Q: I have a 1996 Corvette that gave me a recurring fault code (P0171 - "System Too Lean, Bank One") for more than a year. The code reader on my scanner indicated that the fuel pump was defective, so I replaced the pump and filter, but the P0171 code still occurred. I then took the car to three different shops and had an array of sensors installed—again, to no avail.
Finally a friend told me about a Corvette expert in my area. I took the car to his shop, where he checked it out with his own scanner. After about five minutes he told me the MAF sensor was not reading correctly. While I was skeptical of his diagnosis, I allowed him to install a new factory sensor on the condition that if it didn't fix the problem, I was under no obligation to pay him. Well, it worked, and the code hasn't been back since.
What I'd like to know is how he figured out the problem so quickly, and whether I can test my other Corvettes (an 1984 and a 1990) for the same issue. I'm sure other readers have experienced similar situations and would like to know how these systems work.
A: There are a wide range of seemingly unrelated driveability complaints that can be attributed to a Mass Air Flow (MAF) sensor that is not performing correctly. The P0171 "System Too Lean, Bank One" fault code you experienced is only one of the many fault codes a MAF sensor can set when under- or over-reporting the amount of air flowing into the engine.
Let's start with your earlier Corvettes, which are equipped with a Manifold Absolute Pressure (MAP) sensor, and work up to your 1996, which uses a MAF sensor. We will discuss how all of these sensors operate, how to test them, and common problem areas.
Most C4 and newer Corvettes use a MAF sensor to determine how much air is being pulled into the intake manifold. The exceptions are the 1984 model as well as the 1990-1993 editions, which all used a MAP-based, or "speed-density," system. In 1994 the Corvette went back to the MAF-based configuration but retained the old MAP approach as a backup. What follows is an overview covering the earlier, MAP-equipped vehicles.
Manifold Absolute Pressure (MAP) / Speed Density Theory of Operation
A speed-density system uses a MAP sensor to measure intake-manifold pressure and compares that reading with the atmospheric pressure outside the intake.
This information—coupled with the Manifold Air Temperature (MAT), Engine Coolant Temperature (ECT), and engine rpm—is used by the engine controller to determine the amount of air entering the cylinders.
The MAP is a three-wire sensor. One wire sends a constant 5-volt reference voltage from the engine controller to the sensor itself. It's usually labeled "C."
The next wire is the voltage returning to the engine controller from the MAP sensor. This is known as signal voltage, and is usually labeled "A."
The third and final wire is ground, and is usually labeled "B."
Testing the MAP Sensor
You can test for proper input voltage using a Digital Volt Ohm Meter (DVOM), also known as a multimeter. This test is best performed with the key in the On (engine off) position.
With your DVOM in DC (Direct Current) mode, the reference voltage should be 4.5 to 5 volts. This tells us that the engine controller is capable of supplying voltage to the MAP sensor, and that the engine controller and wiring are functioning properly.
If there is no reference voltage, you'll need to refer to a wiring diagram and back-probe the reference-voltage wire at the engine controller. If there are 5 volts coming out of the controller, ohm-check the wire all the way from the controller to the MAF sensor. You're looking for a cut or burned wire.
If there is no voltage coming out of the engine controller, the controller is most likely faulty.
Testing for a Ground
Begin by unplugging the connector from the MAP sensor. With your DVOM still in DC mode, back-probe the black wire with the black DVOM lead.
Now connect the DVOM red test lead to the battery's positive (+) post. Your DVOM should show either "Battery Voltage DC" or "0 Volts."
If the DVOM does not register battery voltage, that means there is no ground. In this case, refer to a wiring diagram and back-probe the ground wire at the engine controller. If there is no ground coming out of the controller, ohm-check the ground wire from the controller to the MAP sensor. Again, you're looking for a cut or burned wire.
You'll also need to check to make sure a good ground is being provided to the controller itself. This is a common failure, usually found behind the right-side cylinder head, or on the left side of the engine by the oil-sending unit. There is also a ground under the vacuum booster on some models.
A ground problem will sometimes set multiple fault codes such as MAP sensor, Throttle Position Sensor (TPS), and others depending on the year of the vehicle
Testing for Signal Voltage
To test for proper signal voltage with a DVOM, the MAP sensor must be connected to the wiring harness. This test is best performed with the key in the On (engine off) position and a vacuum pump hooked to the sensor.
Note the voltage with no vacuum, then slowly apply vacuum using the pump. The voltage should steadily decrease.
- 0 vacuum = 4.7 volts
- 5 inches of vacuum = 3.9 volts
- 10 inches of vacuum = 3.0 volts
- 20 inches of vacuum = 1.1 volts
If the DVOM displays decreasing voltage during this test, this confirms that the MAP sensor is functioning properly.
If the DVOM does not register any voltage on the signal wire, and the previous tests revealed no problems, this confirms that the MAP sensor has an internal failure.
If the DVOM does not display decreasing voltage, the MAP sensor has an internal failure.
If the voltage drops to 0 and then returns to normal during the test, or jumps around erratically, this could signify a faulty MAP sensor. Some sensors will read properly when the engine is cool and only fail when it warms to operating temperature. As a result, this test may need to be rerun when the sensor is hot. Also note that tapping on the MAP sensor to simulate driveline vibration could produce a failure.
Common MAP-Sensor Problems
- The vacuum line should be inspected for leaks, which can lead to incorrect sensor readings. Speed-density systems are extremely sensitive to vacuum leaks.
- The MAP sensor's O-rings, located on the vacuum-inlet nipple, should be inspected to see if they are missing or not sealing properly.
- If the engine has low compression, it can cause low or erratic vacuum. This can in turn lead to a rich fuel-mixture condition.
- Inspect the MAP sensor's connector for a broken locking tab. This can cause the connector to loosen, triggering an intermittent false reading.
Common Symptoms of a MAP-Sensor Failure
- Rough idle
- Engine stalls under load
- Black smoke coming from the exhaust
- Hard starting/long crank times
- High engine rpm
- Engine may stall when it starts or when coming to a stop
- Loss of power under acceleration
- Check Engine light with a MAP-sensor fault code
Checking Fault Codes
If a MAP sensor fails, the Check Engine light may turn on, and one of the following Diagnostic Trouble Codes (DTCs) may be present:
- P0106: MAP System Performance
- P0107: MAP Sensor Circuit Low Voltage
- P0108: MAP Sensor Circuit High Voltage
The foregoing covers the diagnostic procedures and common problems associated with the speed-density/MAP-based systems on your earlier Corvettes. Next month we'll cover MAF-sensor diagnostics and problem areas.
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