Piston Slap: An STI With A "C. Diff"???

Sajeev Mehta
by Sajeev Mehta
piston slap an sti with a c diff

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.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.

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3 of 11 comments
  • HankScorpio HankScorpio on Oct 03, 2012

    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.

  • R H R H on Oct 04, 2012

    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?

    • Rum Rum on Oct 05, 2012

      Not a cheap sensor either, was it? like, $250? I've had nightmares about breaking the thing when I'm changing the diff oil.

  • ToolGuy CXXVIII comments?!?
  • ToolGuy I did truck things with my truck this past week, twenty-odd miles from home (farther than usual). Recall that the interior bed space of my (modified) truck is 98" x 74". On the ride home yesterday the bed carried a 20 foot extension ladder (10 feet long, flagged 14 inches past the rear bumper), two other ladders, a smallish air compressor, a largish shop vac, three large bins, some materials, some scrap, and a slew of tool cases/bags. It was pretty full, is what I'm saying.The range of the Cybertruck would have been just fine. Nothing I carried had any substantial weight to it, in truck terms. The frunk would have been extremely useful (lock the tool cases there, out of the way of the Bed Stuff, away from prying eyes and grasping fingers -- you say I can charge my cordless tools there? bonus). Stainless steel plus no paint is a plus.Apparently the Cybertruck bed will be 78" long (but over 96" with the tailgate folded down) and 60-65" wide. And then Tesla promises "100 cubic feet of exterior, lockable storage — including the under-bed, frunk and sail pillars." Underbed storage requires the bed to be clear of other stuff, but bottom line everything would have fit, especially when we consider the second row of seats (tools and some materials out of the weather).Some days I was hauling mostly air on one leg of the trip. There were several store runs involved, some for 8-foot stock. One day I bummed a ride in a Roush Mustang. Three separate times other drivers tried to run into my truck (stainless steel panels, yes please). The fuel savings would be large enough for me to notice and to care.TL;DR: This truck would work for me, as a truck. Sample size = 1.
  • Art Vandelay Dodge should bring this back. They could sell it as the classic classic classic model
  • Surferjoe Still have a 2013 RDX, naturally aspirated V6, just can't get behind a 4 banger turbo.Also gloriously absent, ESS, lane departure warnings, etc.
  • ToolGuy Is it a genuine Top Hand? Oh, I forgot, I don't care. 🙂