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Discussion Starter · #1 ·
I have a 2009 Cobalt and several weeks ago I got a "low tire pressure" warning, saying that one of my tires was just 12 lbs. Because the tires all looked fine, I assumed that I had a problem with the pressure sensor. But when I checked with a good tire gauge, I saw that one tire indeed was inflated to only 12 pounds! That made me a believer in the tire pressure monitoring system on the car.

But today when I got in the car, I got a tire pressure warning, and the display showed that all four tires were inflated to only 24 pounds. I checked, and sure enough, the tires were under-inflated. Thinking that today's cold snap had caused the pressure to drop in the tires, I re-inflated them to 30lbs, but after driving for about 20 minutes, the pressure rose to 35lbs.

I have two questions: first, these tires are sort of 'low profile' 16" tires (not those bicycle tires some of the "low riders" use, but not like 'normal' tires either). Does the fact that these tires apparently hold less volume of air cause these big fluctuations in the pressure as temperature rises/falls? And second, should I fill the tires to 30lb cold, even though when they get hot the pressure may go up to 35lbs or so? In other words, is it better for the tires to be over-inflated or under-inflated?
 

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fill the tires to 30lb when they are cold. the sticker on the door usually even says the word "cold" by the number.

it is totally normal to have tires fluctuate 5 psi up or down from being hot/cold.
 

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I'm fighting the same issue (Primarily with the right front). I think the valve-stem cores are bad... but I'm not really looking to fix it since my winters will be going on soon.
 

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Yes quite normal. All of the psi numbers on the inside of the doors are for cold psi. The max pressure on the side of your tire is also cold psi. Running 35 will not hurt anything. Believe me, I run mine at 40 cold psi.
 

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The 17 inch Pirellis that came on my Cobalt call for 35 lbs cold. The tires are filled with Nitrogen and should stay more stabile than with air however, recently on a weekend trip to the NC mountains where the air was thinner and much colder my low presure warning also came on. My car will cycle through all the tire presures and it showed that they had 28 lbs in them. I had to drive nearly 30 minutes before they heated up enough for the light to go off, which it did after they got over 28 lbs. I called my Chevy dealer and they asked me to bring it in and added nitrogen to bring all tires up to 37 lbs hot.
Sometimes those alarms are more trouble than they are worth but mostly I'm glad we have them. I still can't figure why your's did not alarm when pressure dropped below 28 lbs and waited till you were very low. Both of my late model Chevys showed the alarm when tires went at or below 28 psi.
 

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The 17 inch Pirellis that came on my Cobalt call for 35 lbs cold. The tires are filled with Nitrogen and should stay more stabile than with air however, recently on a weekend trip to the NC mountains where the air was thinner and much colder my low presure warning also came on. My car will cycle through all the tire presures and it showed that they had 28 lbs in them. I had to drive nearly 30 minutes before they heated up enough for the light to go off, which it did after they got over 28 lbs. I called my Chevy dealer and they asked me to bring it in and added nitrogen to bring all tires up to 37 lbs hot.
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Nitrogen should not fluctuate that much, that is why we use Nitorgen on aircraft tires. In Afghanistan, we would service the tires and only time we had to service them again was when we replaced them. Sounds like either you have a leak or they did not use nitrogen.
 

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Switch to Nitrogen thats what i use.. the nitrogen molucules are bigger then air molucules so if u were to have a very small hole in ur tire (pin hole) it wolnt leak as fast and also the pressure will not fluctuate from heat and cold air as much so you wolnt have to deal with your pressure light coming on
 

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nitrogen is just a hassle IMO.

and the TPM system does not set off your alarm at a certain PSI. It goes off at a certain percentage drop or gain. so if you have the cobalt model that calls for 35 psi like mine, or the model that calls for 30 psi. the alarm is gonna go off at different pressures. I can't remember what the percentage is but that's how they work.

These things are not that big of a deal. everybody freaks out about them because the don't understand how they work. If you learn how they work and how to reset them it's no big deal.
 

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Discussion Starter · #15 ·
Thanks!

Thanks very much for all the advice - I may not have made it clear, but I don't think I have a leak, because when the temp goes up, the pressure goes back up as well. I just wasn't expecting a 5-7 pound difference between cold and hot. I do like the tire pressure monitors, I've never been very good about checking my tire pressure and this will make me keep them properly inflated. I think the reason for the sudden drop in pressure was that we had a cold snap here. In Texas, temps can vary by 50 degrees from daytime to night at this time of year. I just didn't want to inflate the tires so that they were over-inflated on a warm afternoon -

Thanks again, I may try the nitrogen thing if i can find someone out here in the boondocks who can do it for me -
 

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Fill to GM recommended specs via label in driver door. Pressure stated is "cold". (sitting overnite or not driven for at least 4 hours). Adjust and check with a reliable digital gauge. Check at least once a week if outside temps are varying greatly (this time of year). As you stated, looks can be very deceiving! You will gain pressure as tires warm from driving. Disregard these pressures. Be concerned with accurate factory pressure maintenance,,, cold.

423
 

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Note to Finnscobalt... Yes I agree with you about the fluctuatation. I'm a old time pilot and I thought that same thing. My Chevy dealer is a local dealer for NitroFill and they charge 42.95 plus tax to remove the air and replace it with nitrogen including the spare in their new cars. Since I have no idea if it was correctly done I have to trust them. I did ask the service manager that question and was told that the presure still did change during temp. changes and also altitude. My tires were a little low before the trip about 32 psi so I think they dropped at least 4 psi and my temperture drop was from 70f down to 43f.
I'm interested to hear other NitroFill users experences. Tom
 

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$42.95 to fill tires with nitrogen? Ambient air IS 78% nitrogen! What a scam. You know the old saying..."A fool and his money are soon parted."
Check you tires (cold) regularly with an accurate gauge and adjust to factory specs.

423
 
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