Random Misfire - I'm stumped??!

Do a leak down test. Most likely, you have a failed head gasket. Symptoms and mileage are consistent with the failures reported.

 

Sounds like a HG. Just passing a compression test does not mean the HG is OK.

You can do a "block test" for combustion gases in the engine coolant reservoir.

If there is a sealing failure between the cylinders (one or more) and the coolant passages in the head/block then cooling system pressure may force some coolant into the cylinder(s), leading to misfire. If you fill the reservoir, and notice it overflowing, that would be due to the combustion gases increasing pressure in the cooling system.

If you notice the problem when cold, it could be due to the residual cooling system pressure after a drive gradually causing coolant to leak into the cylinder. After some driving, the coolant is expelled and the cylinder warms up to more easily burn the coolant, and warming up may help the failing seal do its job better.

Another test would be to carefully check for coolant in the cylinders. Fill the reservoir, drive the car until fully warm. Then park it. Next day, without running the engine, probe in the spark plug holes to see signs of coolant. Paper towel on a stick, endoscope will do it. Blasting compressed air in and seeing what comes out (visually or blocking with a paper towel), or cranking the engine with the plugs removed and seeing if coolant droplets spray out, will all do it.

If that last test does not show coolant, and you want to try harder, you could pressurize the cooling system with the engine cold, and hold that pressure for a couple of hours, then test for coolant seepage as described.

EDIT - And a leakdown test as mentioned by @The Critic would be very quick and definitive if there is a good sized leak for all the different options of HG leaks. If the leak is from the cylinder to the cooling system, and the leak is big enough, the leakdown test would show bubbles coming through to the coolant reservoir. The testing I describe would be aimed at a cylinder-to-cooling system leak only, and would be more likely than a leakdown to find a marginal leak.

And also, when "HG" is said, of course that is the most common head failure on these cars, but it could also be a cracked head, valve issue, etc. Hopefully a failed test would lead to head removal at which time exact diagnosis would be easy.

 

please keep us posted, all the best!

 

Unfortunately these things are becoming famous for their leaky headgaskets. That’s most likely your unfortunate reality it’s honestly a damn shame on Toyota’s part

 

@Jozplatte (and all): I haven't read through this line-by-line, but don't think you've posted your miles (odometer)?

 

2.2 million miles on original head gasket, one hell of a run.

 

So many try replacement coils and plugs, I can't recall it ever solving the problem. Without fail it's the head gasket.

 

Well, "without fail" might be a little strong, considering most of the causes of misfires are common across gasoline engines, and we certainly have threads here about those things causing misfires in Prii, and even that old AECS article about the Gen 1 that the owner let go for scrap because of what turned out to be a spark plug. (The title of astrolink's thread was something like "I think I'm going to sell" and clean injectors fixed that one.)

I suspect part of what goes on is that a lot of people who experience misfires knuckle down and diagnose the cause and fix it and then don't have misfires, and then they don't feel compelled to post on PriusChat about it because they figure they only did what anybody would have done. So maybe more of the posts that end up here will turn out to be HGs rather than any of the usual simple causes.

Still a shame to talk anybody out of checking the simple causes in case they haven't yet, on the spectre of it being a HG.

 

New injectors are nice on a old Prius, I put new ones in my old 2004 and it seems to run a good bit better.

My experience with a Prius HG was my sisters 2010 a few years ago. We tried everything before resigning to the HG issue at 190k. We ended up replacing the engine ourselves because it was also drinking oil like mad.

We replaced it with a 2013 Prius plug-In engine with only 40k. Everything worked out, worked fine for about 2 years. Well about 60k later it’s burning oil and got a leaky headgasket. I was shocked! we ended up putting one of those shitty headgasket sealers in. For the last 20k it’s been fine. It was mixing the oil and coolant pretty bad. The leak is mostly fixed now.


At this point we are waiting for it to die. It’s still on its OG battery with over 275k now, so who knows what will happen next.

it does make us like the old gen2 even more! They never had such issues. Our last 2005 with over 340k until it was totaled.


Anyway.. keep us updated and I hope everything goes well!

 

That and two coolant quarts low was pretty definitive. Glad you got off cheap and it has lasted a year. I like a borescope inspection to avoid excessive parts cannon activity.