A friend had a supposedly failed throttle body in an '02 Prius, replaced out of warranty so he got to keep the old one. We gave it a quick eyeball and nothing looked amiss, so he loaned it to me to evaluate in more depth and also take to one of the tech-training seminars. It's a good opportunity for another photo essay on Prius parts. So here's the item in question, from the top. We see the two PCV fittings facing upward, the vacuum tit for EVAP to the [car's] rear, one of the small elbows for the coolant path, and the return-spring assembly. Interestingly, one PCV enters above the butterfly, and one enters below.
The underside [ignore the roll of tape; it's just jamming the butterfly open for illustration]. Both coolant-loop elbows are now visible, along with the position sensor. The lower part of the throat is a little bit crapped up, but nothing really major. Still, we're quite aware that a dirty throttle body can cause Prius no-starts, especially in Classics. But we'll look for a more obvious failure first.
The position sensor potentiometer comes off with two screws through slotted holes that allow for angle tweaking. The pot has its own return spring; its little ears ride firmly on the metal pins of the shaft bracket for a zero-backlash coupling.
The motor housing is next, revealing the drive motor itself and the geartrain. Nothing special about this, because all the motor really does is hold the flap open against the return springs based entirely on position-sensor input. If needed, the motor power can be reversed to help the butterfly close quickly, but without any drive the assembly doesn't waste any time springing closed by itself anyways. [This is clear from the way the motor actively fights you in both directions if you power up and then try to gently move the throttle by hand, so the ECM must have a full H-bridge inside.]
The geartrain basically just falls out. There isn't a whole lot of grease on the gears or pivots [there was a bit more on mine when I cleaned it at ~ 45K miles]; the idler post is actually pretty dry. But not such that it appears to restrict motion at all. The motor clips into a little plastic terminal assembly when installed. It really looks like all of this was designed to be serviceable, such as replacing the drive motor or a gear, but Toyota's service manuals don't acknowledge that at all. The '04+ manual even removes the section on TPS adjustment that the '01 manual has, even though it's the exact same thing -- I guess in '04, you just throw the entire $700-plus thing out. That's idiotic, but nonetheless apparently what's happened here.
There is an adjustable hard-stop at the fully closed position, presumably to prevent the butterfly edge from actually contacting the throat at almost 90 degrees and jamming. But the flap gap as set here is quite thin, implying that gunk stuck on the flap edge could still cause some adhesion. I could feel just a little bit of that when fully closing against this stop, so that's something to suspect as a problem cause. The dual return spring assembly on the other end of the shaft causes it to default to this slightly-open "limp" position.
Nonetheless, thorough testing is needed. The prime candidate is a dirty TPS pot, followed by a failed or flakey motor. Both can be "scratch tested", by sending a small DC current through them and using a speaker to detect any abrupt changes in current. I use this for testing audio and lighting cables, which are notorious for going bad in hard-to-diagnose ways; running this through all three pins of an XLR in series and then wiggling the cable and ends finds that sort of thing really fast.
Testing the pot. Both wipers, from both ends. Sounds absolutely clean; no dropouts and just a very gentle sighing as the resistance reaches the lowest value and lets the highest current through. [That test was kept brief, to avoid heating the pot element...]
Same test on the motor. Apply tester, turn gear slowly and listen for anything indicating bad brushes or commutator. But it too is almost completely quiet, and upon application of a little more current it pulls torque very evenly in either direction. So the motor is fine. It seems to be about 4 ohms at stall.
Desperation time. Plug the whole harness piece in with the motor and scratch-test while wiggling that mess around, to see if the supply wires or connector are bad. Nothing amiss. So there's not a damn thing wrong with this, in my estimation. Toyota has sold this poor fellow a bill of goods. Or at the very least, an implicit refusal to clean the throttle body in response to a well known common- pattern problem. Hopefully the problem is not in his engine or ECM harness and temporarily "fixed" due to movement, but with the potential to recur. Ask me, the real problem is in Torrance CA. Brilliant engineering on the other side of the pond, abysmal support on this side.
Besides the slight butterfly stickiness at full close, I spot this inside the motor housing. I doubt that the butterfly in question emerged from here, however, sticky or not. Actually I can't imagine how a bug got in there at all, since the cover fits pretty closely, but this is nowhere near the mechanism and is just a mild curiosity and fodder for semi-bad puns.
One can effectively do this same teardown to the geartrain for service
without completely removing the throttle body from the car which would require
opening the coolant lines. See this article for details.