference in P0304 code; ditto with spark plug
-bought and installed new fuel injectors with hope that old fuel had clogged up system (first added seafoam and put in new gas in tank).
-put a metal plate/shim at junction between EGR cooler and EGR tube running to intake manifold to block air from EGR system; my assumption is that if there is no air going to intake through EGR the P0304 code will stop because misfire caused by EGR system will stop. Result: No difference. Also, if engine has only 16K seems very strange that EGR system would already be blocked up.
-replaced the water pump with new OEM pump. The old pump was definitely seized. Perhaps from accident?? very strange. P0261B Code went away with new pump installed.
-did a simple compression test. 115-120 on all four cylinders cold engine, no oil added (seemed to be normal spec from reading around on Prius chat); initial reading on #4 cylinder a bit lower but after a few cranks very similar to other cylinders
-did leakdown compression test. Some slight leak-down/pressure difference on #4 compared to other cylinders but my brother, who is aircraft mechanic, did not seem to think it was that significant a difference
-used my brother's professional boroscope (GREAT!) to check inside cylinders. #1, 2, 3 all look normal and quite new. Cylinder walls still have the lightly visible hash marks from the cylinder boring process. When we flipped the camera up 180 degrees we could see the bottom of the head and the edge of the gasket - all looked very clean on # 1,2,3.
-on #4 there was very light trace of what appeared to be leak coming down cylinder wall. It looked like a shadow. Looking up at gasket there was tiny trace of oil/residue different than the other cylinders.

-my assumption is that I have eliminated all the other likely possibilities and that the likelihood is a head gasket needed because of a water leak into #4 from a water passage, maybe occasionally leaking over into #3
-my questions: The thing has 16,000 miles - What the heck!? Of course it has been in an accident and the water pump was bad, maybe from the accident. The new owner (after the accident and repair) seems clueless so he could have driven it for several hundred miles with bad water pump and it caused the head gasket failure. To show his level of car knowledge, carfax indicated that he went to a garage to have sparkplugs changed (I assume he hoped this was problem of engine knocking). Indeed, there is one new/different sparkplug on #4. The owner claimed he did not know why engine was knocking but obviously he had tried to do some simple fixes. My guess is that he was quoted $$$$ for a cylinder head repair from the dealer and just decided to dump the car.

-my worry: scammers took a good, new engine out and sold it and put in a bad engine. Everything about the engine appears to be new, however, especially boroscope check and general bolts in place/torqued down/no bolts missing. Usually scam artists leave behind obvious clues (ask me how I know from a 1997 VW Passat TDI nightmare : < ). Everything looks very good about the engine other than the bad knocking. If I accelerate/higher rpms the car runs great. But If it turns out the pistons/rods or block is bad I will simply pull it out and replace with Japanese imported one.

What am I missing? I'm ready to dive in and do the cylinder head gasket. After pulling the head I'll have my machine shop check it out. Doesn't look too difficult a job (at least compared to my other current project, which is rebuilding a VW 1.9 ALH TDI engine and sticking it into a 1997 VW Eurovan camper, and swapping in a 5 speed manual). I LOVE working on Toyotas compared to VW's! Anyway, I digress.

What am I missing here?

Thanks for any clues you can give me....
Ken

 

yes, because it has a salvage title Toyota won't honor the warranty. Once I sort out the engine issues I'll dive in the back and check to see what is going on with the battery. It could be as simple as the fact that the car sat for over two years without being started or it could be that the guys at the shop swapped out the new battery for an old one with the hopes of making some money.

 

Thanks for the reply. I've had to suspend trying to solve my mystery for a few weeks while I finish up my semester of teaching. The body shop did the work over three years ago and just said it came in with a front end collision, which is what was indicated on the carfax report. It is true that they could have swapped in another engine while they had it in, but that would be a heck of a lot of work for the money they would get for it. Also, given that there seem to be no missing or loose bolts from the work that I have done inspecting it, it seems unlikely - though certainly possible. Re the possibility of it having 316K instead of 16K, and odometer tampered, anything is possible. But I have bought a few cars over the years and there are ways to generally tell if a car's odometer has been tampered/swapped or it has been in a flood. Carpet condition, wear on the brake/gas pedal, faded paint, oil leaks, etc.. usually help with figuring out age, and there doesn't seem to be any of that. Carfax can be cheated, but any kind of transfer/title report should have been in there and it is pretty rare for someone to keep a car for 300K. In any case, I'm almost finished with my semester of teaching and will hopefully have a chance soon to get into the car and figure things out...