Engine - Driveability (Surge)/Emissions

88chevy14

GMC NUMBER: 88-T-141

GROUP. 6E - Engine Emission

DATE: August, 1988

CORPORATE
NUMBER: 836511R
SUBJECT: ENGINE DRIVEABILITY (SURGE)

MODELS: 1987-88 C/K, R/V TRUCKS EQUIPPED WITH 7.4L TBI ENGINES (VIN CODE N)


THIS BULLETIN IS BEING REVISED TO INCLUDE PART NUMBER AND SPECIFICATION FOR THE VACUUM HOSE. THIS BULLETIN CANCELS AND SUPERCEDES TRUCK SERVICE BULLETIN 88-T-81, DATED 3/1/88, ALL COPIES OF WHICH SHOULD BE DESTROYED.
Some 1987-88 C/K, R/V trucks with the 7.4L TBI engine may experience customer comments of poor driveability (surge) on acceleration and at road speed. 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:

FIGURE 1:

Service Procedure:

1. Check MAP sensor using the procedures on page 3-74/3-75 of the 198 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.

FIGURE 2:

7. Using GM6165M rubber hose (bulk P/N 9439274) and standard 1/4" vacuum fittings make the vacuum harness shown in Figure No. 2 and connect as shown. Usage of incorrect specification hose can result in vacuum hose collapse and cause a "Service Engine Soon" light.
Notice: On trucks not equipped with an accessory vacuum fitting in the manifold install fitting P/N 14088336 and use caps P/N 343350 to plug any unused port.
8. Reinstall electrical connection.
For repairs performed under warranty use:
Labor Op: T7500 and
Labor Time: .3 Hour