1991 Chevy Corvette - Fuel Pressure Problems

Technically Speaking

James Berry Nov 6, 2013 0 Comment(s)
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Scan-Tool Diagnostics

If fuel pressure seems to be good, it’s time to connect a scan tool and check for fault codes. If a fault that could affect the fuel system is present, you’ll need to address it at this time. If not, with the key in the On position and the engine not running, select the “View Data” section to determine the sensor values and voltages for items related to the fuel system.

One of the first items to look at is the voltage for the throttle-position sensor, or TPS (Image E). This reading should be approximately 0.50 to 0.54 volts. If the engine controller sees more than approximately 0.60 volts, it may assume that the engine is flooded and the driver has pressed the accelerator to clear the flooded condition. In this scenario, it will restrict fuel flow to help resolve the perceived problem. This is known as the “clear flood mode.”

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It’s possible to adjust some TPSs by loosening the torx screws attaching the sensor to the throttle body and rotating the sensor until proper voltage is achieved. Some sensors are not adjustable and may need to be replaced if the closed-throttle voltage exceeds 0.60 volts.

Next, with the key still in the On position and the engine not running, rotate the throttle smoothly to wide open and monitor TPS voltage. It should progressively climb without dropping and be between 4.5 and 5.0 volts at WOT. If the voltage drops during this test, suspect a faulty TPS or wiring.

Your next step is to monitor the mass-airflow sensor, or MAF. Most C4 Corvettes use a MAF to determine how much air is being pulled into the intake manifold. The exceptions are the ’84 and ’90-’93 models, which use a speed density system known as Manifold Absolute Pressure (MAP). In 1994 the Corvette went back to the MAF-based system but used the speed density approach as a backup. Before we go on, let’s take a look at how these systems operate.

Manifold Absolute Pressure Sensor (MAP) Operation

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This system uses a sensor called a MAP, which measures the manifold absolute pressure and compares it with the atmospheric pressure outside the intake manifold (Image F). This information, along with the manifold air temperature (MAT), coolant temperature (CT), and engine rpm, is used by the ECM to determine the amount of air entering the cylinders. A MAP sensor has a vacuum line that runs from the sensor to the intake manifold. This line should be inspected for cracks, which could lead to incorrect sensor readings.

The TPS and the MAP are both three-wire sensors that share some of the same qualities:

One wire sends a constant 5-volt reference voltage from the engine controller to these sensors when the key is in the Run position.

One wire returns the voltage from the sensor to the engine controller.

One wire is ground.

Mass Airflow Sensor (MAF) Operation

This system uses a sensor called a MAF, which contains an extremely fine wire inside its housing. The ’85-’89 C4 engines use a Bosch sensor that heats the wire to 100 degrees (C). The ’94-and-later C4 models use an AC/Delco sensor that heats the wire to 200 degrees (C). The amount of current required to reach the desired temperature is monitored by the engine controller. More airflow will cool the wire, requiring more current to heat it. By constantly reviewing these parameters, the engine controller can precisely calculate how much air is passing through the intake.

A simple test for either of these sensors is to use a scan tool to monitor voltage with the engine running. Note the voltage at idle, then snap the throttle and look for a voltage change. If a change occurs, the sensor is most likely functioning properly. Most good scan tools will have a specification section listing the range in which each sensor should perform.

Coolant Temperature

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On a cold engine, use your scanner to verify that the coolant-temperature reading at the engine controller is approximately the same as ambient (Image G). If not, suspect a faulty coolant-temperature sensor or wiring, or the engine controller itself. Note that while these are not common failure areas, they should nevertheless be checked.

The foregoing might sound like a lot of work, but if you take your time, use common sense, and work step by step, you should be able to find your problem and help get your Corvette off the ether.


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