LACK OF POWER/TRANS HOT MODE INST MEMCAL/CHECK THERMAC DOOR. GMC Truck GMC C Pickup

SUBJECT: LACK OF POWER/TRANSMISSION HOT MODE (INSTALL MEMCAL/CHECK THERMAC DOOR)

MODELS: 1991-92 TRUCKS WITH 4.3, 5.7, 7.4L ENGINES

CONDITION

Some owners of 1991-92 trucks with 4.3, 5.7, 7.4L engines and 4-speed automatic transmission (4L80E) may comment on the following conditions:

4.3L (RPO LB4 with MT1 transmission) 5.7L (RPO L05 with MT1 transmission) ------------------------------------------------------------------------- 1. Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds.

7.4L (RPO L19 with MT1 transmission) ------------------------------------ 1. Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds.

2. Tach needle flutters on decelerations.

3. Catalytic converter odor.

CAUSE

4.3/5.7L with MT1 transmission ------------------------------ 1. Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gears, entry temperature is too low.

2. Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power.

7.4L with MT1 transmission -------------------------- 1. Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gear, entry temperature is too low.

2. Engine fuel calibration is not optimal.

3. At idle, the engine goes into an open loop condition which does not provide precise fuel control, possibly leading to catalytic converter odor.

4. Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power.

SERVICE PROCEDURE

4.3/5.7L with MT1 transmission ------------------------------- 1. Transmission hot mode entry temperature raised from 252 degrees Fahrenheit to 270 degrees Fahrenheit. Install MEMCAL per attached charts.

2. Check Thermac door in air cleaner for correct operation. Door must open and close freely with no sticking or binding.

7.4L with MT1 transmission -------------------------- Install MEMCAL per attached charts for corrections 1-3.

1. Transmission hot mode entry temperature raised from 252 degrees Fahrenheit to 270 degrees Fahrenheit.

2. Engine decel fuel calibration revised.

3. Engine idle revised from open loop to closed loop.

4. Check Thermac door in air cleaner for correct operation. Door must open and close freely with no sticking or binding.

SERVICE PARTS INFORMATION

In case of limited inventory, parts will be placed on 400 control to waive VIP surcharges. Only verifiable emergency VIP orders will be accepted. GMSPO will make every effort to obtain parts. However, the part will be shipped premium transportation at dealer's expense and all other order types will be placed on backorder until the 400 control is removed.

WARRANTY INFORMATION

Labor Operation: T0500 - MEMCAL Labor Time: 0.6 Hr. on C/K, RN, and P models 0.5 Hr. on G models

1991-4.3L / RPO LB4 -------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ----- ---- --- -------- --------- ------- C/R-20 MT1 All All 16174152 BAJB 4151 K/V-20 MT1 All All 16174153 BAJC 4161 P-30 MT1 GT5 4.10 All 16174154 BAJD 4171 P-30 MT1 HC4 4.56 C6G 7,600# 16174155 BAJF 4181 P-30 MT1 HC4 4.56 C6N 8,200# 16174155 BAJF 4181 P-30 MT1 HC4 4.56 C6U 9,000# 16174155 BAJF 4181 P-30 MT1 HC4 4.56 C7A 10,000# 16174155 BAJF 4181 P-30 MT1 HC7 5.13 C6U 9,000# 16174155 BAJF 4181 P-30 MT1 HC7 5.13 C7A 10,000# 16l74155 BAJF 4181 P-30 MT1 HC4 4.56 C7E 11,000# 16174156 BAJH 4191 P-30 MT1 HC7 5.13 C7E 11,000# 16174156 BAJH 4191 P-30 MT1 HC7 5.13 C7G 11,500# 16174156 BAJH 4191 G-30 MT1 All All 16174157 BAJJ 4201 -----------------------------------------------------------------------------

