Good news: The throttle position sensor is adjustable, and one of the few components in modern engines that is. Which is good because of the issues created by a worn or malfunctioning TPS are debilitating. What does it take? Not a whole lot, actually.
It is important to get to know the function and servicing of this sensor for a couple of reasons. As with any electro-mechanical moving part, they have been known to malfunction or ultimately just plain wear out. Besides that, they are often victims of improper service procedures. These procedures will either result in damage to the sensor, or impair its range of operation. In any event, a malfunctioning, damaged, or misadjusted TPS will cause a variety of driveablility symptoms, often accompanied by a check engine MIL (malfunction indicator light) displayed on the instrument panel.
The TPS does just what its name implies; it provides input to the engine management computer regarding the position of the throttle. How the computer processes that data depends on other prevailing conditions: engine speed, load, vehicle speed, engine and ambient temperatures, and so on. This information from the TPS is especially critical for proper startup and idle, as well as smooth throttle response.
There are a few different TPS design types: switch, potentiometer and combination. The switch type, not surprisingly referred to as the throttle position switch, cycles on and off through its range. It is usually "on" (has electrical continuity) at throttle close, or idle. Just above idle it is "off" (loses continuity), and stays that way until wide open throttle is reached, where it is then "on" again. While operating in the middle area, where the switch is off, other sensor inputs provide the computer with the data it needs to maintain smooth throttle response and good driveability. This type of sensor is usually adjustable.
The potentiometer type of TPS is a variable resistor, effectively supplying a gradually increasing supply of voltage to the computer as the throttle is gradually opened. Usually the voltage operating range is from about one-half a volt at idle, to five volts at wide-open throttle. These sensors are usually not adjustable.
The combination type employs elements of the switch and "pot" type of sensor. Early designs were often adjustable, later designs are not.
Stop and Adjust.
You may already be imagining what kinds of failure and maladjustment scenarios are possible with the different types of sensors. We'll get there momentarily, but no discussion of proper diagnosis and adjustment of the TPS would be right without first addressing the throttle stop, and throttle cable adjustments.
The reason that these adjustments are so important is that if they are incorrect, that is they are holding the throttle plate open beyond specification, the TPS will not be able to give an idle (throttle closed) signal to the computer. Driveability symptoms that result include poor startup, engine stall after startup, poor idle, engine stall at idle, poor throttle response at throttle "tip-in" or possibly even engine "ping." Besides those things, you may also discover that you are unable to access the vehicle on-board diagnostic tests, and/or unable to properly set the ignition timing!
While it may be true that adjusting the TPS may correct some of these symptoms caused by a maladjusted throttle cable or stop, the throttle plate would still be admitting more than its share of air at throttle close. This will cause the idle air control valve to operate out-of-parameter, which will lead to similar, if not new and unusual problems. If the TPS is not adjustable, the throttle cable and stop adjustments are even more critical.
Rules of Thumb.
It's best to consult the service manual for specifics on any adjustments that we are about to discuss, but here are a couple of rules of thumb: To make sure that the throttle cable is only as tight as it needs to be, have a friend push the accelerator pedal to the floor (with the engine off, please!) and then adjust the cable so that the throttle plate just reaches the fully open position without stressing the cable.
As for the throttle stop adjustment, while it may look like an idle speed adjustment, if it has a direct effect on throttle position, don't use it as such. Set it so that the throttle plate is only held open enough to prevent binding of the throttle plate in the bore. At that position, the throttle cable should have at least a little slack in it, so that the throttle can consistently return to its stop. Do not proceed until these relationships are correct.
Regarding TPS adjustment, most TP sensors are adjusted at throttle close. Generally speaking, if the other adjustments are O.K., and the sensor is otherwise fully functional, getting this adjustment correct will finish the job.
Setting switch-type sensors will require the use of an ohmmeter. On some European models however, a pronounced click from the sensor should be heard if the throttle is opened just a bit from fully closed, and again at wide open throttle. Adjust the switch so that this occurs, but it's still a good idea to double-check with the ohmmeter. Combo-type sensors are usually adjusted the same way as switch-type sensors.
Setting potentiometer-type sensors will require the use of a voltmeter. You'll really need the service manual for the exact voltage spec, as well as the correct wires to connect up to. This type of sensor is usually set with the ignition on, engine off.
So, armed with your service manual and proper tools, adjusting the throttle position sensor should lead to a better running, more efficient engine.