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Subaru Legacy, Subaru Outback, Sedan, Wagon
#1 of 34 2009 Subaru Legacy Limited - TPMS Light
Feb 17, 2010 (6:50 am)
The TPMS light is pretty persistent on my dashboard over the last few weeks. I've been measuring the air on a cold tire before it's driven per the door jamb on the car:
Fronts 33 psi, Rears 32 psi.
All of the tires are within 1-2 lbs of the recommended limit, with the exception of the right rear tire (passenger side rear tire), which came it at a surprising 25 psi.
Several times, I have added air to the tire to bring it to the 32 psi, and it will last for 1-2 days, and the light will return.
I've inspected the tire with a flashlight, run my hands over the treads, and I can't find any debris or other issues that could indicate a leak (like hitting a nail, etc.) So, this post is more of a "what exactly does my TPMS sensor do on my car" question.
Does the TPMS have the ability to "release" the valve to let out excess air pressure? My thought is that if the sensor is faulty, and it thinks that there's too much air, if it's releasing some to get it in check. I've had a relative that had this issue on their `08 Nissan Sentra and it turned out that the sensor was faulty.
I'm not sure if the Subaru ones are as complex or complicated, and either way I will be bringing it up to the dealer when I bring it in for my 18,750 mi service in a week or two.
Any thoughts? Thanks.
#2 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [pilot1226]
Feb 17, 2010 (9:56 am)
As far as I know, the TPMS is simply a "measure and report" system, and not interactive or self adjusting in any way. While it is integral with the schrader valve, standard parts like a valve core and seals should be there preventing air from exiting. Most likely you simply have a leak. Could be a bead or rim problem, or the TPMS/valve itself, but it shouldn't be losing air by design.
#3 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [pilot1226]
Feb 17, 2010 (10:02 am)
I learned to spray soapy water on the tire and look for the bubbles to indicate a slow leak. Air it up first, though.
Could be a nail that stayed in, and only leaks when the car is moving, or when the tire temps rise.
#4 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [fibber2]
Feb 17, 2010 (1:47 pm)
Can you explain the bead/rim and how it affects the tire pressure?
Reason I ask is because either myself or my wife hit a pretty serious pothole in April of `09 and the sidewall bubbled. We had the tire replaced shortly after that and we were told the "rim was bent bad, but we will use a bead sealer on it" to fix the problem. I was told that if the sealer didn't work, we would have to purchase another rim, which we were given an estimate of around $1000. (To which, I replied, better fix it with the bead sealer!)
Not another word was spoken about the rim and we've had no issues until the cold weather started up around November of 2009, in which case I started this routine of putting more air in the tire.
I know for a fact I've had some rotations since then, but I'm not sure if it's the same one as the one that was replaced. I can see if it has a different DOT number and see which tire it's on. But otherwise, yeah, I was thinking a slow leak from a nail that's still imbedded in the tire.
I'll go grab a flashlight and go take a peek at the tires now.
So, my last question is, what exactly does bead sealer do, when is it used, and is it a permanent solution, or is it possible I need to get more of it reapplied assuming that the rim in question is the issue?
I wonder if I could find a rim from a junker nearby, if it came down to that.
#5 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [pilot1226]
Feb 17, 2010 (3:19 pm)
The bead is essentially the contact between the rim and the tire. When air pressure is applied to the inside of the tire, it forces the tire against the lip on the rim and creates a seal. When the rim's lip is bent, it can result in uneven pressure on this sealing point and allow air to leak. A bead sealer acts as a gasket between the tire and the rim to help ensure a seal. It does not actually fix the bent rim, but it can prevent the tire from leaking at that point.
It is entirely possible that the leak is coming from this rim and cold weather would certainly exacerbate it. When things get cold, they shrink.
OEM rims are ridiculously overpriced. If you can find a rim off a wrecked example for a reasonable cost, that would be the way to go. Otherwise, you could likely get a full aftermarket set for much less than $1000. It is not a time-critical issue, though, as long as you are aware of it and continue to check the pressure in that tire.
#6 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [pilot1226]
Feb 17, 2010 (3:27 pm)
I think it is likely that you have identified the cause...
The bead seal is where the stiffened edge on the tire has to make an airtight seal against the inner lip of the metal wheel/rim (alloy or steel). In the old days, or on most bicycles, the air is held in by an inner tube. "Tubeless" technology relies on a rubber to metal seal to do the same. During mounting, a slippery water based glue helps promote the seal, and from then on, air pressure holds it all in place.
If the wheel no longer has a perfect mating surface, a more potent glue is used. But if everything is too compromised, large changes in shape (think winter) could pull it very slightly apart. Or, it could have even developed a hairline fracture that is giving way over time. If it was visibly damaged in the pothole incident, you should be able to see and identify which one. If it is this rim, you need to replace it, NOW! Please realize that you are playing with fire. A sudden a blowout and crash will ruin your whole day.
I would expect the cost of an OEM alloy wheel probably runs about $300 - $450. You might be able to find one cheaper on-line. Steel wheels are under $100. A quote of $1000 sounds crazy!
#7 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [xwesx]
Feb 17, 2010 (3:30 pm)
Hey Wes! I got interrupted, and when I finished and posted, you got there first. Only thing I disagree with is the urgency. Not knowing why it is losing pressure would prompt faster action on my part. Then again he does have TPMS to tell him when disaster is imminent.
#8 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [fibber2]
Feb 17, 2010 (4:29 pm)
Thanks for the info. I'll check the number on the tires tomorrow in the morning and see if it's the one that was replaced. We were told that the rim was bent - we didn't see it. The only visible markings we saw on the rim itself was the distinctive pothole marking from when the tire "rubs" against the rim, leaving a black mark.
I'm a pretty big fan of the OEM rims and I'd prefer to keep them all consistent, though I did see on some websites that I can get them individually for around 125-175.
On the off chance that this is a different wheel we're talking about, I'm slightly irked at the fact that this is now the second Yokohama I've had an issue with in the last year and a half I've owned the car.
#9 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [fibber2]
Feb 17, 2010 (4:29 pm)
Yeah, I caught that, and it is a good point. I was not considering the possibility of a compromised rim (in other words, the aluminum alloy is cracked). That could definitely give out in a catastrophic manner especially if the leaking is a result of the fatigue growing worse and not a result of winter shrinkage compromising the bead.
#10 of 34 Re: 2009 Subaru Legacy Limited - TPMS Light [pilot1226]
Feb 17, 2010 (4:32 pm)
It may be that the quote you received was for a set of four, not a single rim. I would think the price you listed of $125-175 is much more accurate for a single rim. If you go with OEM, there would be no reason to replace the whole set.
Of course, if you hit a pothole, wouldn't two tires/wheels be affected (front and back)? Maybe the problem now is from the tire on that side that was not replaced the first time around.