MAF SENSOR SYSTEM DIAGNOSIS
MASS AIR FLOW (MAF) SENSOR SYSTEM DIAGNOSIS CODES 33, 34 AND 36
VEHICLES AFFECTED: 1986-87 F AND Y SERIES VEHICLES EQUIPPED WITH LB9 ----------------- OR L98 ENGINES
Some vehicles equipped with 5.OL or 5.7L port fuel injected engines may experience "Service Engine Soon" lights with some combination of codes 33, 34 and/or 36 stored. This bulletin summarizes diagnostic procedures to be used in trouble shooting these codes.
All 1986 model year vehicles should be updated to the latest service PROM per bulletins: 86-657-106 (Y cars) or 86-650-109 (F cars) and checked for "Service Engine Soon" light reset prior to further diagnosis. In the case of 1986-87 F car or 1986 Y car, the MAF power relay should be checked to assure that it is part number 10067925. If it is not, then it should be replaced with part number 10067925. Please do not substitute.
Diagnosis
CODE 33 (MAF SENSOR VOLTAGE HIGH)
1. Inspect to determine that part number 10067925 relay has not inadvertently been installed as a burnoff relay or incorrectly connected as a burnoff relay. (reference Bulletin 86-6-127)
NOTICE: SOME SERVICE PARTS BOOKS REFLECT THIS INCORRECT USAGE.
For proper relay parts usages, please refer to the relay usage chart provided at the end of this bulletin. No substitutions, should be made.
2. With the engine fully warmed up and operating in closed loop, check the airflow at 2000 RPM in neutral using a Tech 1. If it is below 16.0 gm/sec for a 5.OL or below 18.0 gm/sec for a 5.7L engine and the block learn value is greater than 138, replace the MAF sensor.
If code 33 still resets, refer to section 6E of the appropriate service manual for further diagnostics.
CODE 34 (MAF SENSOR VOLTAGE LOW)
1. Inspect the plenum side of the throttle blades for deposits. If even a small amount of deposits are present, clean the throttle body per Bulletin 87-650-109.
2. Thoroughly inspect the vehicle for sources of air entering the engine without passing through the MAF sensor. Specifically, look for a cracked, cut or loose duct between the air meter and the throttle body, poor inlet runner to plenum or manifold sealing, a split PCV hose or any type of vacuum leak. A large quantity of air does not necessarily have to be involved. If any leaks are found, repair the condition, clear the codes and check if the code resets.
3. Check resistance of the MAF power relay contacts per bulletin 87-650-108. The resistance should be less than 1 ohm across closed contacts. If the resistance is greater than 1 ohm replace the relay.
4. Check the minimum air rate and compare to specifications found in Bulletin 87-650-109. If the air rate is below specifications, please reset by connecting a vacuum gauge to port A on the throttle body, increasing the air rate by turning the minimum air screw until one inch of vacuum appears on the gauge and then reducing the minimum air setting by one half turn of the screw. If Code 34 still resets, refer to section 6E of the appropriate service manual for further diagnostics.
CODE 36 (MAF BURNOFF)
1. Inspect to verify that the MAF power and burnoff relays currently on the vehicle agree with the usage chart provided at the end of this bulletin.
2. Check the relays for proper function per bulletin 87-650-108. Also, check for PCV plugged condition per the same bulletin.
3. Check all electrical connections carefully.
If Code 36 still resets, refer to section 6E of the appropriate service manual for further diagnostics.
RELAY USAGE CHART
YEAR MODEL POWER RELAY BURNOFF RELAY ---- ----- ----------- -------------
86 Y 10067925 10094701 86-87 F 10067925 10094701 87 Y 14089936 14089936
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.
