Just bought a set of Michelin Defender tires three months ago and they keep losing air. I check them every week and at least one is down 5-8 psi by the end of the week. No visible punctures or damage that I can see. Is this normal or do I have a defect?
That kind of pressure drop is actually pretty common and doesn't necessarily mean defective tires - most tires lose a bit of air naturally over time, and temperature swings can cause noticeable weekly fluctuations too.
Before assuming the worst, I'd get them checked at a tire shop with a machine; sometimes the leak is coming from a tiny puncture you genuinely can't spot with your eyes, or occasionally from a faulty valve stem.
If they find nothing wrong, it might just be the normal behavior of those particular tires in your climate, but if there's an actual leak, catching it early beats dealing with a flat later.
Michelin Defenders are solid tires, so if they're genuinely losing that much air, the issue is worth investigating beyond just normal seepage. The thing people don't always realize is that a slow leak can come from places other than the tread - the bead seal (where the tire meets the rim) is a common culprit, especially if your wheels weren't perfectly clean when they were mounted or if there's any corrosion starting on the rim.
I'd get them checked at the shop where you bought them. Have them pull the tires off and inspect the beads and rims carefully, and also have them look at the valve stems since those can weep air too and are cheap to replace. If the rims look fine and the stems are good, then yeah, it could be a defect in the tire itself - small punctures that don't show up obviously can definitely happen. Since you're only three months in, you should absolutely be covered under warranty if there's a manufacturing issue.
One other thing to verify: make sure you're checking pressure when the tires are cold (before you've driven or at least a few hours after). Tire pressure fluctuates with temperature and driving, so checking after a hot day versus a cool morning can give you readings that are off by several psi and make you think you have a problem when you don't. But if you're being consistent with your checks and genuinely seeing that kind of drop, get them inspected. Five to eight psi per week isn't normal.
Before you assume it's the tires themselves, check your valve stems - a lot of people overlook those, and a slightly worn or corroded stem can cause exactly this kind of slow bleed without any obvious damage.
If the stems look fine, could also be worth pulling the tires off and checking the bead seal where the tire meets the rim, since sometimes debris or corrosion there creates a slow leak that's hard to spot visually.
I've dealt with this exact frustration myself, and the previous answers are on the right track but missing something important! Temperature swings definitely play a role, but 5-8 psi weekly is honestly more than the normal drift most tires experience. The valve stems are a great place to start checking - I'd grab them with your fingers and feel for any moisture or corrosion, then try hand-tightening the cap. If that doesn't help, the issue could be the bead where the tire meets the rim, especially if the wheels weren't perfectly clean when they were mounted or if there's even a tiny amount of dirt or corrosion there. You might also want to get them professionally inspected at the shop where you bought them - three months in is still warranty territory for Michelin, and they should be able to pinpoint whether it's the tire, valve, or rim situation!
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