Backfire through TBI 1992 GMC 3500 454 engine

 Auto Repair Questions, GMC  Comments Off on Backfire through TBI 1992 GMC 3500 454 engine
Apr 112017

backfire through TBI 1992 GMC 3500My 1992 GMC 3500 started developing a skip under load. So I did the normal spark plugs, wires, complete distributor, coil, test drove and still skips under load. Truck has 100,000 miles on it, so I replaced the fuel filter then fuel pump. Test drove and still skips at times under load. Now starting to pop up through TBI unit. Any ideas? Have made sure timing is set correctly. Did compression test and all cylinders are at between 145-150.


Backfire through TBI

A pop through the TBI is equivalent to a backfire through TBI.

Anytime an engine backfires through the intake it is from spark occurring while an intake valve is still open. Now this can be from pour timing or an open valve. Since you have done a compression test and received excellent results it makes you want to focus on the ignition timing. I would double check to make sure you have set your timing correctly. Doesn’t hurt to check it more than once. Therefore I have added the timing adjustment procedure below for convenience.

Sticking Intake Valve

I have had sticking intake valves do this from carbon build up on the valve from an RV that had set for a while. It would intermittently cause a valve to stick open. I too would get good compression readings but after testing and retesting found that intermittently one of the engine cylinders compression was ZERO and then would go back up(engine has 14,000 miles). I tried SEAFOAM with no luck. Tried SEAFOAM 4 more times each time driving 200 miles and then changing the oil. Eventually it cleared up on its own after 4,000 miles of driving.

Setting and Adjusting Engine Ignition Timing

  1. Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.
    NOTE: Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.
  2. Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.
  3. Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
  4. Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won’t work with HEI.

    WARNING Never ground the HEI TACH terminal; serious system damage will result.

  5. Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley or damper notch isn’t aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.
    NOTE: TDC or Top Dead Center corresponds to 0�B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.
  6. Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.
  7. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.
  8. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
  9. Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.
  10. Stop the engine and disconnect the timing light.
Feb 222017

1999 GMC JimmyThe parking brake will not release, pedal stuck. It worked for hours, but could not get it. I did not want to do too much without guide. any tips ? thanks


The Parking brake system consists of the a brake lever, two cables, adjuster and brake shoes. In order to determine if the lever itself is at fault disconnect the cable and test.

Park Brake Lubrication

Clean and lubricate the park brake lever assembly using Lubriplate GM P/N 1050109 or the equivalent.

Plastic coated parking brake cables do not need periodic lubrication. However, before performing service that involves the adjuster, perform the following steps:
•Clean the exposed threads on each side of the nut.

Important Lubricate the threads of the adjusting rod before turning the nut
•Lubricate the threads of the adjusting rod using Lubriplate GM P/N 1050109 or the equivalent.

Parking Brake Lever Replacement


For complete step by step instructions get an auto repair manual today.

Parking Brake Shoe Adjustment

The park brake must be adjusted any time the park brake cables have been replaced or disconnected, if the park brake shoes have been replaced or if under heavy foot pressure the pedal travel is less than half the pedal total travel. Before adjusting the park brake, check the condition of the service brakes.

  • Raise the vehicle and support the vehicle with safety stands.
  • Remove the wheel and tire assembly.
  • Take off the caliper and the rotor.
  • Remove the park brake cable from the park brake lever.
  • Adjust the shoe diameter using the adjuster nut.
  • Turn the adjuster nut clockwise to increase the diameter until the rear wheel will not rotate without excessive force in a forward direction.
  • Connect the park brake cable to the park brake lever.
  • Install the caliper and the rotor.
  • Install the wheel and the tire.
  • Adjust the rear park brake cables.
  • Install the wheel and tire assembly.
  • Remove the safety stands and lower the vehicle.
Feb 152017

2002 GMC Safari AWDyes i was told that i could take my transfer case (that is a two wire plug-in to encoder) and use a 6 wire transfer case but just change the encoder motor on it with the one off of my… and put the two wire encode on that transfer case.. will this really work…. will the pcm still read and work right. i no it has to be able to read the pwm ,pm ,rpm,vss, and hell who no;s i don’t thats y im here asking hope some one give me some info. on this thanks gene


Since you are still using the same electronics, I don’t see any reason why it wouldn’t work. The computer won’t even know you have changed anything and should work just fine.

Jan 302017

2002 GMC Safari AWDI was tracing some wires from my transfer case and ended up under the dash driver side.. any ways I found a fuse box just above the info. plug the box has 4 fuses in it there marked micro 1,2,3,4 can any one tell me what they are and what they run (or do ) my Haynes book don’t tell me sh_ _   but these cheap books never really do tell you or show you much .. thanks


I have to agree with you on the Haynes manuals. These days they are not half as good as they once where. If you want Up to date vehicle specific information I recommend an Online Auto Repair Manual. They have more information than you could ever need. They include a full set of wiring diagrams and repair estimator. Well worth the money in my opinion.

On to the fuse box. The fuses should be covered in your owners manual. To find the specific page you can look up “fuses” in the index at the back of the manual. If you do not have a manual you may view yours online for free.

Jan 232017

2006 GMC EnvoyDue to oil pressure gauge at 0, replaced oil pressure sensor 1.5 years ago, fixed problem until 2 weeks ago. Replaced sensor again, fixed for 1 week but then again read 0. Oil level is ok, engine sounds good and runs fine.It is a  2006 GMC Envoy, 5.3 liter, 156k miles. Suggestion? thanks


If there is a ticking noise, the oil pressure may actually be low oil pressure. If the engine seems normal other than the light being on lets dig deeper. The sensor has been giving you some issues which is normal. I would think since you have had to replace it several times there may be an issue with the wiring or the connection at the sensor.

What to Try First, Low Oil Pressure

Have someone sit in the vehicle and watch the oil light/gauge. Perform the wiggle test by wiggling the wires and the connection at the oil pressure sending unit. You may refer to the wiring diagram to help trace any wiring issue you encounter.

Oil Pressure Sensor Wiring Diagram

low oil pressure sensor wiring diagram 2006 GMC Envoy

Jan 232017

2002 GMC Safari AWDMy all wheel drive is not working. I just have rear wheel drive. I need to find out how the all wheel drive is wired. Where dose the transfer case get powered from. How does the all wheel drive stay in all wheel drive and how dose the wiring run (to what or from what relay)?



All Wheel Drive Wiring Diagram 2002 GMC Safari


All Wheel Drive Description

All Wheel Drive 2002 GMC Safari

Power is delivered from the transmission (1) to the input shaft (2). The input shaft (2) is splined to the rear output shaft (7). To deliver the power to the rear propeller shaft (9), the power is constant through the rear output shaft (7). When power is required for the front propeller shaft (11), a command is sent to the encoder motor (13). The encoder motor (13) rotates the shift detent lever (12), which is cam shaped. The cam action moves the clutch lever (3). The clutch lever (3) pivots on the clutch lever pivot studs, and moves toward the clutch apply plate to engage the clutch. As more pressure is applied to the clutch apply plate, the clutch discs are compressed. Using the inner clutch discs, which are engaged with the clutch hub (4), and the outer clutch discs, which are engaged with clutch housing (5), the power flow is delivered to the clutch housing (5). The clutch hub (4) is splined to the rear output shaft (7). The clutch housing (5) rotates on a needle bearing on the rear output shaft (7). The chain drive sprocket (6) is splined to the clutch housing (5). The power flows from the drive sprocket (6), through the chain (9), to the chain driven sprocket. The chain driven sprocket is splined to the front output shaft (10). The power flow is delivered to the front propshaft (11) through the front output shaft (10).

New Venture Gear 136 Automatic Transfer Case

During normal driving situations, the Auto 4WD mode is active. During the Auto 4WD mode, the transfer case shift control module monitors rear wheel slip speed, based on the inputs from both the front and rear propshaft speed sensors. When the vehicle experiences a rear wheel slip condition, the transfer case shift control module sends a pulse width modulated (PWM) signal to an electronic motor, transfer case encoder motor. This motor rotates the transfer case sector shaft, applying a clutch pack. This clutch pack is designed to deliver a variable amount of torque, normally delivered to the rear wheels, and transfers it to the front wheels. Torque is then ramped up to the front wheels, until the front propshaft speed sensor matches that of the rear propshaft speed sensor. Torque is then ramped down, until torque is completely removed from the front wheels or until rear wheel slip is once again detected. The process then repeats.

View the list of major components that make up the automatic transfer case (ATC) system below.

SERVICE Indicator (4WD/AWD) Lamp

The SERVICE indicator (4WD/AWD) lamp is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD) lamp is controlled by the transfer case shift control module via a Class 2 message or by a Service Indicator Control circuit.

Transfer Case Encoder Motor

The transfer case encoder motor consists of a permanent magnet (PM) DC motor and gear reduction assembly. It is located on the left hand side of the transfer case. When activated, it turns the sector shaft of the transfer case to shift the transfer case and to apply the clutch that applies the front propshaft. The encoder motor is controlled with a pulse width modulated (PWM) circuit provided by the transfer case shift control module. This circuit consists of a power supply relay, Motor Control A, and supplies voltage to the motor. The Motor Control B circuit is a PWM driver that varies the duty cycle to control the amount of current through the motor to ground. The transfer case encoder motor can be turned ON and OFF using a scan tool. You may also monitor Motor Control A and B circuits using a scan tool.

Transfer Case Shift Control Module

The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information. This information is provided to the transfer case shift control module via Class 2 data bus from the powertrain control module (PCM). The transfer case shift control module monitors front and rear propshaft speed as well as controlling the operation of the transfer case encoder motor assembly.

Transfer Case Speed Sensors

There are three speed sensors mounted on the transfer case. 2 speed sensors are mounted on the rear output shaft and one on the front output shaft. Each speed sensor is a permanent magnet (PM) generator. The PM generator produces an AC voltage. The AC voltage level and number of pulses increases as speed increases.

Front Propshaft Speed Sensor – The transfer case shift control module converts the pulsating AC voltage from the front transfer case speed sensor to front propshaft speed, in RPM to be used for calculations, and to monitor the difference between the front and rear sensor speed. It is also used in the AUTO 4WD mode to determine the amount of slip and the percent of torque to apply to the front axle. The front propshaft speed can be displayed with a scan tool.

Rear Propshaft Speed Sensor – The transfer case shift control module converts the pulsating AC voltage from the rear transfer case speed sensor to a rear propshaft speed, in RPM to be used for calculations. The rear propshaft speed can be displayed with a scan tool.

Vehicle Speed Sensor – One of the 2 speed sensors on the rear output shaft is the vehicle speed sensor (VSS) input to the powertrain control module (PCM). The PCM sends this information to the transfer case shift control module via the Class 2 serial data bus.

Dec 302016

2006 GMC SierraHello, I have a 2006 GMC Sierra 1500 5.3L Ext cab, the AC intermittently working. It will always work when I first start the truck and then blow warm air. It will blow cold shorter times during hotter days. The compressor is not running when it blows warm. I have checked the charge on the system and it is good. I have replaced the low pressure switch and checked the continuity is good. Not sure what the next step would be, the relay module or the High pressure switch? or icing up the line and needs purged? or the wire connection at the low pressure switch?


AC Intermittently Working

My initial thoughts would be to check the high speed fan and or the high speed fan relay. The system is charged and the compressor is cycling as it should. So lets look to make sure the fan is cooling the Freon in the condenser.

It may be possible that the Electronic Blend Door Actuator is not functioning properly which would only allow warm air to pass through vents. The Air Conditioning lines going into the firewall would be cold if this is the case. Most common to replace actuator.

Dec 232016

2005 GMC YukonI have a 2005 GMC Yukon XL that I have just replaced the fuel pump in. After getting the old pump out and installing the new one, reconnecting all the hoses and lines and getting the tank back up in place, I turned the key to the on position about 4-5 times to re pressurize the system. After that the truck started right up. Ran perfect. The wife used to it to run a few errands, and later ran to the store, still no problems.

Next day, I went to run to the gas station and it didn’t start right up. I just turned the key to the on position a couple times and it started. As I proceeded to drive down the road, I stepped on the gas pretty good and it like fell flat. It wouldn’t rev up. I left it run as I ran into the store and on my way back it seemed to slowly rev up more and more.

