engine hesitates/misfire/shakes when gas engine comes one

What do they call it; the seven stages of grieving?

Engine randomly starting after EGR replace | PriusChat

 

High quality dual view borescope is the only definitive test for hg leaks on gen3s. These leaks seal up in seconds and therefore don’t present combustion gasses in the coolant or obvious coolant level changes.

If it were plugs, coils or injectors the problem would not go away quickly or be sporadic. They would present on one cylinder and move if you moved the plugs, coils or injectors.

Egrs sticking open at startup usually cause stumbling but not bad enough to lose sync with the transaxle. A 10 minute free test is to block the egr valve and intake pipe temporarily to exclude the egr.

Many shops have seen hundreds of hg leaks with exactly your symptoms. Borescope.

 

When I first got my 2011, it started having those types of issues. It also threw the codes for one of the O2 sensors, which had a pretty chewed up cable. It also had only about 30K miles, and replacing the O2 sensor fixed my issues. I doubt that's your problem though.

With your miles, it's almost certainly a head gasket. Have someone else start it, and watch the exhaust. Do you see a bit of white smoke (steam)? Do you smell coolant (sweet smell)? The exhaust gas in the coolant blue/yellow test only works after it gets pretty severe. Yeah, if it turns yellow, that's definitely the problem, but if it stays blue, a head gasket can still be the problem.

 

The borescope will show a piston much cleaner than the rest. High mile pistons will have a black coat of carbon which is normal. A piston where you can see the metal in places or throughout has been steam cleaned by coolant. By the time you hear it when switching from electric to gas after warmup you are dealing with an advanced case.


Shudder video

“Hey Walt… they want to wait and see if it gets worse…”

Car Care Nut HG Borescope at 7:40

 

Those test don't work on a Prius.
Using a bore scop, and putting pressure on the cooling system so you can see
the coolant leaking past the damaged head gasket.
And seeing clean piston tops from the steam cleaning....