2012 v Spark Plug/Coil Removal

I'm guessing the issue is similar to the regular 3rd gen Prius: you may have to remove the bottom of windshield trim, wipers, wiper motors and the metal cowl (what you refer to as "sheet metal" in your intial post. Yeah, I know, just nuts.

@NutzAboutBolts has a very nice video on this, pinned in the 3rd gen maintenance forum. Hopefully it's similar enough to be helpful. If I didn't lose count, it's the 13th one down, here:

Nutz About Bolts Prius Maintenance Videos | PriusChat

 

Have you cleaned the throttle body and MAF sensor? There are lots of helpful directions and videos available by searching.

 

I made the spark plug socket by removing the rubber insert from a spark plug socket, cutting it in half, and inserting it in the correct-sized deep socket (I think it's an Imperial, non-metric size you need. Weird).

I've heard that the reason for the rubber insert is to keep the ceramic from cracking. Which may be possible, if you're a nincompoop. In my experience, the rubber insert does two things: It holds the plug when you insert it into the spark plug cylinder, so it doesn't fall in, and it holds it when you remove the plug, so you don't need pliers. If the insert holds the plug too tightly, the socket will get stuck in the ignition cool shaft. What you should do is test your spark plug socket by attaching the extension(s) you will use to it, then fully insert the spark plug into the socket. Hold the very end of the extension and pull the plug out of the socket. If the plug comes out, good. But if the extension separates from the socket, the rubber insert is too tight. Don't use it until you fix it by pressing it further in or cutting it down a little.

Are you losing oil? Because a blown head gasket will likely leak oil.

Before looking for the head gasket, there are some steps you can follow:
1) If you or a friend has Techstream, check for diagnostic fault codes. If you have an old Windows laptop, consider buying one of these babies. About $20 on eBay. There are a ton of vendors selling them.
2) On the intake side, Prius has an EGR valve which vents exhaust gas into the intake. It can become clogged. This is a royal pain because the problem can be the EGR valve (expensive), the pipe between the EGR and the intake manifold, or the intake manifold/throttle body itself. Here is a video of the EGR pipe cleaning procedure. It is very simple. If there is clogging in the EGR, then the pipe will probably look pretty bad. Mine just had a light coating of caked-on crud, which I brushed out, but it wasn't slimy nor was the pipe remotely obstructed.

Toyota has a recall on rough startup caused by faulty EGR valve. No mileage limit until next March. warranty enhancement; P0401 EGR Valve | PriusChat Maybe that applies to your car?

 

Is this new? Or did you have signs?

 

Any indy shop that works on Toyota's can do the work. Replacing a head gasket on a Prius isn't particularly different than replacing one on any other Toyota.

 

@OKPriusV, are you out of the powertrain warranty?

FWIW, I've heard head gasket replacement is so costly it's more practical to swap the engine. Also: you're in the States? And maybe id what state?

 

Did you have the new intake installed.? Did you have some minor start up knock. Or the longer knocking.