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GMC engine repair questions and answers

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  • Paul
    Paul - 2007 GMC Sierra 2500 HD - Engine - 10/19/2014
    P0171 & P0174 DTC coming up
    What would cause a P0171 & P0174 DTC to come on only at idle?
     
    • Jimm
      10/19/2014 Jimm
      P0171 = System Too Lean (Bank 1)
      Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes.

      This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code.

      A code P0171 may mean that one or more of the following has happened:
      •The MAF (Mass Air Flow) Sensor is dirty or faulty
      Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
      •There could be a vacuum leak downstream of the MAF sensor
      •Possible cracked vacuum or PCV line/connection
      •Faulty or stuck open PCV valve
      •Failed or faulty oxygen sensor (bank 1, sensor 1)
      •Sticking/plugged or failed fuel injector
      •Low fuel pressure (possible plugged/dirty fuel filter!)
      •Exhaust leak between engine and first oxygen sensor

      Possible Solutions:
      A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you're on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:
      •Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with MAF-specific spray cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling.
      •Inspect all vacuum and PCV hoses, replace/repair as required
      •Inspect all hoses and connections in the air intake system
      •Inspect and/or test the intake manifold gaskets for leakage
      •Check for a dirty fuel filter and proper fuel pressure
      •Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
      •If you have access, you may want to run a smoke test

      A code P0174 may mean that one or more of the following has happened:
      •The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors can leak the silicone potting material used to protect the circuitry.
      •There could be a vacuum leak downstream of the MAF sensor.

      Possible solutions include:
      •In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with MAF-specific spray cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling.
      •Inspect all vacuum and PCV hoses, replace if necessary
      •Check for a dirty fuel filter and proper fuel pressure
  • Mark Judd
    Mark Judd - 1994 GMC Safari - Engine - 10/18/2014
    Why does vehicle idle well but misses and hesitates and eventually die at higher r.p.m.
    basic tune up with plugs wires rotor and cap were installed and although it helped some vehicle is still performing same as above
     
    • donnydonnyd
      10/17/2014 donnydonnyd
      Could be a fuel filter problem.
    • Jimm
      10/18/2014 Jimm
      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific code for more assistance. There may be a fault code stored in the OBD memory which may not be displayed as a check engine light.

      Possible Causes of Engine Hesitation or Stumble:

      Dirty fuel injectors (cleaning the injectors often fixes this)
      Dirty air filter (change filter)
      Dirty / clogged fuel filter (change filter)
      Bad MAP (manifold absolute pressure) sensor
      Bad TPS (throttle position) sensor
      Bad or dirty MAF (mass airflow) sensor
      Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
      Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
      Bad gasoline (fuel contaminated with water or too much alcohol)

      Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

      Dirty or worn spark plugs
      Bad plug wires
      Weak ignition coil
      Wet plug wires
  • Steve
    Steve - 2001 GMC Sierra 1500 - Engine - 10/16/2014
    What would cause the battery to completely drain while the truck is off. I replaced the old battery with new one
    I replaced old battery with new one. By the next morning my new battery was dead. What would cause it
     
    • Zpaxson
      10/16/2014 Zpaxson
      Did the new battery completely drain? Your battery could have been running low already. A battery will always drain a small amount when the car is off, but that is normal. If you left an accessory light on that would cause a faster drain. To test it with a voltmeter you could remove the negative battery terminal, hook a lead up to the battery and one to the cable. You should read a small number around 50 mV. If it is larger you have a drain and need to start pulling fuses until the voltage on the meter drops. Then you know what circuit you need to look further in depth to.
  • Steve
     
    • alex
      10/14/2014 alex
      a bad alternator to a bad ground. are you sure the battery is going dead. unhook the cables and charge the battery when you hook the cables back up see if they spark bad a little is ok after that if it does stick a amp clamp on it and stat pulling fuzes untill the amps drop down.
  • Steve
    Steve - 1995 GMC Safari - Engine - 10/12/2014
    Van fluctuates when idle. Goes dead at stop. What could be problem?
    Starts fine...after driving awhile goes dead at stopping and starting high millage
     
    • Jimm
      10/12/2014 Jimm
      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific code for more assistance. There may be a fault code stored in the OBD memory which may not be displayed as a check engine light.