Fuel Problem

Later that afternoon we went to leave and took like 15-20 minutes to get started. And every time I would put it in gear it would die. So I left it till the next day and now I can’t get it to start, sounds like it wants to but wont. I can still here the fuel pump turn on when I turn the key to the on position but, it sounds like its got air bubbles in it?? Like its pushing air. I ended up replacing the pressure line because I couldn’t get the old fuel line out of the the plastic connector. It took some time but I did end up finally getting the old return line out and was able to use the original plastic connector.

I just wondering if that original return connector isn’t holding pressure? Or if I have some other line that isn’t seated properly?? Or, do I have a faulty fuel pump or a clogged fuel filter?? Any help would be greatly appreciated! Thank You!!


Fuel Problem Test

Anytime you have a “crank no start” problem you first want to determine what the engine is not getting. Once you know for sure its not getting spark or fuel you can concentrate on that system. Since you are pretty certain you have a fuel problem. A quick test can be done. Spray starting fluid in the air filter while someone cranks the engine over. Hold the gas pedal at least half way while doing so. If the engine fires right up, you have proven there is a fuel related problem. About the only way to get air into the system would be if the fuel tank was empty or the fuel pump was faulty.

Fuel Filter

The fuel filter should be replaced anytime the fuel pump is replaced. It is  a normal maintenance part and will only help the situation. Of course the fuel system needs to re pressurized after replacing the fuel filter similar to what you did when you replace the fuel pump earlier. The in-line fuel filter is located on the rail behind the composition sensor for the 5.3L engine. The 4.8L,6.0L and 8.1L engines do not have an external fuel filter, just the one in the tank connected tot he fuel pump.

Fuel Pressure Testing

Fuel Problem Pressure Gauge

When dealing with the fuel system it is important to know the fuel pressure of the system. Without the correct fuel pressure the fuel injectors will not fire. The fuel system pressure should be between 55-62 psi for gasoline engines and is slightly lower for ethanol engines at 48-54 psi. Connect the fuel pressure gauge tot he fuel rail test port located under the engine sight shield. In conclusion when the fuel pressure is below the specified reading some restriction may be involved. Consequently replace the fuel filter and see if the reading increases. If not the fuel pump may need to be replaced as a result.

Dec 162016

GMC SafariMy 2000 GMC Safari van clunks and jerks when I stop or start. And does the same when I turn the wheel all the way either direction.


Difficult to guess without some testing. I would recommend placing the front end securely on jack stands. Turn the wheels from side to side to try and reproduce the issue. Inspect all moving components. Look for loose ball joints and wheel bearings while still listening for clunks. Wiggle the tire from top to bottom and side to side while someone looks for anything loose.


With the vehicle on the ground, in park and the engine running turn the wheel all the way in either direction. Doing this should help recreate the issue and allow someone on the outside to listen for the problem area. Once the area is located a more in depth inspection will be necessary.

Dec 082016

2007 GMC SierraKeep getting a ‘stabiliTrak service and service traction’ error message. The ‘brake pedal sensor switch’ has been replaced and now the message appears only when the engine is idling for more than a few minutes (disappears once I start driving). What could cause the message to keep appearing?


My first guess would be a faulty sensor or sensor connection. However some hands on testing would be needed to locate the issue.

This vehicle is equipped with a BOSCH ABS/EBD/TCS/VSES brake system. The electronic brake control module (EBCM) and the brake pressure modulator valve (BPMV) is serviced separately. The BPMV uses a 4 circuit configuration to control hydraulic pressure to each wheel independently.

The following vehicle performance enhancement systems are provided.

    • Electronic Brake Distribution (EBD)
    • Power Brake Booster Solenoid Vacuum Supply
    • Traction Control System (TCS)
    • Vehicle Stability Enhancement System (VSES)

StabiliTrak® Indicator Light

Stabilitrak LightIf equipped, this warning light should come on briefly when the engine is started.

