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  #1  
Old 04-28-2012, 01:21 PM
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Need help with tire-size related transfer case question

Went through this about a month ago and have a couple questions for the tech gurus here:
My 2005 4.4 sport needed new tires so ordered new Conti CrossContacts online - from 2 different places, unfortunately. I also moved up a size to 265/50/19 front and 295/45/19 rear. Rears came first, fronts came 3 WEEKS LATE! Mounted rears as I was running on a spare. Here's what happened:
DSC kicked in and reduced power at 80mph and/or when taking curves at 70mph+. I guess cpu freaked out and thought I was losing traction. Then transfer case would howl, but couldn't site a pattern. Also observed an OSV issue with smoking tailpipes after prolonged idle. Coincidental timing but curious if related to tire size difference taxing my transfer case.
Mounted new front tires and no more DSC issues or howling, although very paranoid over EVERY LITTLE NOISE, thinking it may be caused by the bigger tires. Also noticed the gas mileage seems to have gone to sh1t!
Can anyone tell me if I might have damaged anything in my drivetrain from running mismatched diameter tires for those 3 weeks. I drove roughly 6 or 7 hundred miles. Couple dealer techs said should be no problem. I'm kicking myself for changing tire sizes. Sometimes a great deal can bite you in the ass!!
And would my coincidental OSV issue or MPG reduction be related? I'm still blowing white-ish, blue-ish smoke after prolonged idle or stop and go traffic. (gotta love L.A. traffic)
ANY input would be great!! Thanks

Last edited by boynamedsoo; 04-28-2012 at 01:48 PM.
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  #2  
Old 04-28-2012, 02:06 PM
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You will have applied some additional stress to the transfer case, and accelerated the wear, but if it is quiet now once you have matched tires there shouldn't be any long term effects. There would have been two different potential impacts from the situation you describe: there would have been increased wear in the transfer case continually, whether the DSC lights were on or not (evidenced by the whining noise) and when you crossed the threshold of different wheel rpm (high speed cornering could do it, as you were already partly there on different tire sizes) the DSC would have engaged, which means applying the brakes and potentially reducing throttle. Primary wear there is on the brake pads. CPU didn't freak out, it did exactly what it is designed to do, which is to save the driver from himself.

If the tires are heavier (and bigger ones often are) then you will see a reduction in fuel efficiency, but it shouldn't be dramatic.

I don't see any connection between your smoke out the tail pipe and the tire mismatch. Do you know that it has anything to do with the OSV or is that just a guess? It could be, but there are other things that cause oil consumption (like valve stem seals) and the OSV problems are often evidenced at the start of winter, when there is a buildup of condensation, and freezing of the condensate happens. It can get plugged without freezing. What is your oil consumption like?
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Old 04-28-2012, 03:20 PM
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Thank you, JCL, for your post.

You're right, the cpu didn't "freak out," I did, followed by the paranoia over every little noise and nuance difference. After hours and hours of researching tires, and sizes, and differences in radii and diameters, I was comfortable with my decision to up-size. Until the delay, then the subsequent noises.

Is there a way to quantify the stress and wear and long term effects of my episode? Probably not, right?

As for the gas mileage, I have known these trucks to be gas hogs to begin with but 13 mpg from 15 seems on the low side. What other factors might reduce gas mileage, other than heavier tires? Could the possible cause of the tailpipe smoke be a factor?

And the tailpipe smoke never happens on a morning start up, cold morning or otherwise. I noticed the smoke after idling for more than 5 minutes or during stop and go after about 10 minutes in. I've experimented with varying the acceleration from idle and the stop and go and noticed less or no smoke with slow, smooth starts, but plumes and plumes with "happy" acceleration.

I go to a BMW "specialist" that I trust and he was the one that initially diagnosed the OSV. And assumed the same after researching this forum for similar issues. And this being my 5th bimmer, the oil consumption does not seem out of the ordinary. I would estimate <quart every couple months.

If not the OSV, am I looking at BIG repairs to possibly consider a new car?
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Old 04-28-2012, 11:34 PM
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No way to quantify the stress.

Gas mileage will have more to do with driving style than anything else.

If your mech thinks it is the OSV then have him fix it. That is cheaper than if it is valve seals. Worth doing it first.
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