1992-4.3L / RPO LB4 -------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ----- ---- --- -------- --------- ------- C/R-20 MT1 All All 16169737 AZJA 9381 K/V-20 MT1 All All 16169741 AZJB 9391 P-30 MT1 GT5 4.10 All 16169749 AZJD 9411 P-30 MT1 HC4 4.56 C6G 7,600# 16169753 AZJF 9421 P-30 MT1 HC4 4.56 C6N 8,200# 16169753 AZJF 9421 P-30 MT1 HC4 4.56 C6U 9,000# 16169753 AZJF 9421 P-30 MT1 HC4 4.56 C7A 10,000# 16169753 AZJF 9421 P-30 MT1 HC7 5.13 C6U 9,000# 16169753 AZJF 9421 P-30 MT1 HC7 5.13 C7A 10,000# 16169753 AZJF 9421 P-30 MT1 HC4 4.56 C7E 11,000# 16169757 AZJH 9431 P-30 MT1 HC7 5.13 C7E 11,000# 16169757 AZJH 9431 P-30 MT1 HC7 5.13 C7G 11,500# 16169757 AZJH 9431 G-30 MT1 All All 16169745 AZJC 9401 -----------------------------------------------------------------------------

1991-7.4L / RPO L19 ----------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ----- ---- --- -------- --------- ------- C-20 MT1 GT4 3.73 All 16175204 BAMM 5031 C-20 MT1 GT5 4.10 All 16175214 BAMP 5051 C-30 MT1 GT4 3.73 C7A 10,000# 16175204 BAMM 5031 C-30 MT1 GU6 3.42 All 16175204 BAMM 5031 C-30 MT1 GT5 4.10 C7A 10,000# 16175214 BAMP 5051 C-30 MT1 GT5 4.10 C7E 11,000# 16175214 BAMP 5051 C-30 MT1 HC4 4.56 C7A 10,000# 16175214 BAMP 5051 C-30 MT1 HC4 4.56 C7E 11,000# 16175214 BAMP 5051 C-30 MT1 GT4 3.73 C7E 11,000# 16175214 BAMP 5051 C-30 MT1 All C5B 15,000# 16175254 BAMZ 5131 P-30 MT1 All C7M 14,100# 16175224 BAMS 5071 P-30 MT1 All C7N 12,300# 16175224 BAMS 5071 P-30 MT1 All C7P 8,600# 16175224 BAMS 5071 P-30 MT1 All C7S 14,500# 16175224 BAMS 5071 P-30 MT1 All C7C 10,500# 16175238 BAMW 5101 P-30 MT1 All C7E 11,000# 16175238 BAMW 5101 P-30 MT1 All C7J 11,800# 16175238 BAMW 5101 P-30 MT1 All C7L 12,000# 16175238 BAMW 5101 G-30 MT1 GT4 3.73 C6P 8,600# 16175228 BAMT 5081 G-30 MT1 GT4 3.73 C6S 8,900# 16175228 BAMT 5081 G-30 MT1 GT4 3.73 C7A 10,000# 16175228 BAMT 5081 G-30 MT1 GU6 3.42 All 16175228 BAMT 5081 G-30 MT1 GT5 4.10 C6P 8,600# 16175238 BAMW 5101 G-30 MT1 GT5 4.10 C6S 8,900# 16175238 BAMW 5101 G-30 MT1 GT5 4.10 C7A 10,000# 16175238 BAMW 5101 G-30 MT1 GT4 3.73 C7C 10,500# 16175238 BAMW 5101 G-30 MT1 GT4 3.73 C7Q 10,200# 16175238 BAMW 5101 G-30 MT1 GT5 4.10 C7C 10,500# 16175238 BAMW 5101 G-30 MT1 GTS 4.10 C7Q 10,200# 16175238 BAMW 5101 -----------------------------------------------------------------------------

