AUTO Q AND A: Testing a throttle position sensor
Question: I own a 1993 Corsica 2.2 liter 5 speed with 146k miles. The car is well maintained as I am a former mechanic. Here is my problem. When I leave off the throttle quickly, the car bucks. The condition is enhanced by a cold engine or cold a...
Question: I own a 1993 Corsica 2.2 liter 5 speed with 146k miles. The car is well maintained as I am a former mechanic. Here is my problem. When I leave off the throttle quickly, the car bucks. The condition is enhanced by a cold engine or cold ambient temperature. Do you think it would be worthwhile to replace the throttle position sensor? As much as it is against my grain to be a parts changer, I hesitate to spend hard-earned dollars on expensive diagnostics on an older vehicle. At a Blue Book of $1,200, the car is one minor disagreement away from being declared a total loss. Any hints that you can give would be greatly appreciated.
Answer: I agree changing parts is a lousy way to fix a car. Let's test that throttle position sensor, which is quite easy to do. TP sensors provide important input to a vehicle's engine management system, indicating not only throttle position, but rate of change. A flaky TP sensor can cause weird engine symptoms and/or erratic transmission shifting. Your TP sensor, like most used during this era, is of the potentiometer type, which contains a mechanical arm and a sliding electrical contact which varies electrical resistance. This design is simple, but long life isn't its strong suit (newer sensors are a more reliable non-contact, Hall effect type). After perhaps 75K miles of use, the sliding contact can begin to skip, causing a momentary glitch in the sensor's output. The engine control computer interprets the glitch as a sudden throttle movement, and makes an unfortunate correction, which could be what you're feeling (a very brief stumble during acceleration is a more common symptom).
Testing a TP sensor is easy if you have a basic digital voltmeter and a T-pin or two. I recommend the Craftsman model 82344 (Sears item # 03482344000) to my students as it's easy to use, inexpensive, and has great features for its price. Start by locating the sensor, which is attached to the engine's throttle, in line with the throttle shaft. There will be three wires entering its connector. With the key on, engine off, one wire contains five volts, another has very close to zero, and the third perhaps .5 volts. Gently insert a T-pin into the back side of the still-attached connector, shadowing each wire, until it seems to bottom. With your meter's black lead connected to a bare metal engine part, touch your meter's red lead to the T-pin, sampling the voltage. Move the T-pin as necessary to locate the wire containing about .5 volts.
This is the TP sensor's signal output, and it should vary smoothly between approximately .5 at idle and 4.5 volts at full throttle as the throttle is opened and closed (key-on, engine off). Try sweeping the throttle very slowly, perhaps a dozen times, and watch the digits closely for any abnormalities on the way up and down. A glitch can be so brief, it's difficult to see, and may also pass unnoticed by the vehicle's on board diagnostic system (no code or check engine light, a graphing multimeter or oscilloscope is much better for TP testing but they're pricey for a home user). If the voltage momentarily stumbles, the TP sensor requires replacement (easy: two screws and about $50).