Flat spot under acceleration

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  • #580129
    Peter BonneyPeter Bonney

      Hi all, 1st post but have been watching ETCG for a year or so. Love the videos 🙂

      Anyhoo, I have a 2005 2.0i Mazda 6, under acceleration i have a flat spot/underpower, it only happens between 1500 & 2200 RPM. After the flat spot i get a surge of acceleration and the car drives normally.
      I 1st started to notice it shortly after i gave the car a mini service and replaced the plugs (with the NGK plugs recommended by Mazda) when i checked the plugs i’d not nipped them up tightly enough, so tightened them again, still the same.
      Then i changed the plug leads and it went away, but in the last week or 2 its started to do it again, after watching the Thermostat replacement video the othe day/week, Eric said if the thermostat is stuck open it can cause problems. I’m wondering if this might be the cause of my problem – I’ve got a thermostat (non genuine) today, but its a pretty big job to attempt at the side of the road (in the rain looking at the forecast) so i was after any other suggestions before i get on with swapping the stat.

      Oh, i’ve been monitoring when the flat spot happens (to see if it ties in with a stat issue) and it seems worse 1st thing in the morning when its cold, and the engine does seem to be running cold – temp gauge, is usually pretty close to the cold side – it is working because the needle moves when i’m in traffic. It also seems to happen a less when its up to temp

      Thanks for your advice in advance – sorry this is so loooooooong!! :huh:

    Viewing 6 replies - 1 through 6 (of 6 total)
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    • #580137
      college mancollege man

        Try doing a power balance test with the injectors.
        If the power balance test shows a cylinder or so with no change.
        confirm the spark with the spark tester You could have a failing
        ignition coil.

        EricTheCarGuy 1EricTheCarGuy

          If you come up to operating temperature in good time, then the thermostat is not likely to be the problem. College man’s advice is pretty good and there’s also a LOT of information in the article he posted about addressing performance problems. You might want to run through those first before you go to the thermostat.

          Keep us posted.


            If the engine is not reaching operating temperature it can cause many different problems so it should be replaced. I would remove the plugs again and check them for the correct gap and make sure there are no cracks or black lines running vertically on the porcelain part of the plug. (Terminal End) Recheck everything you removed when completing the service you performed especially the air intake tubing and ducting. Be sure there is no damage to the air intake tubing as that could cause incorrect Mass Airflow Sensor operation.

            Peter BonneyPeter Bonney

              Thanks all,
              Tried the power balance, and got an RPM drop on each cylinder – however, the engine was hot(when it happens least) and at idle. Is it worth wedging the throttle open at 1500 rpm on a cold engine and repeat (when it happens most)

              Wysetech – I didn’t touch the gap when i fitted the plugs, they are NGK Iridium plugs and it says on the box not to alter the gap in case it damages the iridium (and at £10 a plug I was happy to follow the advice!) For the record its these – (Not sure if these are the exact plugs, but its the same range of plugs)


                If you checked the gap then you can almost rule that out but I have seen gaps closed right up when they came out of the box. Sometimes we accidently drop them also and that might close the gap and we didn’t take the time to look before installing. Just trying to cover all bases.

                Peter BonneyPeter Bonney

                  I visibly checked the gap, didnt use any gauges though – will try your other suggestions, checking the plugs etc.

                Viewing 6 replies - 1 through 6 (of 6 total)
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