Dec 082016

2007 GMC SierraI keep getting an engine light for my gas cap. It is not a GM part, which costs $35. Is there a sensor for the gas cap that can be disconnected?


The check engine light comes on when a Diagnostic Trouble Code has been detected. A trouble code for a small evap(evaporative system) leak comes up and the most common reason for this code would be a loose or failed gas cap. If you have just filled the fuel tank I would recommend tightening the cap and driving it a few days to see if it doesn’t cycle back off. The Gas cap can be tested with the proper equipment. Also the evap leak can be located with a smoke machine.

Check Gas Cap Message

 The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.

There is not a sensor that can be disconnected that pertains to the gas cap.

EVAP System Operation

 The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test

 This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.
Nov 242016

GMC SafariI’m trying to figure I have brake lights and directional light no nightlife no night light I change the receptacle with only two wires in the with only two wires in the rain and what am I doing wrong?


Check the fuses and then the wiring for good ground and current. Then check the switch, wiring harness and connections for no night light.

fuse box diagram 1986 GMC Safari Van - No Night Light

Light operation – No Night Light

The headlight switch is a push button switch to turn the lights on and off. It is is located on the left side of the instrument panel. A rheostat dial, located with the headlight/parking light switch, is used to control the illumination of the instrument panel. It is also used to control the night light.

A dimmer switch (part of the combination switch), to control the Hi and Lo beam operation, is located in the steering column; the lights are changed by pulling the lever toward the driver.

Nov 232016

2012-GMC TerrainThe entire display and radio quit working, all fuses are good,? thanks


The fuses should be Hot at all times. This means you should see battery voltage at each fuse on both sides of the fuse. Double Check these fuses.

Radio Quit – Check These Fuses First

  1. F5DA – 10 amp – located in the instrument panel fuse block(center console)
  2. F31DA – 30 amp – located in the instrument panel fuse block(center console)
  3. F7DA – 5 amp – located in the instrument panel fuse block(center console)
  4. F15DA – 10 amp – located in the instrument panel fuse block(center console)
  5. F19DA – 10 amp – located in the instrument panel fuse block(center console)
  6. F9DA – 20 amp – located in the instrument panel fuse block(center console)
  7. F33DA – 10 amp – located in the instrument panel fuse block(center console)

If all the fuses show good and but you do not see battery voltage at the fuse, trace the wiring going tot he fuse and repair. If you see battery voltage at the fuses on both side, check wiring going to the display and radio. Check the connections. Repair as needed. If wiring and connections check out, it may necessary to replace the radio and or the display.

You may want purchase a full set of wiring diagrams to assist in tracing the circuit.

Nov 202016

1998 GMC SonomaCan the lower intake manifold gaskets be replaced without removing the upper intake from the lower? Or can I pull the entire intake as an entire unit?


Yes. The intake manifold may be removed as an assembly. Do not remove the specific intake manifold components unless component service is required.

intake manifold

Intake Manifold Removal

  • Remove the evaporative emission (EVAP) canister purge solenoid valve harness.
  • Push the quick disconnect clip and hold in place.
  • Pull outward on the harness elbow.
  • Take off the engine coolant temperature (ECT) sensor wire connector (if equipped) from the engine wiring harness bracket.
  • Remove the lower manifold bolts.
  • Remove the manifold assembly.
  • Discard the lower manifold gaskets.

Important The intake manifold may be removed as an assembly. Do not remove the specific  manifold components unless component service is required.
Do not allow dirt or debris to enter the fuel system. Ensure that the ends of the fuel system are properly sealed.
Do not disassemble the Central Sequential Fuel Injection (SFI) unit, unless service is required.

• Inspect the lower intake manifold for the following:

– Cracks or damage

– Damage to the gasket sealing surfaces

– Damage to the threaded bolt holes

– Restricted exhaust gas recirulation (EGR) system passages

– Restricted cooling system passages

Oct 312016

1990 GMC SierraI just had a mechanic put in a used rebuilt set of gears in the rear end of my 1991 GMC 1/2 ton 4×4 truck. The problem we are having is when you drive it, it starts out fine then there is a jolt and it acts like it wants to go in revere.
He brought it back to his shop and picked it up and spun the wheels, they seemed fine and then Wham, loud noise and the wheels stop and wanted to go in revere. He is stumped he has never came across this before what could of happened?


Is the Driveshaft disconnected when spinning the wheels up in the air?

