Question about 2004 Pontiac Grand Am

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2004 pontiac grand am replaced O2 sensor, spark plugs, plug wires and fuel pressure regulator. starts easily. idles very rough. When accelerating car runs well. When cruising on freeway with engine under light load engine misses. Any ideas what to look for? Thanks

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  • ksouthoc Nov 12, 2010


    Please take a look below. I added some additional information. Thanks
    I have a 2004 pontiac grand am. Replaced O2 sensor, fuel pressure regulator, plugs and wires. Engine idels rough starts easy. idle speed is all over the place. When accelerating under load engine runs well. When cruising on freeway with engine under light load engine misses. Sitting at stop light engine idles very rough and you can smell CO inside. Any suggestions?

    Also replaced the idle sensor that controls the air valve position

    NOTE: With engine running I disconnected the plug to the air flow measurement (not sure exactly what it is called) The engine idle speed jumped to about 1500 and engine smoothed out. With engine still running I plugged it back in and the engine speed did not change. Turned off motor and restarted and it was back to a slow rough idle.

    Thank you



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Idle air control motor(IAC). EGR valve. And vacuum leaks to include the brake booster and if you have a plastic intake, that too. this is a start there are other things that will have the same symptoms. let me know

Posted on Nov 12, 2010

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  • Randy Ohler Nov 13, 2010

    That is called the mass air flow sensor. This could be your problem, but before you change it( high dollar) Was there any codes on the computer, Was the check engine light on. What I woould do is have the codes erased, and start from there. Run the car and when the check engine light comes on , have it checked , get the codes and description. I know some , but there are to many to remember. let me know

  • ksouthoc Nov 13, 2010


    the check engine light is on. The code says multiple cylinder misfire. Not other code can be found. i found a MAF on line for about $55 plus freight. When the fuel pressure regulator failed there was gasoline everywhere. When I took off the rubber boot from the air filter to the intake manifold about a cup of gas poured back into the air filter box. Could gas have gotten into the MAF? Is there any way to test the MAF to see if it has failed?

    Thank you


  • Randy Ohler Nov 13, 2010

    This is the way a maf works. the are two thin wires that you can see in the maf. one mearsures the temperture of the air coming into the hole, the other is heated up to 200 plus degrees. the faster the air comes in cools dow the heated wire, thus giving the effect of how much air (in cubic feet per minute) that is traveling thru it. telling the computer, so the computer adjusts the fuel to meet a 14.7 to 1 ratio. If you ave a vacuum leak between the maf and exhaust system it will throw off the maf reading( not pulling enough air) This will cause your random misfires. I have checked many maf sensor codes and have not replaced one yet. The only way I check the maf is against a new one. You can check it with an ohm meter. Check the resistance on each lead, check all leads with all leads. meaning that if there are eight leads comming off the sensor,then you haveto measure it 64 times. the numbers can be off alittle, but not much. Look at the wires that I mentioned and see if they are broken (You may have to use a magnifing glass). If not then I would look for vacuum leaks before I would put out any more money. Also just to say I clean maf sensors with highly flamible liquid, because they can get contaminated with dirt and oil, and this cleans them the best. let me know



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Try a few if these ideas...

