By on October 3, 2012

TTAC Commentator robstar writes:

Hello Sajeev+Piston Slap Audience,

A few weeks ago I saw something strange with my lightly-used-never-abused 2005 Subaru WRX STi — A “R. Diff Temp” started flickering on the dashboard. This particular day it was quite cold and it came on as I reversed out of the garage backward down a sloping driveway. After going one block (in forward, not reverse) with this behavior, I turned the car off and let it sit for a minute. After turning the car back on, this didn’t happen again for about 3 weeks — until after the mechanic changed the oil. I drove the car back to the mechanic who looked under the car & thinks the sensor is going bad (7 year old car, 45k miles, ~ full synthetic oil change 3 times/year). He said he can’t see any sign of an Rear Diff leak so it should be OK to drive home (40 miles). After the first 7 miles or so mostly city the light turned off & was completely off for the last 33 or so miles all the way home. The only prior issue I’ve had with this car was an engine valve spring at 10k that was replaced under warranty.

I’ll admit — I’ve been a bad on the maint side, however this car sees about 80% highway usage typically at 55-65mph. No clutch dumps, no redlining — rev matching when downshifting……I pretty much baby it. So why did I buy this car? For fun in the snow.

I am going to have him actually swap out the R. Diff fluid when he replaces the sensor, but do you guys think it’s simply just a bad sensor or some other problem?

To make things easy, the car has NEVER been modified (Yes, it’s probably the only Stock WRX STi that exists). The only parts different from stock are that I now have winter tires (stock size) on steel rims (stock size) for the winter here in Chicago.

Thanks Sajeev!

Sajeev answers:

Is there a chance that, during the fluid change,  the sensor was (unnecessarily) removed? And over-torqued? And it broke? Oh wait, the “after the mechanic changed the oil” remark was about the engine, not the differential. Right?

Sensors can go bad at any point, that’s the beauty of a somewhat fragile/complex piece of electronics living in a rather brutal environment.  I can’t Google a good reason for why your sensor is out of range, so maybe the control module is at fault. Or maybe the ground is dirty and no longer doing its job.  Or the wiring harness is damaged somewhere. This thread on NASIOC looks pretty comprehensive for your diagnostic needs.

So there you have it. Good luck with all that.

EDIT: A far superior title came to me, thanks to the B&B.  Explanation here.

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11 Comments on “Piston Slap: An STI with A “C. diff”???...”

  • avatar
    Athos Nobile

    “Yes, it’s probably the only Stock WRX STi that exists”

    Nah, I see plenty of stock WRX and STi cars down here.

    Actually, yesterday I saw one driven by a señora while I was putting petrol. Bone stock.

  • avatar

    Funny, “C. Diff” has a totally different meaning in the medical community.

    The jokes just write themselves for the small cadre of medical types on TTAC…..

    • 0 avatar

      “Yeap, I have an sti with a c. diff”

      Thanks, I’m here all week.

    • 0 avatar

      I saw that, too. At least in this case, it looks like you can lower the C.Diff using a rotary control. Potential medical breakthrough?

    • 0 avatar

      Wow, just wow. Ya know, you’d like to think you can cover every angle in the 1-2 hours budgeted to write these Piston Slaps…and then this happens.


    • 0 avatar
      kid cassady

      The newest touted experimental treatment for Clostridium difficile infections involves a fecal enema, which is just as it sounds – some poop lovingly donated from one of the patient’s relatives is introduced to their GI tract, allowing the natural bacterial flora to proliferate and suppress growth of the foreign bacteria.

      Perhaps this new-found knowledge could be employed to help this gentleman and his Subaru.

  • avatar
    comrade slow

    uneven tire wear or mismatched tires can cause the differentials to overheat… so says the manual. Or possibly it’s a bad sensor/bad connection. If it really has been overheated, I would think you will see evidence when you flush the fluid.

    … and go put some miles on that car while you are at it! STi’s are built to be flogged daily.

  • avatar

    I do think it was an engine oil change, but the sensor does have to be removed to change the rear diff fluid because it is crewed into the fill hole.

    If you haven’t already changed the fluid do so when you replace the sensor. Be sure to use the LSD fluid that is required.

  • avatar

    This happened on my 07 Tribeca. On the top of the rear diff, there is an electrical connector. It is for the sensor. I pulled off the connector, cleaned up the contacts, reinstalled. Have not seen the light since. The contacts were slightly corroded, which would explain why it was intermittent.

  • avatar

    OP here>

    So I had this sensor replaced on March 1st of this year and this seems to solve the issue. I had the rear diff fluid changed when the sensor was changed.

    Around mid-august the same problem happened AGAIN. I took it back to my indy mechanic who talked to Subaru and Subaru replaced the part under warranty. I’m now on my 3rd rear-diff temp sensor and the problem hasn’t reoccurred.

    So far so good since. Hoping this one lasts at least a couple of years.

    @HankScorpio> It was a while back but I’m pretty sure that this was checked by my indy. If it happens again I’ll make sure he checks.
    @Mbella> Did it. I replaced the rear-diff fluid with whatever subaru had specced, royal purple brand.
    @comrade slow> I did have 2 rear tires replaced a few years after buying the car (sub 20k miles, split over 2 entire sets of tires), but this happened WAY after. In any case, I had switched to 4 new identical tires prior to this problem happening.
    @doctor> Now if only the dccd was cdc instead….
    @athos> i’ve met a few other STi owners and the first question I always get is ‘what mods have you done?’. When I tell them ‘none’ their jaw drops open. Midwestern thing?

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