      Possible Causes of Engine Hesitation or Stumble:

      Dirty fuel injectors (cleaning the injectors often fixes this)
      Dirty air filter (change filter)
      Dirty / clogged fuel filter (change filter)
      Bad MAP (manifold absolute pressure) sensor
      Bad TPS (throttle position) sensor
      Bad or dirty MAF (mass airflow) sensor
      Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
      Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
      Bad gasoline (fuel contaminated with water or too much alcohol)

      Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

      Dirty or worn spark plugs
      Bad plug wires
      Weak ignition coil
      Wet plug wires
  • evelyn
    evelyn - 1999 GMC Yukon - Engine - 10/12/2014
    Why engine loses power when accelarated after changing plus,wires and fuel filter?
    ran fine before just had a lil mis every now and then. wires were bad and fuel filter was full of black gas
     
    • Jimm
      10/12/2014 Jimm
      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific code for more assistance.
      Note: there may be a fault code stored in the OBD memory which may not be displayed as a check engine light.

      Possible Causes of Engine Hesitation or Stumble:

      Dirty fuel injectors (cleaning the injectors often fixes this)
      Dirty air filter (change filter)
      Dirty / clogged fuel filter (change filter)
      Bad MAP (manifold absolute pressure) sensor
      Bad TPS (throttle position) sensor
      Bad or dirty MAF (mass airflow) sensor
      Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
      Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
      Bad gasoline (fuel contaminated with water or too much alcohol)

      Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

      Dirty or worn spark plugs
      Bad plug wires
      Weak ignition coil
      Wet plug wires
  • evelyn
    evelyn - 1999 GMC Yukon - Engine - 10/12/2014
    Why engine loses power when accelarated after changing plus,wires and fuel filter?
    ran fine before just had a lil mis every now and then. wires were bad and fuel filter was full of black gas
     
    • Jimm
      10/12/2014 Jimm
      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific code for more assistance.
      Note: there may be a fault code stored in the OBD memory which may not be displayed as a check engine light.

      Possible Causes of Engine Hesitation or Stumble:

      Dirty fuel injectors (cleaning the injectors often fixes this)
      Dirty air filter (change filter)
      Dirty / clogged fuel filter (change filter)
      Bad MAP (manifold absolute pressure) sensor
      Bad TPS (throttle position) sensor
      Bad or dirty MAF (mass airflow) sensor
      Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
      Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
      Bad gasoline (fuel contaminated with water or too much alcohol)

      Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

      Dirty or worn spark plugs
      Bad plug wires
      Weak ignition coil
      Wet plug wires
  • evelyn
    evelyn - 1999 GMC Yukon - Engine - 10/12/2014
    Why engine loses power after changing plugs wires and fuel filter
    i had a little miss and bad plug wires before. now it loses power when eccelarated. i went by auto zones requirments for the plug gap which was .060 .the fuel filter i took off had black color to the gas that came out of it.
     
    • Jimm
      10/12/2014 Jimm
      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific code for more assistance.
      Note: there may be a fault code stored in the OBD memory which may not be displayed as a check engine light.

      Possible Causes of Engine Hesitation or Stumble:

      Dirty fuel injectors (cleaning the injectors often fixes this)
      Dirty air filter (change filter)
      Dirty / clogged fuel filter (change filter)
      Bad MAP (manifold absolute pressure) sensor
      Bad TPS (throttle position) sensor
      Bad or dirty MAF (mass airflow) sensor
      Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
      Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
      Bad gasoline (fuel contaminated with water or too much alcohol)

      Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

      Dirty or worn spark plugs
      Bad plug wires
      Weak ignition coil
      Wet plug wires
  • smills98823
    smills98823 - 1996 GMC K1500 Suburban - Engine - 10/11/2014
    Leak by canister by the firewall. Also A/C lines underhood are freezing up without using A/C.
    Just within the past few days a leak has developed at a connection to a canister by the firewall on the passenger side of the truck. Then just today 10-11-2014 the A/C lines frosted up a lot. All this is going on while the A/C is not in use. It also died on me at a ...
     
    0 answer
  • smills98823
    smills98823 - 1996 GMC K1500 Suburban - Engine - 10/11/2014
    Leak by canister by the firewall. Also A/C lines underhood are freezing up without using A/C.
    Just within the past few days a leak has developed at a connection to a canister by the firewall on the passenger side of the truck. Then just today 10-11-2014 the A/C lines frosted up a lot. All this is going on while the A/C is not in use. It also died on me at a ...
     
    0 answer
  • Clay
    Clay - 2005 GMC Sierra 1500 - Engine - 10/9/2014
    I've changed the thermostat. It is still running hot. Ny heater stopped blowing hot after I changed the thermostat.
    It started doing it on rare periodical times. Now its does it parked out stop and go traffic. Any ideas. Please help.
     
    • Jimm
      10/9/2014 Jimm
      The most common cause of overheating is because of an external leak from a water pump or a radiator or heater hose. The way to find the leak is to wait till it is overnight cold and maybe just pour water in and see if the leak is large enough to see it simply leak out from where it has a leak. if you pour in a half gallon or more and you still see no leak its time to pressure check the system.

