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Acceleration Hesitation!Posted 11/8/1999By Jim Watson
Vehicles: 1987 through 1991 Cadillac, Chevrolet models Beretta, Corsica, Celebrity, Cavalier, Lumina (not vans), 2.8L V6 MFI, 3.1L V6 MFI. Symptom: Hesitation/bog on acceleration. Key on/engine off. MAP sensor values are incorrect. Problem: PCM misinterprets sensor values. Typically MAP and/or TPS sensor voltages lock at approximately 2.0 volts on the scan tool during acceleration. Voltmeter readings at the PCM sensor terminal show active voltage increase. Test and Fix: Scan TPS and MAP sensors and compare with actual voltmeter readings on those sensors. If they do not agree, replace the PCM if all connections are good.
Background To accomplish this, an analog-to-digital (A/D) converter is mounted on the board in the PCM. Its task will be to sample the sensor input and turn that value into a digital signal. This A/D converter does a time-share among the analog sensors. The scanner information has important value since it would have already been through the A/D converter and been to the processor as an input. It shows what information the processor used in determining fuel and ignition control. In general, normal key on/engine off values on the scanner or at the PCM connector are: TPS less than 1.0 volts and the MAP approximately 4.7 volts at about 800 feet above sea level. This MAP value is also used as a BARO value for base fuel delivery. When the A/D converter failure occurs, the values seen only on the scanner will be less than what is expected. For example: TPS .78 volts or a closed throttle and MAP at 1.9 volts. This will tell the processor that the vehicle is at an altitude some level above Mt. Everest. Since not much O2 exists at that altitude, very little fuel will be required for a start. The processor would toggle the injectors. However, it would be too lean to start because it would have more O2 than the processor knew about. Another tip-off to an A/D converter failure is if, when the throttle is moved to WOT, maximum TPS voltage seen on the scanner is the same value as the MAP sensor would be locked low at, even though when backprobed the voltage would appear to be normal on the DVOM. If the failure occurs after a start-up, the engine will run well at idle because both TPS and MAP values are expected to be at a low voltage level. TPS will be less than 1 volt, indicating closed throttle and MAP will be around 1 volt, indicating low pressure in the manifold. As the engine accelerates from a stop under a load, both values should rise depending on the rate of throttle opening a load being pulled. In most cases during a WOT acceleration, the TPS will be greater than 4.5 volts and the MAP value will be near BARO value. This combination provides good acceleration and power. However, if the A/D converter fails at 1.9 volts, the processor will assume the rate of acceleration is considerably less, resulting in less fuel being delivered to the engine. Then it will go into a lean hesitation/bog or in some cases, if the acceleration is held long enough, a stall condition will result. The value of the voltage seen on the scanner during failure does vary. It may be 1.9 sometimes and at other times, 2.6 volts. Monitoring the scanner while backprobing the PCM input is the only true method to determine the failure. These values need to be within tenths of a volt for correct operation. The failure can be very intermittent. Monitoring scanner and DVOM values must be done during the failure mode when the problem is present.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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