1991-7.4L / RPO L19 ------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ------ ---- --- ------- ---- ---- R-20 MT1 GT4 3.73 All 16175228 BAMT 5081 R-20 MT1 GU6 3.42 All 16175228 BAMT 5081 R-20 MT1 GT5 4.10 All 16175238 BAMW 5101 R-30 MT1 GT4 3.73 C6U 9,000# 16175228 BAMT 5081 R-30 MT1 GT4 3.73 C6Y 9,600# 16175228 BAMT 5081 R-30 MT1 GT4 3.73 C7A 10,000# 16175228 BAMT 5081 R-30 MT1 GU6 3.42 All 16175228 BAMT 5081 R-30 MT1 5X2 3.73* C7A 10,000# 16175228 BAMT 5081 R-30 MT1 GT5 4.10 C6U 9,000# 16175238 BAMW 5101 R-30 MT1 GT5 4.10 C6Y 9,600# 16175238 BAMW 5101 R-30 MT1 GT5 4.10 C7A 10,000# 16175238 BAMW 5101 R-30 MT1 HC4 4.56 C6U 9,000# 16175238 BAMW 5101 R-30 MT1 HC4 4.56 C6Y 9,600# 16175238 BAMW 5101 R-30 MT1 HC4 4,56 C7A 10,000# 16175238 BAMW 5101 R-30 MT1 5X3 4.10* C7A 10,000# 16175238 BAMW 5101 R-30 MT1 SX4 4.56* C7A 10,000# 16175238 BAMW 5101 R-30 MT1 GT4 3.73 C7C 10,500# 16175238 BAMW 5101 R-30 MT1 HC4 4.56 C7C 10,500# 16175238 BAMW 5101 R-30 MT1 5X2 3.73* C7C 10,500# 16175238 BAMW 5101 R-30 MT1 5X3 4.10* C7C 10,500# 16175238 BAMW 5101 R-30 MT1 5X4 4.56* C7C 10,500# 16175238 BAMW 5101 K-20 MT1 GT4 3.73 All 16175267 BANC 5161 K-20 MT1 GT5 4.10 All 16175276 BANF 5181 K-30 MT1 GT4 3.73 C7A 10,000# 16175267 BANC 5161 K-30 MT1 GU6 3.42 All 16175267 BANC 5161 K-30 MT1 GT5 4.10 C7A 10,000# 16175276 BANF 5181 K-30 MT1 GT5 4.10 C7L 12,000# 16175276 BANF 5181 K-30 MT1 HC4 4.56 C7A 10,000# 16175276 BANF 5181 K-30 MT1 HC4 4.56 C7L 12,000# 16175276 BANF 5181 K-30 MT1 GT4 3.73 C7L 12,000# 16175276 BANF 5181 V-30 MT1 GT4 3.73 C6W 9,200# 16175267 BANC 5161 V-30 MT1 GT4 3.73 C7A 10,000# 16175267 BANC 5161 V-30 MT1 5X2 3.73* C7A 10,000# 16175267 BANC 5161 V-30 MT1 GT5 4.10 C6W 9,200# 16175276 BANF 5181 V-30 MT1 GT5 4.10 C7A 10,000# 16175276 BANF 5181 V-30 MT1 GT5 4.10 C7E 11,000# 16175276 BANF 5181 V-30 MT1 HC4 4.56 C6W 9,200# 16175276 BANF 5181 V-30 MT1 HC4 4.56 C7A 10,000# 16175276 BANF 5181 V-30 MT1 HC4 4.56 C7E 11,000# 16175276 BANF 5181 V-30 MT1 5X3 4.10 C7A 10,000# 16175276 BANF 5181 V-30 MT1 5X3 4.10 C7E 11,000# 16175276 BANF 5181 V-30 MT1 5X4 4.56 C7A 10,000# 16175276 BANF 5181 V-30 MT1 5X4 4.56 C7E 11,000# 16175276 BANF 5181 V-30 MT1 GT4 3.73 C7E 11,000# 16175276 BANF 5181 V-30 MT1 5X2 3.73* C7E 11,000# 16175276 BANF 5181

--------- *DUALLY (Dual Rear Wheels) ----------------------------------------------------------------------------