      Here is a short list of possible causes for overheating;

      Low coolant level
      Leaking coolant
      Improper coolant concentration (not at recommended 50 / 50 mix of anti-freeze and water)
      Clogged radiator hoses
      Clogged heater hoses
      Clogged radiator passages
      Clogged heater core
      Faulty thermostat
      Faulty water pump
      Faulty radiator cooling fans
      Faulty radiator cooling fan switch (coolant temperature sensor)
      Faulty / worn serpentine drive belt
      Faulty / worn drive belt tensioner
  • Clay
    Clay - 2005 GMC Sierra 1500 - Engine - 10/9/2014
    I've changed the thermostat. It is still running hot. Ny heater stopped blowing hot after I changed the thermostat.
    It started doing it on rare periodical times. Now its does it parked out stop and go traffic. Any ideas. Please help.
     
    • Jimm
      10/9/2014 Jimm
      The most common cause of overheating is because of an external leak from a water pump or a radiator or heater hose. The way to find the leak is to wait till it is overnight cold and maybe just pour water in and see if the leak is large enough to see it simply leak out from where it has a leak. if you pour in a half gallon or more and you still see no leak its time to pressure check the system.

      Here is a short list of possible causes for overheating;

      Low coolant level
      Leaking coolant
      Improper coolant concentration (not at recommended 50 / 50 mix of anti-freeze and water)
      Clogged radiator hoses
      Clogged heater hoses
      Clogged radiator passages
      Clogged heater core
      Faulty thermostat
      Faulty water pump
      Faulty radiator cooling fans
      Faulty radiator cooling fan switch (coolant temperature sensor)
      Faulty / worn serpentine drive belt
      Faulty / worn drive belt tensioner
  • RAlph
    RAlph - 1996 GMC C1500 - Engine - 10/8/2014
    Oil leak at oil pan .. can pan gasket be replaced without pulling pan completely?
    5 qt capacity
     
    • Jimm
      10/8/2014 Jimm
      Depends on the available clearances - between the engine and the lower crossmember. Also, the oil pump is typically replaced at the same time the oil pan is lowered - cheap insurance.

      Check the various on-line vendors; www.autopartswarehouse.com, www.RockAuto.com, www.partsgeek.com, to list only a few. For example, they (RockAuto) list the oil pan gasket set from around $12.70 for your vehicle.
  • Mikep
    Mikep - 2003 GMC Yukon XL 1500 - Engine - 10/7/2014
    Check engine light on
    Was filling up with gas drove away with Mosel and hose left I'd gas tube . Check engine light now on . Replaced gas cap
     
    • Jimm
      10/6/2014 Jimm
      The check engine light is located on the dash and stays on if the computerized engine control system detects an engine performance or emissions problem.

      First step - have the stored fault codes or check engine light scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA, etc. Then, post a reply as 'add answer' with the specific code for more assistance. There may be a fault code stored in the OBD memory which may not be displayed as a check engine light. There are literally hundreds of possible fault codes and therefore numerous reasons for the MIL condition.

      This step is important - retrieving the stored fault code and relaying this detail - will provide good information to assist in diagnosing and repairing the issue you are having.
    • Davo Cue
      10/6/2014 Davo Cue
      what is your question?
    • Jimm
      10/7/2014 Jimm
      I believe the question is: why is the CEL (check engine light on)? Please refer to the original answer and first step for detail.
  • celticnight
    celticnight - 1997 GMC Yukon - Engine - 9/30/2014
    Yukon help no start no check engine light
    I have a 97 yukon 5.7L it won't start no spark down the line. No check engine light. I tryed hooked up a obd2. Powers it but wont read it. Fuel pump dose not come on but in the fuel pump relay if I bypass the ecm port it turns on. I checked all fuses and thay are good.
     
    • jdl
      9/30/2014 jdl
      Did you check the ecm B fuse under the hood? Not only check the fuse, check voltage on fuse circuit, hot all the time. My hearing isn't much good, I can't hear the fuel pump run with engine cranking.

      Also, there shows to be a fuel pump/oil pressure switch. It's a redundant power supply to the fuel pump in parallel with relay. If switch is functional once you crank it enough to bring up oil pressure and the switch closes, the fuel pump will run, regardless of relay.

      During initial prime, no crank, the pcm sends voltage to control side of fuel pump relay, the ground for control side of relay is constant ground. If no initial prime because computer doesn't send voltage to control side of relay, Wiring circuit problem, voltage or ground, relay problem, computer problem.

      Does the battery have a full charge?