ENGINE SURGE ON ACCELERATION OTHER DRIVEABILITY COMPLAINTS

MODELS: 1987-88 C/K, RN TRUCKS WITH 7.4L TBI ENGINE (VIN CODE N)

Some 1987-88 C/K, R/V trucks with the 7.4L TBI engine may experience customer complaints of poor driveability (surge) on acceleration and at road speeds. If normal diagnostics finds nothing wrong, it may be necessary to reposition the Manifold Absolute Pressure (MAP) sensor. Repositioning of the MAP sensor will prevent fuel vapor and/or gasoline from entering the MAP sensor under heavy throttle conditions. Vapor or gas in the MAP sensor causes the sensor to supply lower than normal voltage readings to the ECM. The ECM, when receiving low voltage readings from the MAP sensor, thinks the engine is rich and leans out fuel, causing engine surge. If it is necessary to reposition the MAP sensor, the following service procedure should be followed:

SERVICE PROCEDURE:

1. Check MAP sensor using procedures on page 3-74/3-75 of the 1988 Fuel and Emissions Manual X-8836. Replace sensor if necessary.

2. Disconnect vacuum tube assembly and electrical connection from MAP sensor.

3. Remove MAP sensor from upper plastic bracket (see illustration, View B). Care must be taken to avoid breaking bracket.

4. Remove sheet metal screw holding upper plastic bracket to lower metal bracket, and retain plastic bracket.

5. Add one additional 3/16" hole to plastic bracket and two 1/8" holes in air cleaner, using revised bracket as hole template (see illustration, View C).

6. Mount bracket so MAP sensor vacuum connection faces down toward engine, using two 8-32 x 1/2 sheet metal screws (P/N 447164).

7. Reconnect vacuum tube assembly by reversing from as-shipped orientation (see illustration, View D.)

8. Reinstall electrical connection.

For repairs performed under warranty use:

Labor Op: T7500

Labor Time: .3 hrs with Trouble Code 92

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.