I picked up my Cooper S at the end of July. The next morning, I checked the tire pressure to make sure they were inflated properly. If I remember correctly it was only a couple of PSI off. So I pumped it back up to spec and re-initialized the tire pressure monitoring system. Fast forward to today. I checked the pressure with my trusty dial gauge and all four tires read just shy of 20 PSI! At first I thought the tire monitoring system would pick it up but then I remembered the system only measures the difference between the tires. So in this case, I don't think the sytem would detect the loss of pressure since all four tires had the same pressure reading.

I know I'm guilty of not checking more often. I was really diligent when I had my previous car. I guess the monitoring system made me more complacent. But I'm clueless as to how all four tires could loose so much all at the same time in two months. Heck, I had snow tires stored in my garage during the warmer months and when I put them on in the winter, they only lost just a couple of PSI.

I did a quick check of the treads and I didn't see any damage. Hmmm...I'm gonna go back into the garage and look more closely. If the tires are OK then the only thing I can think of is the valve stems. But what are the changes of getting 4 bad stems?

 

My experience is that all tires tend to lose pressure, some more than others.

Even with the tire pressure monitoring systems, most manufacturers still recommend checking the tire pressure periodically. There is no guarantee the monitoring system will not malfunction.

If your tires are still wearing acceptably, count yourself lucky and start checking more frequently.

I would still have thought the system would have notified you that the tires were low.

 

I think the system only notifies you if there's a pressure difference on one of the wheels, not when all four are losing pressure at the same time.
I'd also check the air pressure gauge. did you use the same gauge? also tires tend to lose psi over time, temperature drop etc.

 

Great job checking your tire pressures! I've programmed it into my Automotive Wolf software, and check them every 30 days. Often, I do mid-monthly checks too just because.

First rule of thumb is to use a very high quality tire pressure gauge. Throw away any and all of the pencil-type gauges like you get in the .99 cent bin at the check-out lane. This is the number one cause of wierd readings.

I'm presently running 34psi front and rear on my 18" Dunlop runflats (JCW) and find they drop down to 32psi over the course of a month.

Another thing to be sure, check your tires cold. Preferably in the morning when the tires have been allowed to cool down over night. The temperature of the tire and the outside temperature can make a difference in the air pressure reading.

Elevation above sea level could also be a factor, so check in a consistent location.

Give it another couple of weeks, and if your tires hold air better, chalk it up to the tire gauge. If they drop again, you know there's something wrong.

 

I just checked again after I got home and all four tires lost about 3 PSI! I checked about an hour after I got home. Then with a flashlight, I got on my hands and knees and checked each tire. I could not find anyting wrong. The sidewalls have no apparent damage. No hissing sound is coming from the valve stem. It's either I ran over a 2x4 full of nails and all four tires just happened to run over a nail or I have a bad gauge. I have a good quality dial type with a reset button. Hmmm...I noticed that during the winter, the dial goes down slowly after you get a reading. But only during the colder months. Tomorrow I'm gonna pull the car out of the garage and re-check all the tires in direct sunlight. Then I'm gonna try another gauge.

I'm stumped.

 

This too can falsify your readings as the black sidewalls absorb the sun's radiant heat - inacuratley raising your pressures. Start with an acurate measuing device .

Bear in mind each 10 degress of ambient air temperature change = 1 lb of inflation pressure. Figue also in the average loss 1 lb a month.

 

i use a pen-type I bought like 10 years ago when they were still pretty good quality. I get consistent readings from it.

Anyway, it is not unusual for tires to change pressure before and after a run. typically 3-6psi depending on ambient temps and how you drove the car.

You should check your pressures in the morning while your car was garaged inside for at least 3-4 hours. top off to 34psi (that's what I use for 1-2
people in the car) and check once every week especially if you are
running low profile tires.

In the winter, I use higher psi as the tires do not warm up enough to get to
its ideal pressure which is about 39-40psi for me.

 

OK...the car has been sitting in the garage all last night and all day today. I bought a new dial type gauge and checked the pressures. It was waaaay off. So I'm thinking the gauge was defective.

I guess the moral of the story is, if you dropped your tire gauge, it may be broken. Carry on :smile: