Mass air flow troubleshooting

Your egr circuit needs to be cleaned

 

This is like one of those freeway pileups that never ends.

Toyota should man up...

 

Cleaning intake manifold and egr cooler

 

I would love to see the reasoning behind tankyuong's pronouncement.

 

170k miles?

 

The simplest way to see where you're at, is to check the degree of carbon build up in the EGR pipe, a stainless steel connecting pipe between the EGR valve and intake manifold. Watch @NutzAboutBolts video #16 here:

Nutz About Bolts Prius Maintenance Videos | PriusChat

Two or three other videos linked there too, for the full cleaning of the intake manifold, full EGR clean, and Oil Catch Can install.

Good thread:

EGR & Intake Manifold Clean Results | PriusChat

Another:

Oil Catch Can, Eliminate that knock! | PriusChat

Somewhat tools worth having:

E8 Torx socket (mandatory)
E6 Torx socket (optional, but good to have, to remove the throttle body studs from intake manifold)
3/8" ratchet wrench, regular and long handle, flex head, you can never have enough
1/4" ratchet wrench, or 3/8" to 1/4" reducer
Ratchet extensions: you can never have enough
Long needle nose piers, straight and bent tip
Ratcheting 12mm box wrench (optional, but makes disconnection of the EGR cooler from exhaust easier)
Torque wrench (3/8" and 1/4" both good to have)
Floor jack and safety stands (or ramps): basically some method to raise front, if you need to take underpanel off, which you may need to, both for access and to recover dropped items.

 

Maybe this:

 

You said you unplugged the MAF sensor and everything ran great? Have you considered changing the MAF sensor since I didn't see you mentioning a bit about changing that part?

Edit: I just noticed you mentioned a bit about that part. If you said that when you disconnected the sensor it ran great and you changed the sensor and everything was bad again, have you considered checking the harness? It might be the harness that has something wrong since we might be able to be certain that it has something to do with the sensor. Since the sensor is brand new and the problem is still there, might as well check the harness.

 

More likely what's happening is that without the MAF sensor the engine only runs in "open loop" mode. And the OP's problem is only occuring with "closed loop" mode. In open loop (used when the Prius is warming up), the air/fuel ratio isn't monitored, and the PCM uses some default guesses for how much fuel to mix in. Once the coolant and air/fuel ratio sensor are warmed up, the PCM uses the air/fuel sensor as feedback to precisely control the ratio. (In order to know how much fuel to mix with the air, it needs to know how much air, and hence MAF is required.)

One of the things that happens in closed loop is the use of EGR, and in practice it tends to kick in right around 25mph. (It's dependent on engine load, not the vehicle speed.) That's not a slam dunk diagnosis, but at OP's mileage there's zero doubt EGR cleaning was necessary anyway.

 

I don't know if you're still having this issue or not, however, I just had the same issue with my 2013 Prius and there appears to be a small 2 hose valve behind the Air Filter box and mine was, some how, disconnected. I plugged my hose back up and added small hose clamps to prevent another "episode", as it were and my car runs like a champ. The hose in question, for me, looked to be a vacuum hose that ran to the throttle body and it was pulling in un-metered air which caused us to run too lean and was running like pure shit.