After years of service, gunk can accumulate on the throttle plate and airway and cause the engine to fail to start. In my case, it would frequently happen in the morning with temperatures around 30F (0C) and high humidity from rain or snow. But when I took my car in for an ECU upgrade that was covered under a Toyota TSB, the service writer gave me a can of throttle cleaner and it worked. This page describes what it takes and a link to the more through Good Prius Friend Hobbit's throttle body refurbishment page.

Test Throttle Plate Sticking

There is a fairly strong spring easily seen under the air cleaner between the engine and a heater hose:

A sticky throttle plate should be tested in the coolest part of the day before the engine is first started, the early morning.

You can check for a stuck throttle by rotating the spring, in this case I'm using a finger to pull the bottom of the spring towards the front. It is perfectly safe to get a good hold of the spring to try and rotate it:

If the throttle plate is sticking, it will be very difficult, even impossible to rotate the throttle plate and it needs to be cleaned. You'll notice I have my block and transaxle pan heater plug in that area. Also, my car has an experimental breather hose that leads to the transaxle to reduce dirt and grit in my transaxle oil.

This is a good time to open the two front clips and one side clip holding the air filter cover and inspect the filter. If it is full of pollen and dirt, either clean it with a vacuum hose or plan to replace it.

Getting Ready

You will need a long handled brush and a catalytic converter safe, throttle cleaner. The long handled brushes can be found in many automotive parts stores. My local Toyota shop sells a kit that includes the throttle cleaner, pour-in tank injector cleaner, and a vacuum hose cleaner:

Just pour the injector cleaner in the tank and you're done with that part.

This photo shows the part number and a close up of the brushes:

Cleaning The Throttle

First open the air cleaner and remove the air filter:

There is an airflow sensor at the front and I try to avoid spraying it with cleaner.

If you can open the throttle, look down to see if there is oil in the manifold indicating the engine oil level has been too high:

Do not worry about removing the oil, just take a note to make sure that in the future the oil changes leave it below the "F" mark. This can be accomplished by using just 3.5 quarts of oil instead of four. If you have someone else change the oil, buy four quarts and ask them to use your oil and return three empties and one half full.

Especially if the throttle plate is stuck, spray away:

My throttle plate would not open so I put the nozzel tube beyond the air flow sensor and sprayed about 1/3d of the cleaner. Having nothing to lose, I started the ICE and the powerful throttle motor opened it up. After ICE shutdown, I could rotate the throttle spring and finish cleaning the throttle area without spraying the air flow sensor.

A long handled, narrow brush like this one is perfect for cleaning the plate and area where the shaft meets the airway:

Be sure to rock the throttle plate open and closed while spraying with cleaner. The goal is to get all of the sticky gunk out without risking the airflow sensor.

Follow Up

I prefer to clean the throttle once a year, in the fall, when I change the oil and use the fuel injector cleaner. I also change the air filter, a quick and easy way to eliminate the summer pollen and dust from the air filter. However, it would probably work just as well to do this every other year since my 2003 Prius didn't have the problem until 2006. Note that the Prius throttle body has a number of parts not touched by solvent and brush. Good Prius Friend Hobbit has an excellent page addressing how to remove, disassemble, and clean all parts.