By on June 4, 2014

Geoff writes:

Sajeev, I enjoy your TTAC contributions very much. I have a 2002 Dodge Ram with a 5.9 liter V8. Starts every time and idles fine initially. But just when it transitions over from the cold start sequence to Normal running it starts to act as if it is gasping for air.

If I give it a stab of WOT or if I shut it off and restart everything is fine for the rest of the time I drive it which makes me think O2 sensor. But looking at the O2 output it remains high when the engine is stumbling but after the WOT or restart the O2 sensor signal begins cycling up and down as it should. It will throw a CEL if I let it keep going once the stumble starts – but I can’t remember the code at the moment.

Since I know how to replace a starter the restart is my “fix” but any pointers on where to go next? After it throws the light the engine stops sputtering and the idle returns to normal.

Sajeev answers:

“But I can’t remember the code at the moment.”

Writing to an automotive help column without posting the CEL code is like dating a gal/guy that’s a total jerk, but not pretty/handsome enough to justify the psycho jerk-i-tude. You never do this, unless you know your audience both loves you and knows your vehicle like the back of their hand.

Since you asked for pointers, here’s my short list to anyone posting on Piston Slap or some forum where noobs get flamed:

  • Year, Make, Model and relevant options (completed)
  • Mileage (needed)
  • Previous, relevant service history (kinda needed)
  • Engine Codes scanned with your scan tool or the free service at a parts store (mandatory)
  • Research the code here. (definitely appreciated)
  • Ask about your next step on the diagnostic tree after giving us a good slice of the branch.

Without following the above pointers, I can only guess.  So what the hell: there’s a problem with the EGR system, as it happens after warmup.  Maybe it’s a bad sensor/actuator, or some vacuum lines are toast.  Maybe the intake manifold and the EGR need a good cleaning.  But I’ll go with the EGR valve itself, because there’s zero accountability on my side!

Off to you, Best and Brightest!

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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21 Comments on “Piston Slap: The Importance of Enlightened Diagnoses...”


  • avatar
    RS

    They should teach the above pointers in high school.

  • avatar
    Halftruth

    I can see the argument for both EGR and 02. Could also be a bad temp sensor though.
    While the issue I saw was not corrected with a restart, it was similar to this.
    Anyway, we can guess all day. Get us the code and we can start to dig.

    • 0 avatar
      CoreyDL

      I was thinking MAF sensor error.

      • 0 avatar
        Blue-S

        It can’t be a MAF issue, as the 5.9L doesn’t have one. Like most Chrysler products, it has a MAP sensor. I’m pretty sure that the ’02 Ram odometer will display P-codes. You should be able to get the odometer to display codes by cycling the key ON-OFF-ON-OFF-ON.

        • 0 avatar
          CoreyDL

          What is the difference? I’m learning things here.

          • 0 avatar

            MAP sensor senses air pressure in the intake manifold, MAF senses incoming air pressure. MAF is usually more accurate.

            IIRC, if you have a MAF you still have a MAP-style sensor, they just call it a BAP.

            Enjoy googling that. :)

          • 0 avatar
            CoreyDL

            Oh I have seen Halle Berry and that other woman already. They used to play that on Comedy Central often.

            Thanks for the info.

          • 0 avatar
            Blue-S

            A Manifold Absolute Pressure (MAP) sensor is used to measure the air pressure inside the intake manifold. The ECM uses this input to determine how much load is on the engine, and as part of the method to determine intake air density. Intake Air Temperature (IAT) and Crankshaft RPM (CKP) are also used, as well as a programmed Volumetric Efficiency (VE) lookup table. This is called the speed/density engine management strategy. The popular “Megasquirt” DIY engine management system is a speed/density type. By contrast, the Mass Air Flow (MAF) sensor (usually with integrated IAT sensor) is used to measure the mass of air entering the intake manifold. The MAF sensor is the primary “engine load” input on systems so equipped. Most engines will have either a MAF or MAP sensor as a critical input in order to determine the quantity of fuel needed by the engine. Some MAF equipped engines may also have a MAP sensor on the intake manifold which is primarily used to perform certain diagnostic functions, although the MAP may have an influence on fuel quantity as well..

          • 0 avatar
            golden2husky

            Blue-S: good description of injection system designs. One diagnostic key with speed density designs is what happens when the MAP sensor fails. Typically with Mopar systems, the limp mode doesn’t work. The car will start because there is a preprogrammed value the ECM uses. Once the car fires and the ECM is “aware” that the car is running, it looks to the MAP sensor for a value to take the place of the preprogrammed value. When the sensor is bad, the car stumbles and dies immediately after startup for this reason – there is no valid MAP signal. It will restart and die over and over. Interesting to note that if a good sensor is disconnected when the car is off, upon startup the limp mode value does work and the car will run. But if it fails on startup – the typical Mopar failure mode, your dead. I kept a spare in my trunk for that very reason.

  • avatar
    punkybrewstershubby aka Troy D.

    It could be any number of a hundred DIFFERENT THINGS…

    Oh look, my tire is flat. Hmm, it is lacking of air. Could it be a nail, leaking thru the bead, leaking thru the wheel? Dunno.

    Geesh.

  • avatar
    ezeolla

    Since it is a Chrysler product you can read the CEL on the odometer. Once the CEL comes on, turn the truck off. Cycle the ignition from off to on (but not to start, you don’t want to restart the truck) 3 times and it will show the code

  • avatar
    YetAnotherHobby

    Geoff here.

    Sorry about that Sajeev – I got your response and I have been trying to get the bloody thing to throw a code but it just chugs and chugs then settles into a nice idle without throwing a god blasted code. When it begins you can hear an increasing hiss of air at the intake, like it’s sucking air as hard as it can – which made me think IAC…but that’s fine. It’s gotta be temp related because when the weather is cold it will throw the code and if I remember correctly after it’s been huffing and puffing for 5 minutes straight it’s usually some meaningless misfire in cylinder “x” – no surprise since it’s probably running lean as hell. During the event it has no power but if you flutter the gas pedal you can keep it going, barely. I have an AutoTap and the O2 sensor signal is high the entire time it’s misbehaving. After I restart the 02 sensor trace cycles up and down like it should. That is why I am thinking O2….but I never get an o2 code and it passes emissions with flying colors every year.

    Mileage – 82K

    Recent related service, or service occurring around the time it started – none. This has been going on literally for years – I have just gotten used to it. The EGR has never been serviced so that’s actually a good starting point.

    I truly am sorry – I never expected you to use the letter without the CEL – I just didn’t know I had a deadline to meet! I will post the code if I ever get something. I at least should have replied to let you know I was working on it!

    Keep up the good work (no snark intended).

    Geoff


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