Throttle Position Sensor Symptoms on a Buick LeSabre

by Richard Rowe
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Image by, courtesy of JOHN LLOYD

The LeSabre shares at least one thing in common with the Chevrolet Corvette and Ford Mustang: it is in the small club of cars that have been in continuous production by the same manufacturer for over 40 years. In a bid to stay current, General Motors began producing the first primitive electronic fuel injection units in the late 1970s, a refinement that quickly made its way to the LeSabre. Though these systems have undergone constant refinement over the years, certain components do go bad from time to time.

The TPS Sensor

The TPS (Throttle Position Sensor) is attached directly to the throttle body valve shaft on most LeSabres but is actuated by the throttle cable on older cars. This sensor uses a potentiometer (similar to a household light-dimmer switch) to tell the computer how far the throttle is open. This sensor services several important systems, including the fuel injection and transmission.

Unsteady Idle

The most common symptom for a failing or maladjusted TPS is an unsteady idle. The vehicle's idle may fluctuate by as much as 1000 RPM in park, as the computer constantly adjusts air-fuel ratio for its best-guess at intake volume.

Engine Stalling

This generally occurs when rapidly decelerating to a standstill, though it can also happen while idling. Again, without accurate information, the computer cannot deduce how much fuel to inject, so it averages the amount used in the last few moments of operation. When the throttle suddenly snaps shut, the engine is starved of air while the injectors are still delivering part-throttle amounts of fuel. The effect is very similar to rapidly closing the choke on an old manual-choke equipped truck.

Erratic Shifting

In order to increase performance under acceleration while providing comfortable shifting under cruise, electronic transmissions like those used in the car from 1992 and up are programmed to either increase or decrease shift firmness according to throttle position. A malfunctioning TPS can cause the car to accelerate sluggishly, or to shift hard and chirp the tires under normal acceleration.

High or Low Shifting

Acting in tandem with the mechanism that modifies shift firmness, the transmission's governor controls the RPM at which the shifts occur. To extract maximum performance from the engine, the transmission is designed to shift at a higher RPM under hard acceleration. A malfunctioning TPS can cause it to do so under normal acceleration, or to shift low when the right pedal is floored.

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