If the warning light does not come on then, have it fixed so it will be ready to warn you if there is a problem. If it stays on, or comes on when you are driving, there may be a problem with your StabiliTrak® system and your vehicle may need service. When this warning light is on, the system is off and will not limit wheel spin. Adjust your driving accordingly.

This light will also flash when the StabiliTrak® system is active.

If the StabiliTrak® system warning light comes on and stays on for an extended period of time when the system is turned on, your vehicle needs service.

Service StabiliTrak Message

 The message center displays the service stability system message when any one of many VSES-disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.

Service Traction Control Message

 The message center displays the service traction control system message when any one of many traction control – disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.

StabiliTrak Off Message

 The message center displays the stabiliTrak off message when one or more of the following conditions exists.
    • The transfer case is shifted into 4 LO. The EBCM sends a serial data message to the IPC requesting illumination.
    • The driver manually disables the VSES and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the IPC requesting illumination.
    • The estimated temperature of any solenoid coil exceeds an acceptable limit. The EBCM sends a serial data message to the IPC requesting this display.
    • The EBCM detects a failed brake switch. The EBCM sends a serial data message to the IPC requesting this display. A DTC sets when this condition exists.
    • VSES sensor initialization time is excessive. The EBCM sends a serial data message to the IPC requesting this display.
    • Serial data communication between the EBCM and any of several other control modules is interrupted. The EBCM sends a serial data message to the IPC requesting this display or the IPC displays the message when communication with the EBCM is interrupted.
    • The PCM is not able to perform engine torque reduction. The EBCM sends a GMLAN message to the IPC requesting this display. DTCs set when this condition exists.
    • The EBCM detects an excessively low or excessively high ignition voltage. The EBCM sends a GMLAN message to the IPC requesting this display.

Vehicle Stability Enhancement System (VSES)

 Vehicle stability enhancement system (VSES) provides added stability during aggressive maneuvers. Yaw rate is the rate of rotation about the vehicle’s vertical axis. The VSES is activated when the electronic brake control module (EBCM) determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.

The desired yaw rate is calculated by the EBCM using, primarily, the following inputs.

    • The position of the steering wheel
    • The speed of the vehicle
    • The lateral, or sideways acceleration of the vehicle

The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. When a yaw rate error is detected, the EBCM attempts to correct the vehicle’s yaw motion by applying brake pressure to one or more of the wheels. The amount of brake pressure which is applied varies, depending on the correction required. The engine torque may be reduced also, if it is necessary to slow the vehicle while maintaining stability.

VSES activations generally occur in turns during aggressive driving. When braking during VSES activation, the pedal may pulsate. The brake pedal pulsates at a higher frequency during VSES activation than during ABS activation.

VSES Sensors Initialization

 The vehicle stability enhancement system (VSES) sensors values may vary slightly due to differences in temperature, sensor mounting, connector resistances, manufacturing, etc. Since VSES is a very sensitive and precise control system, it is imperative that the electronic brake control module (EBCM) be able to accurately equate a given sensor voltage with an actual unit of measurement. For example, the yaw rate signal of one vehicle may be 2.64 volts at +18.0 deg/sec yaw rate while the yaw rate signal of another vehicle may be 2.64 volts at +17.5 deg/sec yaw rate. Therefore, at the beginning of each ignition cycle, the EBCM must perform an initialization procedure to observe how the VSES sensors are correlated with each other and also to determine what each sensor value is when the applicable unit of measurement equals 0. This voltage is referred to as the sensor bias voltage. Although some activation of the VSES system may occur if required to prior to full initialization, the system does not give optimum performance until the sensors are fully initialized.

The following VSES sensors require initialization:

    • The yaw rate sensor
    • The lateral accelerometer
    • The master cylinder pressure sensor
    • The steering wheel position sensor

When the vehicle speed is greater than 25 km/h (15 mph), full sensor initialization must occur during 3 km (1.8 mi) of driving or 1 km (0.6 mi) of straight and stable driving, whichever occurs first. Although an attempt at initialization may fail due to driving conditions, such as driving on a very winding road, failed initialization is usually caused by a sensor bias voltage which is not within an acceptable range. Often, a DTC sets soon after a failed initialization attempt. The message center displays the stability system disabled message when sensor initialization fails.