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Discussion Starter #1 (Edited)
my 2017 330i xdrive with standard wheels/tires seems to lose tire pressure every 2-3 weeks. Is this something that is normal? Of course it happens more frequently in winter, but still?? I now keep a pump in the trunk.
 

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Check for an embedded nail or screw. It could also be a bad valve stem, or a previous puncture repair that has failed. You could put the tire and wheel in a bath tub, rotate it and look for bubbles.

I had a tire not fully seated on a rim that was leaking since new. It also would vibrate at high speed, even after being balanced.
 

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Pedant and Curmudgeon
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Are you saying that ALL FOUR tires lose pressure? They shouldn't. Probably you will need to have the tires dismounted, inspected and re-mounted to discovery why. Valve stems are a good thing to look at.
 

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Tire pressure goes up or down about one PSI for every ten degrees F in temperature.

It's important to be consistent when checking and topping off the tires. The best time to do it is in the early morning, when the car is completely cooled off, and before sunlight hits one or two tires and heats them up. I record the "from" and "to" pressures, and the ambient temperature when I top off my tires. From this, I can see what my leakage rates are for each tire. If one is leaking down fast, I go looking for an embedded nail or screw.

All of my tires lose about one PSI every month, and that's after accounting for changes in temperature.
 

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What everyone said - inflation pressures will vary 1-3psi based on temperature swings.

However, in my experience, if one tire (or more) regularly loses that every few days it's because of something leaking somewhere (hard-to-see nail etc, cut in tread - glass damage, leaking tire valve stem, etc.) Worst case (you are in NYC) is pothole damage to wheel or tire - hard to see on the inside area of wheel/tire). Another situation I faced was with a tire gauge that had grown "tired" (old) and was often inaccurate - I replaced that one.
For starters get out your spray bottle of soapy water and start investigating.
 

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What everyone said - inflation pressures will vary 1-3psi based on temperature swings.

However, in my experience, if one tire (or more) regularly loses that every few days it's because of something leaking somewhere (hard-to-see nail etc, cut in tread - glass damage, leaking tire valve stem, etc.) Worst case (you are in NYC) is pothole damage to wheel or tire - hard to see on the inside area of wheel/tire). Another situation I faced was with a tire gauge that had grown "tired" (old) and was often inaccurate - I replaced that one.
For starters get out your spray bottle of soapy water and start investigating.
He said every 2-3 weeks. If it’s all 4 wheels it’s likely temperature related.
 

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thanks for all the advice. I will have the valve checked first!
Looks like the problem is limited to the valve on one wheel.
The original poster should also have the tire shop look carefully for an embedded nail or screw. Those can leak slowly as well.
 

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Discussion Starter #10
gonna get spray bottle of soapy water first, if I can't find anything, it's off to my local tire shop for a "dunk".
Thanks everyone, much appreciated!
 
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