Stutter at low throttle openings

Discussion in 'Mazdaspeed 3 Troubleshooting' started by MacheteJames, Sep 3, 2020.

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  1. MacheteJames

    MacheteJames Greenie Member

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    This is a weird one. For the last month or so, I'm noticing this odd stutter at very low throttle openings - essentially, when at cruise or barely accelerating. It feels like a momentary buck or stutter and always happens at between 2 and 3,000rpm. I've checked for vacuum leaks and everything feels tight. I removed and cleaned my MAF sensor, no difference. My plugs have around 8,000 on them and should be fine. No stored codes.

    I went and took a log to demonstrate what I'm seeing, and the throttle position column appears to demonstrate what I mean - my speed holds steady, as my foot isn't moving - but the TP numbers dip down to 7, then up to 12, back to 7, etc several times.

    My first thought is a failing throttle position sensor, or a primary 02 sensor on the way out. Any other ideas? Thanks all.
     

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  2. The_big_dill

    The_big_dill Greenie Member

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    Good diagnostics on your part, it helps.

    I am not familiar with what TPS normally looks like on these cars, but from what I understand, it is not a direct measurement from your gas pedal, that would be you Accelerator Pedal Position (APP). This means that the ECU decides what throttle position you need, mainly to stabilize idle or respond to loads such as A/C.

    There is a history of a poor connection at the throttle body wiring with these cars. See if there is damage to the wiring near the connector. If you don't find anything, double check that the female pins aren't oversized. Finally, try to move around the connector between runs to see if you can get inconsistent results. I have seen people tear apart half their car, when all they had was a poor connection at the TPS.

    Hope that helps.

    Phil.
     
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  3. MacheteJames

    MacheteJames Greenie Member

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    Issue is still ongoing - took it to a shop, and the tech thinks it is VTCS related, as I had a historic fault code triggered by a bad VTCS solenoid that was later replaced, but I'm not sure I agree - datalogging is showing weird, intermittent spikes in STFT that make me think this could be a primary 02 sensor issue. I've attached the log; cells 73 and 75 show what I mean. I'm going to get some voltage readings off of the primary 02 sensor with Torque to see if they are within factory specs.
     

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  4. Sho

    Sho Silver Member

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    My ms6 does this in 6th gear on the highway if barely pushing the throttle to maintain constant speed. Same behavior as in your log. Goes slightly lean and stutters. Goes away with more throttle input.

    I dont have VTCS As I have a JMF manifold.

    I have no boost or vacuum leaks either.

    I attributed it to my larger corksport throttle body. But Who knows
     
    Sho, via a mobile device, Sep 30, 2020
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  5. MacheteJames

    MacheteJames Greenie Member

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    I'm pleased to report that this problem is now fixed. It wasn't cheap, but the stuttering/surging is now undetectable. I replaced the following a few weeks ago:
    • VTCS Solenoid
    • Primary 02
    • New set of stock heat range plugs (was running step colder)
    After this, plus several hundred dollars of dealer diagnostic time (which sucks), drivability has improved drastically. The VTCS system is also now working reliably again and my P2004 code is gone. I want to note that my step colder Denso ITV-22s were already looking a bit cooked after only about 7k miles of daily driving, so back to stock heat range plugs it is. IMO step colder plugs aren't well suited to long periods of driving with minimal or no boost, like I have been throughout much of this year - I must be getting old.
     
  6. The_big_dill

    The_big_dill Greenie Member

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    Thanks for sharing the solution, it's appreciated.

    My money is on the plugs being the main cause. Out of curiosity, was the stutter noticeable when the engine cold and hot?
     
    The_big_dill, via a mobile device, Oct 29, 2020
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  7. Easter Bunny

    Easter Bunny Professional Engineer Motorhead Platinum Member

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    For what's it's worth you should select a heat range for your plugs based on your use of the car not the mods or power level (to a point anyway) if you have colder plugs and commute long distances they are going to foul quickly
     
    Easter Bunny, via a mobile device, Oct 29, 2020
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  8. MacheteJames

    MacheteJames Greenie Member

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    Totally. For a long time, the MS3 gospel held that a bolted K04 car running on 93 should use step colder plugs. I'd always wondered why I would get only 6 to 9 months out of a set. Definitely staying with stock heat range plugs now, given the relatively mundane driving I do most of the time.
     
  9. MacheteJames

    MacheteJames Greenie Member

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    Definitely more pronounced when hot. I wouldn't typically notice it in open loop; only in closed loop and at cruise. It was also never present at about more than 10% throttle (i.e. not ever an issue at WOT).
     
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  10. The_big_dill

    The_big_dill Greenie Member

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    As with anything: grain of salt. I am probably going to get burned for saying this, but I think the OTS maps from COBB get too much flack for what they are. Ofcourse they are nothing compared to a custom tune, but what I read on these forums is a bit too far.

    My heavy footed driving proved that I needed a step colder plugs, like Easter bunny mentioned. But I am pretty sure my fuel economy has suffered due to the gap/colder plugs.
     
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