ENGINE DETONATION (PINGING) OR SLIGHT TIP IN HES.

MODELS AFFECTED: 1982-1985 RWD DE VILLES AND FLEETWOODS 1985-1987 DE VILLES AND FLEETWOODS 1982-1987 ELDORADOS AND SEVILLES

Some 1982-1987 Cadillacs equipped with HT4100 engines may experience engine detonation (pinging) or a slight tip in hesitation. Also, 1986-1987 Cadillacs equipped with HT4100 engines may set a code E48/E048, EGR system fault. These conditions may be caused by carbon deposits in the EGR tubes of the intake manifold. Refer to Figure 1. The carbon deposits restrict the flow of exhaust gas into the intake manifold. To diagnose and repair this condition, perform the following:

1. Verify full system vacuum is being supplied to the EGR solenoid and that the EGR solenoid functions during ECM output cycling.

2. Tee a vacuum gage into the MAP sensor vacuum line.

3. With engine operating in closed loop, transmission in park, and A/C off, disconnect the ISC motor electrical connector. Record the MAP sensor vacuum level.

4. Disconnect the EGR solenoid electrical connector and record the vacuum level again. Reconnect the EGR solenoid electrical connector.

5. If the difference between the vacuum in step 3 and 4 is less than 3.5 Hg, the EGR pipes are restricted. Proceed to step 7.

6. If the difference between the vacuum in step 3 and 4 is 3.5 Hg or greater, the EGR pipes are NOT restricted.

For vehicles that do not set a code E48/EO48, refer to the EGR system diagnosis in Section 6E-C7 of the appropriate Service Information Manual.

For vehicles which set a code E48/EO48, refer to Serviceman Bulletin T-86-52, Group 6E.

For 1986 Eldorados and Sevilles which set a code E48/EO48, refer to Serviceman Bulletin T-88-23, Group 6E.

7. Remove the throttle body assembly referring to the appropriate Service Information Manual.

8. For 1985 DeVilles and Fleetwoods and all 1986-1987 vehicles, remove the EFE grid.

9. Remove the EGR valve.

10. Using an 11/32 inch drill bit, clear the carbon out of the EGR pipes in the intake manifold. Refer to Figure 1, view A.

IMPORTANT: The depth of the EGR pipes is 2-1/4 inches from the throttle body mounting surface of the intake manifold. Refer to Figure 1, view A. There is approximately 1/2 inch of clearance between the bottom of the EGR pipes and the manifold casting plugs.

CAUTION:

For the next step, safety glasses must be worn when using compressed air.

11. Cover the top end of one EGR pipe with a piece of tape. Hold a shop rag over the EGR valve opening on the manifold. Using compressed air, blow through the uncovered EGR pipe to force the carbon debris out through the EGR valve opening of the manifold. Remove the tape from the covered EGR pipe.

12. Install the EGR valve, EFE grid (if equipped), and the throttle body assembly. Refer to the appropriate Service Information Manual.

13. Check for fuel leaks at the fuel feed and return line fittings at the throttle body assembly.

For warranty purposes use Labor Operation T6360 at 1.0 hour.

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.