Engine - Poor Performance At Idle/Deceleration

Bulletin No. T 84-9
File In Group 6C
Number 67
Date Oct. '83
SUBJECT: POOR PERFORMANCE AT IDLE OR DECELERATION - EARLY PRODUCTION 1984 "E, K" CARS WITH DFI

MODELS AFFECTED: 1984 ELDORADO AND SEVILLE

A condition of poor performance, possibly accompanied by an intermittent "Service Now" light on some 1984 DFI equipped Eldorados (before B.P. 602410) and Sevilles (before B.P. 801221), may be caused by an improper MAP sensor mounting.

Electromagnetic interference (EMI) may cause false MAP sensor input to the ECM causing rich engine operation. Poor vehicle performance due to over enrichment would be most noticeable at idle or deceleration, with detection at cruising speeds (55 MPH) being limited. Codes 31, 33 and/or 34 may be set accompanied by an intermittent "Service Now" light. The false input is a result of EMI in the 50 MHZ range (6 Meter Ham radio, TV or Navigation transmitters) conducted to the MAP sensor bracket, which in turn acts as an antenna in close proximity to the MAP sensor electronics.

In order to correct this condition, the MAP sensor bracket must be electrically isolated from the I.P. mounting bracket. The following procedure should be followed when diagnosing and correcting this condition.

1. Verify condition and check for diagnostic trouble codes. Diagnose and repair any codes other than 31, 33 and/or 34 following 1984 Cadillac Service Manual procedure.

2. Remove lower right side dash panel, below glove box, and gain access to the MAP sensor assembly.

FIGURE 1 - MAP Sensor Assembly:

3. Refer to Figure 1. Check the MAP sensor assembly bracket (1) to determine if it is isolated from the I.P. mounting bracket (2). Using a digital multimeter (J-29125), set the meter on the OHM scale and probe between the MAP sensor assembly bracket and the I.P. mounting bracket as indicated in Figure 1.

If the reading is infinite (an open circuit), the bracket is isolated from the sensor assembly and the situation described above is not causing the condition and the 1984 Service Manual procedures should be followed.

If the reading shows continuity as is expected on most cars before the breakpoint ("E" car - 602410, "K" car - 801221), proceed with the following service procedure.

4. Remove the MAP sensor assembly bracket (1) from the I.P. mounting bracket (2) and discard the C-clip (3) shown in Figure 1. Apply three to four wraps of DUCT tape on the I.P. mounting bracket at the point indicated in Figure 1.

Pierce the tape at the hole location and install the MAP sensor assembly bracket using a black plastic push fastener, GMWDD P/N 1605396.

5. Test the isolation of the mounting as described in step 3. An infinite reading (open circuit) should be registered on the digital multimeter.
1984 Eldorados and Sevilles produced after the breakpoint have electrically isolated MAP sensor brackets. When servicing these vehicles, the MAP sensor bracket must remain isolated from the chassis.
For warranty purposes, use Labor Operation T6081 at .5 hour.