LIFT FOOT SHUDDER/VIBRATION IN REVERSE OR IN 1ST 0-10 MPH

SUBJECT: SHUDDER IN 1ST/REVERSE AT 0-10MPH DURING LIFT-FOOT MANUEVER NEW VACUUM MANIFOLD ASSEMBLY

MODELS/YEARS: 1991 NINETY-EIGHT AND TORONADO EQUIPPED WITH 3800 ENGINE (RPO L27) AND HYDRA-MATIC 4T60-E TRANSAXLE (RPO M13)

A vehicle equipped with a 3800 TPI (RPO L27) VIN L engine and a HYDRA-MATIC 4T60-E transaxle may experience lift-foot shudder. This shudder occurs when the driver accelerates from 0 MPH at 1/2 to full throttle and then quickly lifts their foot from the throttle (under 10 MPH). This is caused by the vacuum modulator (the vacuum modulator controls transaxle line pressure) responding too quickly to engine vacuum. A new vacuum manifold block assembly with an orifice will slow vacuum response to the transaxle approximately 1/10 of a second. This will allow the transaxle to have sufficient line pressure to hold the bands and clutches during lift-foot maneuvers.

SERVICE PROCEDURE:

To repair the lift-foot shudder condition, replace the existing vacuum manifold block assembly (located on the intake manifold) with the new vacuum manifold block assembly which incorporates a vacuum orifice. Mark or tag the vacuum hoses so the hoses do not get switched. In the unlikely event that the new vacuum manifold block assembly does not eliminate the lift-foot shudder, refer to the diagnosis section of the service manual.

IMPORTANT: Do not try to modify the existing vacuum manifold block with an orifice. Without the orifice being the correct size or made from the correct material, the orifice may eventually clog and cause a harsh or soft shift concern.

Do not install this new vacuum manifold block assembly with an orifice on a non-3800 TPI (RPO L27) VIN L engine that has an Exhaust Gas Recirculating (EGR) valve. The orifice could become plugged with debris from exhaust gases and cause a harsh or soft shift concern.

SERVICE PARTS INFORMATION:

Part Model Number Description Year Carline Model Usage -------- -------------------- ----- ------- ----------- 24500916 Vacuum Manifold Block 1991 C, E 1YMW, 1YZW w/Orifice Assembly 1YPW {only for (RPO L27) VIN L}

Parts are expected to be available on 12-17-90. Until then normal part orders will not be accepted by GMSPO. Only verifiable emergency VIP orders will be accepted. SPO will make every effort to obtain parts. All parts will be placed on 400 control to waive VIP surcharges. However, the part will be shipped premium transportation at dealer's expense. All other order types will be canceled as incorrectly ordered while the 400 control is in place.

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.