Auto Repair Help


by Kyle McFadden

The engine hesitates when accelerating. The vehicle may stumble or stall when the accelerator is pressed or may exhibit an immediate lack of power.

This symptom may be caused by several items. A defective Throttle Position Sensor (TPS), Mass Air Flow Sensor (MAF), low fuel pressure, or something as simple as worn spark plugs or defective spark plug wires can be the cause.

If the “Service Engine Soon” or “Check Engine” light is on, extract the trouble code using a scanner prior to diagnosing this symptom. If the vehicle is due for maintenance, a complete tune-up should be performed prior to an extensive diagnosis. A complete tune-up including spark plugs, spark plug wires, distributor cap and rotor (if so equipped), fuel filter and air filter will go a long way in correcting the most common problems. If the problem still persists, further diagnosis on the vehicles fuel management system is necessary. The throttle position sensor, and mass air flow sensor (if equipped) should be checked. For an understanding of how these sensors operate, refer to the “Fuel Injection and Computer System” article on this website. On most vehicles, the TPS is a three wire sensor. The wires include a 5 volt reference, a ground, and the output voltage. The output voltage of the throttle position sensor should be checked using a digital volt meter. The voltage output should increase smoothly as the throttle is increased. Consult a vehicle specific wiring diagram for a schematic of the computer system which will include the TPS details. The Mass Air Flow sensor is difficult to test without the proper test equipment. There are many styles of sensors utilizing many methods of output signals. You should consult a vehicle specific service manual for testing procedures.

The computer system and its sensors are sensative devices and can be damaged using improper testing procedures. Never use an older needle type volt meter to measure signals on vehicles equipped with computers. Only digital meters should be used. Use caution when working around hot or rotating engine components.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

3 responses to “Diagnose Engine Hesitates When Accelerating”

  1. tim says:

    grand marqui 1996 when car is in drive foot on break engine idles up and down almost like a delayed pulsating , when in nuetral idles normal, and can this affect the transmission when driving thanks for reply

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