A Tire Pressure Mystery

[kaneohe]

Had a flat tire so took car to Costco to have flat repaired and tires rotated at same time. Marked tires so I could confirm rotation. The next AM before driving the car, I measured the pressure on all tires. I normally run them a few psi above mfg spec so I don't have to refill so often. The spec psi was printed on the Costco order form for the tech to follow if he desired. Apparently he did since the psi on 3 tires was exactly spec and lower than I normally set them.

The mystery part is that one tire , the repaired flat, was 26psi.........3psi low.
Concerned that I had a leaker, I went to Costco and refilled all tires w/ their nice automated setup to my normal pressure (also confirmed w/ gauge). Of course, the tires were slightly warm after driving a few miles but the important thing is that they were all refilled to the same pressure. The next AM, I measured all 4 again and they were all the same. 4 days later, I measured them again and they were within 0.5psi of each other.......so I conclude that the flat was repaired and holding ok (at least over this term).

Question: what happened to produce the 3 psi lower pressure after the flat repair? Since the tires were reset to spec, it seems clear the tech knew what the target was. I would have guessed all 4 tires were filled/reset at the same time so they should have been the same. I would guess my gauge is repeatible within +/- 0.5psi so a measurement should be within
1psi or so, certainly less than the 3psi difference seen.

Any theories?

 

[pb4uski]

Maybe you did have a leaker but the repair settled in and sealed up when you drove from home to Costco the morning after the repair or from Costco home after refilling.

Or the tech messed up when refilling the leaker the day before.

Or its you.

 

[kaneohe]

DW says I says it is never me who messes up (don't know how to put grins on anymore....something's changed).

So what does "repair settles in and sealed" mean? Any cause for concern that it may fail again, perhaps catastrophically, in the long term? Costco suggested that they would look at it if I brought it in but I wasn't thinking of this scenario.........

 

[MichaelB]

Could be the flat has less air pressure to begin with. Was it a cool night?

 

[bld999]

Why not mount the spare and put the other on probation for a week?

Tires with slow leaks, if that's what is going on, go soft slowly, not quickly.

 

[JPatrick]

If you have gone 4 days with no problem, just continue to check every couple of days. The initial issue was likely a tech screwup. Nothing to see here.

 

[kaneohe]

Could be the flat has less air pressure to begin with. Was it a cool night?

Not real cool........recorded the temp in AM when measured but too lazy to look it up......probably around 60F. I'm thinking the temp effect would be minimal and would go the other way.......the other 3 tires were slightly warm.....maybe 2psi increase from the short drive. They probably would have cooled significantly by the 1-2 hr wait but, if anything, would be hotter than ambient.
I assume the tech just drained some gas out of those tires to adjust to spec so, if anything, they were slightly warmer than the repaired flat. I assume the repaired flat would have the ambient temp when filled so would decrease less in pressure with a decrease in temp.

If the gas used to inflate the flat were significantly warmer than ambient, that might explain it?

 

[BUM]

Best advice - Change the air on all four and remeasure every other day.

 

[BUM]

Pressure gauge not accurate? Use more than one - then take an average.

 

[JOHNNIE36]

Put some soapy water in a spray bottle and shoot it around the rim of all the tires AND the stems to see if you can see any bubbles forming. I've seen a lot of leaks around the stems.

 

[imoldernu]

Possibilities:

"Settling in" - especially on aluminum wheels, where the bead doesn't make a perfect seal, when the tire is first inflated, but subsequent driving completes the seal.

Valve leak, that happens when the little valve seal didn't seat properly. Often this corrects itself with a bump, or the next time the tire is inflated. If this is suspected, just tapping the pin usually reseats the valve. Can check with a drop of soapy water.

Tiny leaks, as with tacks or bits of wire often self-seal.

During re-inflation after loss of air, temporarily over inflate by 10 pounds then let air out to desired pressure.

My preferred solution is to use pressure can of flat-fix. The mechanics go crazy when you do this, as it's a bit messy when putting on a replacement tire, but three or four dollars does the trick to save an $80 tire that still has tread remaining... and if the "industry" doesn't like the solution, IMO, they shouldn't sell the product.

MY older cars often begin with "slow" leaks that I fix with a shot of flat fix. Hasn't failed me yet.

 

[REWahoo]

Valve stem fart...