Not hopeful for a solution as the pros are stumped, but maybe this will save others time.

New to me 2018 Sport trim with 48K miles when I bought it. Was noticing a slow air leak on driver side rear (DSR) that would trigger the TPS about 24-72 hours after inflation to spec. TPS seems to trigger about 25-27PSI but the tire once was at 17PSI (inflated back to spec). The rate of the leak was super variable and I didn't see any obvious reason for the leak as the tire had no damage and lots of tread left. At that point I thought valve stem might be it. But the variability confused me as the ambient temp was not changing that much.

Took it to a tire company I had worked with in the past and who are known for the quality of their work. Only thing they found was some gunk built up on rim near the area where the tire bead sits. They had removed it and checked valve stem and placed some sealant and remounted the tire and it passed the submerge in water test. For about 5 days all was perfect.

Took road trip of about 2400mi. A pattern quickly emerged about about 400mi. At speeds well above 65mph I'd travel about 250-300mi and TPS would trigger. If I pulled off within 10min the pressure would be down to 20-25PSI, and the longer I took the lower it would be with lowest about 15-17PSI. This was consistent the whole trip. When stopped overnight for rest I rarely lost pressure or maybe 3PSI (temp/altitude could do this).

Second shop was also trusted by my parents I took car to. Same tests and considered the TPS seal but they should be fine as car is 2018 and there was no air leak there during water overpressure testing.

My best guess there is a very fine fracture in the rim that with the weight of the car and the inertia of the higher speeds causes the air to leak. Around town I've not lost the air hardly at all. But a new used rim is about $230 not including labor costs. I'll try it when my financial situation is better but right now it's an annoyance more than anything. I do have to drive back later this month about 3300mi so I'm sure it will happen again. No real way to troubleshoot as the forces involved are not easy to replicate outside of a lab and the cost to fix the problem is low enough to not make it practical to find out the truth of the matter.

Thanks for your attention. If you think I missed something, know that it's probably me not recalling what the pros told me they did and that it was probably done. Novel ideas about figuring it out are welcome. Cheers!

 

If it were a fine fracture, wouldn't it eventually fail the submerge test?

I don't know how long the submerge test lasted, but if it's a tiny leak, it won't show up for awhile.

My guess is that you have a bead seal leak that exacerbates at higher speeds. Wondering why the gunk had built up there in the first place. Is something accumulating there that isn't supposed to?

 

Not sure about the amount of time either, but it's under an hour for sure. I do know they overpressure the tire (to what level, again not sure, but no more than the rating on the tire and probably less than that as pneumatic is very dangerous thing) and again it's the best they can do as far as testing goes.

The matter of fact way the gunk issue was described I'm guessing this is common and usually not a problem. Same stuff that is made of road grime, brake powder, and motor oil off the road surface. Once the surface of the rim was clean running a finger along would detect dents or cracks and visual inspection of the tire bead should be enough to tell if there is defect or wear which would be very unusual. Yes in my mind the tire could be suspect and I may have the tires rotated before I leave or while on the road, except I think I will request (more expense) to have the tire from the DSR removed from the rim and moved so the rim stays DSR and the tire moves off to where ever the next location is. If they use the X-pattern this should be easy. Will take notes for sure to keep track. Of course if the extra labor is going to shoot above the 230 then it's not worth it.

 

Impossible. Damaged or cracked wheels will leak air all of the time. It isn't possible for a wheel to only leak air at freeway speeds.

I would guess you have a faulty TPS if anything at all.

 

I suppose you could swap tires with another wheel and then see which one leaks. That would tell you if it's the tire or the wheel.

 

No, every time the TPS sensor went off I was checking with a very well rated digital tire pressure gauge and the values I quoted were from those measurements. So we can discount the TPS being faulty, it's doing the job. As to your supposition that damaged or cracked wheels will leak all the time it maybe perfectly true. But I'd say it's a case that at lower speeds the time it takes for the tire to go from say 35PSI (slight overpressure) to the 25-27PSI is on the order of a week to two weeks. But at highway speeds the leak happens in about 2/3rds of a tank of gas or about 3.5 hours, magnitude of difference.

I think the modified tire rotation scheme will hopefully give insight into what exactly is broken rim, tire or both.

 

Not true.

The one variable that would be exacerbated at highway speed would be temperature.
The faster the OP is driving the more heat is generated in the tire, thereby leading to the leak worsening.
Kinda like when they suggest you do a compression test on an engine when hot, vs. cold (a cold engine will always give great results).


OP, I'd suggest you keep a portable air compressor in the car with you until you can afford to have the tire replaced.
good luck