1992-7.4L / RPO L19 -------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ----- ---- --- -------- --------- ------- C-20 *P MT1 All All 16175204 BAMM 5031 C-20 *S MT1 GT4 3.73 All 16175204 BAMM 5031 C-20 *S MT1 GT5 4.10 All 16175258 BANA 5141 K-20 *S MT1 GT5 4.10 All 16175276 BANF 5181 K-20 *P MT1 All All 16175267 BANC 5161 K-20 *S MT1 GT4 3.73 All 16175267 BANC 5161 C-30 MT1 GT4 3.73 C7A 10,000# 16175204 BAMM 5031 C-30 MT1 GT4 3.73 C6U 9,000# 16175204 BAMM 5031 C-30 MT1 GT4 3.73 C6Y 9,600# 16175204 BAMM 5031 C-30 MT1 GT4 3.73 C7E 11,000# 16175214 BAMP 5051 C-30 MT1 GT5 4.10 All 16175214 BAMP 5051 C-30 MT1 HC4 4.56 All 16175214 BAMP 5051 C-30 MT1 All C5B 15,000# 16175254 BAMZ 5131 K-30 MT1 GT4 3.73 C6W 9,200# 16175267 BANC 5161 K-30 MT1 GT4 3.73 C7A 10,000# 16175267 BANC 5161 K-30 MT1 GT4 3.73 C7L 12,000# 16175276 BANF 5181 K-30 MT1 GT5 4.10 All 16175276 BANF 5181 K-30 MT1 H4C 4.56 All 16175276 BANF 5181 P-30 MT1 H4C 4.56 C7M 14,100# 16175224 BAMS 5071 P-30 MT1 H4C 4.56 C7N 12,300# 16175224 BAMS 5071 P-30 MT1 H4C 4.56 C7P 16,000# 16175224 BAMS 5071 P-30 MT1 H4C 4.56 C7S 14,500# 16175224 BAMS 5071 P-30 MT1 H4C 4.56 C3D 14,800# 16175224 BAMS 5071 P-30 MT1 H4C 4.56 C7C 10,500# 16175238 BAMW 5101 P-30 MT1 H4C 4.56 C7E 11,000# 16175238 BAMW 5101 P-30 MT1 H4C 4.56 C7J 11,800# 16175238 BAMW 5101 P-30 MT1 H4C 4.56 C7N 12,300# 16175238 BAMW 5101 G-30 MT1 GT4 3.73 C7N 12,300# 16175228 BAMT 5081 G-30 MT1 GU6 3.92 C7N 12,300# 16175228 BAMT 5081 G-30 MT1 GT5 4.10 C7N 12,300# 16175238 BAMW 5101 G-30 MT1 HC4 4.56 C7N 12,300# 16175238 BAMW 5101

------------ * P=Pickup S=Suburban ----------------------------------------------------------------------------

1991-92-5.7L / RPO L05 ---------------------- SERVICE BROADCAST SCANNER MODEL TRANS AXLE GVW MEMCAL CODE I.D. ----- ----- ----- ---- ------- ---------- ------- C-20 MT1 All All 16173069 BAAX 3001 C-30 MT1 All C6P 8,600# 16173069 BAAX 3001 C-30 MT1 All C7A 10,000# 16173069 BAAX 3001 C-30 MT1 All C7E 11,000# 16173069 BAAX 3001 C-30 MT1 All C5B 15,000# 16158426 AWTS 7981 R-20,30 MT1 All C7L 12,000# 16174245 BAJW 4301 V-20,30 MT1 All All 16173073 BAAY 3011 K-20 MT1 All All 16173073 BAAY 3011 K-30 MT1 All C6P 8,600# 16173073 BAAY 3011 K-30 MT1 All C7A 10,000# 16173073 BAAY 3011 K-30 MT1 All C7L 12,000# 16173073 BAAY 3011 G-30 MT1 All C7L 12,000# 16174245 BAJW 4301 P-30 MT1 All C7L 12,000# 16174245 BAJW 4301

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.