Yes carbon buildup (aka. varnish buildup) on the back of the throttle body plate is common in all multi-fuel injection cars - not just toyota. Here are my comments on the safety report information above based on my experiences over the years on my camry and corollas.
The varnish does buildup over thousands of miles and will cause sticking which is mostly felt after starting a cold car and one wants to start driving the car and the car won't move forward since one senses the throttle not moving (sticking). You can also do this in park and sense this when you step on the gas peddle. This might occur intermittently. After the peddle is pushed down to where the plate releases, it will be ok. When the car is hot or at highway speeds, there is no problem. The varnish may be a little soft but regardless of this it will not stick when hot. This is why the dealerships suggest a $300 throttle body clean with BG cleaner. One could remove the throttle body and clean behind this plate, but this is time consuming.
Although the safety guy was looking only for engineering defects (which is most of the document that repeated the same info different ways), he did say on page 7 that cars with Exhaust gas Recirculation (EGR) valves contribute to the varnish buildup. He did mention the gas vapors may also contribute to the buildup once the gas condenses on the plates. However, this is insignificant considering that gas today contains detergents and has a very low varnish content (which is the other petroleum product residues in the gas - they can't get it all refined out to zero). What the reviewer did forget about is the Pollution Control Valve (PCV) since vapors from the valve cover area is also recircualted back through the throttle body.
Thus, the varnish buildup contributing to sticking comes from multiple sources,
a). PCV valve gas recirculation
b). EGR valve gas recirculation
c). the gas itself.
Moreover, the varnish will also buildup in the idle bypass hole which restricts the amount of air coming into the throttle body when in parkor one is idling at a stop sign. This plugged up hole causes rough idle or stalling. If one steps on the gas pedal the throttle plate opens up letting more air in and the stalling/roughness goes away. Thus as part of the throttle body/plate cleaning to eliminate the sticking, the air bypass holes are cleaned with carb cleaner to get the cars performance back at idle.
The rate of varnish buildup varies on different cars and engine sizes. I haven't experienced the sticking on the 92 corolla, but mostly on the 90 V6 camry. i've seen alot of buildup on a 96 4cylinder camry. From what I can determine, the rate of buildup is do to the palcement of the EGR and/or PCV valve entry point into the throttle body itself. The buildup seems to be faster when either of the valves enter the throttle body right behind the plate itself vs. somewhere in the middle of the throttle body between the engine and the throttle plate. Hence, the various gases from the EGR and the PCV (which contains fuel vapors mixed with vaporized engine oil due to heating) are in high concentration if placed right behind the plate and thus condense on the plate faster. If these are placed further down the throttle body tube, then they have a greater chance of being sucked into the engine and burned before they can cool down and condense on the throttle plate.
In a 96 camry, the throttle plate getting stuck at highway speeds occurred once for me while I and a friend were trying to get his daughters car working again. We had disassmebled, cleaned and put the throttle body back together. We also replace the PCV and hose too. During high acceleration when I tramped on the gas to pass a car, the RPMS got stuck at 5-6K. I put the car in neutral and when stopped on the side of the road I looked at the throttle body to see what was happening. I saw that the throttle itself was stuck open. The tab that rests against the throttle stop screw was hung up on the PCV hose. Releasing this returned the idle back to normal. The reason this occured is that the new PCV hose was cut from fuel hose, but it didn't have the "S" shaped curve like the factory one did. Thus, when the throttle opened, the tab interferred with and got stuck on the hose. I then replaced the PCV hose with an S shaped on from the junk yard and the problem went away. Toyota could of designed the PCV layout such that the hose was furthter away from the throttle plate itself such that this sticking wouldn't occur. Perhaps they will in future models.
For the 90 camry, the idle could stick a little higher due to the cruise control wire that pulls on the throttle plate being slightly misrouted. That is, the cable is anchored on a hangar that holds the cable such that it routes from the cruise control to the throttle plate a certain way. If the cable is moved a little to the left or right of this hanger point, it will put tension on the cable and open the throttle a little buit. Adjusting the position on the hangar eliminates this problem.