Does the rear end have a Locker Unit? Limited Slip? Or Posi-traction?

Sounds like something has backed off and is periodically catching. Remove Drive Shaft. Drain rear end fluid and remove cover. spin and inspect the work. Make sure the axle clips are not backing out or damaged.

1991 GMC K1500 Rear End Cover

rear end

Sep 282016

1999 GMC SafariI Am Unable To Get The Back Hatch Open So The Doors Can’t Open. I Don’t Have Auto Lock & Unlock As It Won’t Work. The Hatch Won’t Open With The Key Either. I Need Info To Remove Inside Door Trim Panel.


Rear Door Trim Panel Removal with Liftgate


Rear Door Trim Panel Removal with Liftgate

Sep 262016

2008 GMC SierraThe air conditioning compressor froze up and can’t afford to fix it right now. How can I drive it without the loud noise or hurt it?


Remove the Air Conditioning belt.

air conditioning belt diagram 2008 GMC Seirra


  • Remove the accessory drive belt.
  • Install a ratchet into the air conditioning (A/C) belt tensioner adapter opening.
  • Rotate the A/C belt tensioner clockwise in order to relieve tension on the A/C belt.
  • Remove the A/C belt from the pulleys.
  • Slowly release the tension on the A/C belt tensioner.
  • Remove the ratchet from the A/C belt tensioner.



Aug 042016

2003 GMC DenaliI had my truck in the shop to get the door fixed. While it was there the battery died due to not being able to shut the door. The mechanic charged it for me and it started fine however when I was in drive or reverse and not giving it gas, it would die. It would restart fine. I also noticed that while in gear and no gas applied the battery gauge would drop. While in neutral or park it idled fine. Today and at a stop sign it died and would not restart. It would turn over but as soon as it caught it died. It started right up with a jump but once the cables where removed it ran for a second or two and died. Took the battery and alternator in and both tested good. What could it be?


Most likely the battery is bad. And odds are the battery is beyond 4 years old anyway. Once a battery is run completely flat it is difficult to get it live much longer. Also jump starting a battery also causes more damage internally. I recommend replacing the battery first.  And yes, I saw that the test said it was good. I have seen many batteries test good that really are not. Usually if the alternator is faulty, the battery (not charging) light will illuminate on the dash display.

Jul 262016

transmission filter changeWhat are the signs of a dirty transmission filter. When  is it time for a transmission filter change?


Shouldn’t be any signs, most transmission filters are changed by age instead of fluid color.

It is recommended that the transmission filter and fluid be changed every 12,000 miles according to the manufacturer. for 3/4 ton trucks and larger. Every 15,000 miles for 1/2 ton trucks.

Use Dexron lll(r) or its superseding fluid type.

Check the level of the fluid at least once a month. The fluid level should be checked with the engine at normal operating temperature and running. If the truck has been running at high speed for a long period, in city traffic on a hot day, or pulling a trailer, let it cool down for about thirty minutes before checking the level.

  1. Park the truck on a level surface with the engine idling. Shift the transmission into  P and set the parking brake.
  2. Remove the dipstick (on newer models, you may have to flip up the handle first), wipe it clean and reinsert if firmly. Be sure that it has been pushed all the way in.
  3. Remove the dipstick and check the fluid level while holding it horizontally. All models have a HOT and a COLD side to the dipstick.

Transmission Filter Change and Fluid 1988 GMC 1500

  • The fluid should be drained with the transmission warm. It is easier to change the fluid if the truck is raised somewhat from the ground, but this is not always easy without a lift. The transmission must be level for it to drain properly.
  • Place a shallow pan underneath to catch the transmission fluid (about 5 pints). Loosen all the pan bolts, then pull one corner down to drain most of the fluid. If it sticks, VERY CAREFULLY pry the pan loose. You can buy aftermarket drain plug kits that makes this operation a bit less messy, once installed.NOTE: If the fluid removed smells burnt, serious transmission troubles, probably due to overheating, should be suspected.
  • Remove the pan bolts and empty out the pan. On some models, there may not be much room to get at the screws at the front of the pan.
  • Clean the pan with solvent and allow it to air dry. If you use a rag to wipe it out, you risk leaving bits of lint and threads in the transmission.
  • Remove the filter or strainer retaining bolts. On the Turbo Hydra-Matic 400, there are two screws securing the filter or screen to the valve body. A reusable strainer may be found on some models. The strainer may be cleaned in solvent and air dried thoroughly. The filter and gasket must be replaced.