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2007 Mazda 6 3.0L V6 automatic with ~80k miles:

  1. Runs "rough," especially while stopped in drive – feels like what I imagine an occasional cylinder misfire would feel like, producing a sort of an unbalanced vibrational shudder.
  2. Recently, in addition to the above, full-load full-throttle feels like it is sporadically cutting fuel to the engine even nowhere near redline and with no traction-control indicator. (Traction control is functional and its light illuminates when it is intervening.)
  3. Behavior has persisted over several tanks of gas. Occurs when engine is both cold and warmed up.
  4. Check engine light is functional but has not illuminated.

My naïve guess would be there's a fuel flow problem. I don't know how to diagnose. If the check engine light is not on could there still be engine codes that would be helpful? Is there a fuel filter that should be replaced? Is there a fuel pump and a way to check that?

Update: I have confirmed that the only fuel filters are in the fuel pump itself. I also found suggestions that driving tanks empty, which I often do, can reduce the pump life by giving it less cooling. Fortunately there is only one fuel pump and it can be replaced without removing the fuel tank. Since it's a $70 part I'm going to try replacing that and note what I find here....

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  • It sounds like it could be a fuel delivery problem. There's no directly replaceable fuel filter, as I believe it is all part of the fuel pump "bucket" assembly. IOW: you'd have to change the fuel pump assembly to replace the fuel filter (if my research is correct). You can always check to see if there is a stored code ... it never hurts. Sometimes the code will store without tripping the CEL if it's something that goes away quickly but is enough to trip it in the first place. Random misfires would pop a code. 80K isn't a lot of mileage, so makes me wonder.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 18, 2021 at 20:32
  • Fuel quality? All tanks from the same place? Some can have water mixed in...
    – Solar Mike
    Feb 18, 2021 at 20:45
  • Any chance that you could hook up a scan tool to read long and short term fuel trims? You may also be able to connect a fuel pressure gauge to see if it’s in spec. A cheap early option is to clean the MAF sensor as it may be under/over reporting airflow causing too much or too little fuel to be injected. Otherwise spark plugs, coils, vacuum leaks etc... anyways please let us know how you get on with troubleshooting this vehicle.
    – ajayel
    Feb 21, 2021 at 5:39

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This one turns out to have been very misleading. Since posting this question the engine roughness got increasingly worse to the point that it felt like it was on the verge of stalling with any load, including just putting the transmission in drive at a stop!

The day I received the replacement fuel pump the check engine light finally came on for the first time. It even began flashing when running at significant engine load. I got to a code reader and found error:

P0301 – Cylinder 1 Misfire Detected

Fix: Swapping the coil on cylinder 1 confirmed it was a bad coil. Replaced that coil. Now the engine is running smoothly. (While at it also replaced all of the original sparkplugs even though they didn't look too bad.)

I find it bizarre that the ECU didn't pick up on the coil failing before. The first signs of trouble in the drive-train showed up 8 months ago and I (now know erroneously) attributed them to a transmission problem. The best guess at that time based on the symptom and known weaknesses with this car model was that the transmission valve body assembly was going bad, but accessing and replacing that would have been too expensive. The power slippage got increasingly worse until just now, and replacing that one coil has mitigated all of those symptoms!

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