Priority Action Part Type Cause
1 Inspect Spark Plug Fouled, Damaged, or Broken Spark Plug(s).
2 Inspect Hose (PCV) Clogged or Collapsed PCV Hoses.
3 Inspect Wireset Worn, Damaged or Faulty Spark Plug Wire(s).
4 Inspect Distributor Worn, Damaged or Faulty Distributor.
5 Inspect Catalytic Converter Clogged or Faulty Catalytic Converter.
6 Inspect Fuel Pump Low Fuel Pressure.
7 Inspect TimingSet Slipped TimingChain or Worn TimingGear(s).
8 Inspect Knock Sensor Improperly Connected or Faulty Knock Sensor.
9 Inspect M.A.P. (Manifold Absolute Pressure) Sensor Faulty M.A.P. Sensor.
10 Inspect Throttle Position Sensor Faulty or Incorrectly Connected Throttle Position Sensor.
11 Inspect Mass Air Flow (MAF) Sensor Damaged, Loose, or Faulty Mass Air Flow Sensor or Circuit.
12 Inspect Air Cleaner Temperature Sensor Damaged or Faulty Air Cleaner Tempature Sensor.
13 Inspect Fuel Injector Pressure Regulator Faulty Fuel Injector Pressure Regulator.
14 Inspect Piston Ring Set Worn or Broken Piston Rings.
15 Inspect Timing Specification Incorrect Ignition Timing.
16 Inspect Fuel System Pressure Incorrect Fuel Pressure Being Delivered to Carburetor or Fuel Injection System.
17 Inspect Fuel Filter Clogged or Dirty Fuel Filter.
18 Inspect PCV Valve Plugged or Damaged PCV Valve.
19 Inspect Valve Burned, Worn, or Sticking Exhaust Valves.
20 check idle stop solenoid - if faulty, could cause rough idling
21 inspect distributor cap/rotor - if burnt, could cause loss of acceleration

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Check the coil packs and EGR:
Exhaust Gas Recirculation System Operation Application of the Exhaust Gas Recirculation (EGR) gases is regulated by the Powertrain Control Module (PCM). It is based on engine coolant temperature, engine speed, vehicle speed, throttle angle and Manifold Absolute Pressure (MAP). The operation of the EGR system is monitored by the PCM, which allows gases to flow at a calculated rate and compares it against MAP sensor and oxygen sensor activity. The PCM anticipates an increase in MAP voltage and an increase in O2 sensor voltage.
Electrically operated vacuum control solenoids used to control the flow of vacuum to the EGR valve, rely on pulse-width duty cycle commands from the PCM to control its operation. Using input values and the operating conditions previously mentioned the PCM completes the electrical circuit for the solenoid to allow vacuum operation of the EGR.
In addition to solenoids controlling vacuum flow, the incorporation of a back pressure transducer regulates vacuum signals independently of electrical signals. When back pressure in the exhaust system is low, the transducer pintle is open and manifold vacuum is allowed to bleed off. As back pressure increases the amount of vacuum allowed to bleed off is reduced resulting in more vacuum being applied to the valve. This allows air pressure to open the valve.
The following symptoms can be caused by a faulty EGR valve due to loose connection, bad grounds, high resistance in the circuit, or opens in the circuit.
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Diagnosis and Testing Secondary Spark Test These spark tester looks just like a spark plug, attach the clip to ground and crank the engine to check for spark 91052p10.jpg
This spark tester has an adjustable air-gap for measuring spark strength and testing different voltage ignition systems 91052p11.jpg
Attach the clip to ground and crank the engine to check for spark 91052p12.jpg
This spark tester is the easiest to use just place it on a plug wire and the spark voltage is detected and the bulb on the top will flash with each pulse 91052p10.jpg
The best way to perform this procedure is to use a spark tester (available at most automotive parts stores). Three types of spark testers are commonly available. The Neon Bulb type is connected to the spark plug wire and flashes with each ignition pulse. The Air Gap type must be adjusted to the individual spark plug gap specified for the engine. The last type of spark plug tester looks like a spark plug with a grounding clip on the side, but there is no side electrode for the spark to jump to. The last two types of testers allows the user to not only detect the presence of spark, but also the intensity (orange/yellow is weak, blue is strong).
  1. Disconnect a spark plug wire at the spark plug end.
  2. Connect the plug wire to the spark tester and ground the tester to an appropriate location on the engine.
  3. Crank the engine and check for spark at the tester.
  4. If spark exists at the tester, the ignition system is functioning properly.
  5. If spark does not exist at the spark plug wire, perform diagnosis of the ignition system using individual component diagnosis procedures.
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Replacing O2 sensor on 2004 Pontiac Grand Am

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