In the early 1980s, General Motors introduced On-Board Diagnostics 1. This was their first reliable, fully computerized system. The earliest models used the computer to alter carburetor fuel flow. This system was failure prone and in the mid-1980s GM abandoned carburetion altogether and deployed fuel injection. From the beginning GM used sensors, one of which is a manifold absolute pressure sensor, or MAP sensor. This is perhaps the most important sensor due to the direct impact it has on engine performance. During the OBD-1 era, if the MAP sensor failed, it was assigned failure code number 34.
Locate the MAP sensor on the vehicle. During the electronically controlled carburetor years the sensor was located inside the vehicle. It was a combination sensor called the MAP-BARO sensor--BARO standing for barometric pressure. This changed with fuel injection and the MAP sensor became a single unit that is mounted on the engine. Since it measures vacuum, it will have a vacuum hose attached, along with a three-wire plug. It is small, rectangular and black.
Inspect the vacuum hose going to the sensor. Many times the hose will collapse or develop holes. This will cause severe engine fueling problems. The car may not start at all if the hose is damaged, or it may start and not idle, or start and smell of raw fuel. Be absolutely sure the hose is in top condition since this is the direct communication link between the engine and the computer circuit.
Check the connector that plugs into the sensor. These are known to fail and actually crumble apart due to the heat. In this case, all communication to the computer is lost. The computer sends a reference voltage to the sensor on one of the wires, typically five volts. The other two wires are a ground and variable resistance wire. The resistance rises and falls as the vacuum increases and decreases. This changing value determines fuel increase or decrease. Replacement connectors are available at auto parts stores. If there is any damage to this important connector, replace it.
Replace the MAP sensor. This sensor is very reliable and usually the problem is associated with the hose or connector. It can, though, be defective. At times the computer circuit will fail and the computer will be at fault, or the wiring between the computer and MAP sensor may be suspect.
