[ekuLUKEkul]

Hey, so I have a slow puncture in one of my front tyres. Normally every 2/3 weeks the flat tyre monitor would alert me that the pressure was low, I would top the tyre up and reinistialise. However the last two times I have checked the tyre and it has been as low as 21psi down from 38psi without any warning. If I look on the iDrive display it still says active and shows all 4 tyres green.

I can't seem to find any definitive information as to what system this car uses (late 2010 e91 lci 325d) I also can't find anything on the forums as most people seem to have the opposite problem of the car reporting low pressure when the tyres are fine.

Does anyone have any clue what the problem could be? Is there simply a sensor that needs replacing?

seperate question: When the system was working correctly the display on iDrive always showed all 4 tyres as yellow as opposed to pin pointing the flat one, is this how it should behave? I always wished it would show the exact tyre.

Thanks a lot for any help!

[jwilalex]

The e9x range has pretty limited tyre pressure warning capability and is based on the time taken for the wheel to rotate (using ABS) unlike he more modern TPMS which actually takes a pressure reading from the valve/sensor.

The system you have will either show green, amber or red depending what it thinks and can't show individual tyre pressures. I've never had an amber show as I check tyres weekly but have had a red appear when a tyre failed.

The thing to consider is if you have run flats fitted they are so stiff on the side wall that even with considerably less air in the rolling circumference remains too similiar to normal settings. If memory serves me right it should warn amber when there's a 10% difference to the others and by virtue be able to show which wheel, but this is not always the case.

Personally I'd just get the slow puncture fixed (assuming leak is somewhere within the contact area of the road). it only costs a few quid so why not?

Anyway info here https://www.moderntiredealer.com/art...-3-series-e-90

[ekuLUKEkul]

Originally Posted by jwilalex

The e9x range has pretty limited tyre pressure warning capability and is based on the time taken for the wheel to rotate (using ABS) unlike he more modern TPMS which actually takes a pressure reading from the valve/sensor.

The system you have will either show green, amber or red depending what it thinks and can't show individual tyre pressures. I've never had an amber show as I check tyres weekly but have had a red appear when a tyre failed.

The thing to consider is if you have run flats fitted they are so stiff on the side wall that even with considerably less air in the rolling circumference remains too similiar to normal settings. If memory serves me right it should warn amber when there's a 10% difference to the others and by virtue be able to show which wheel, but this is not always the case.

Personally I'd just get the slow puncture fixed (assuming leak is somewhere within the contact area of the road). it only costs a few quid so why not?

Anyway info here https://www.moderntiredealer.com/art...-3-series-e-90

Thanks for the replying, yeah I really should get the puncture sorted but aside from that I am just trying to understand why the system isn't working as expected. I did already read that article but couldn't work out what was causing my specific issue.

I should've added but I don't have run flats.

But from my knowledge with this car it has always showed all tyres as green or all tyres as amber if one has dropped enough pressure since it was initialised. Now however all seem to be showing green regardless of actual pressure.

[Andystobbs]

Possible that the changing temperature is making the tyre slightly stiffer and not altering the rolling length.

[ekuLUKEkul]

So I figured it out. Turns out the other front tyre is also dropping pressure (albeit not as much) and both must've been maintaining a similar enough rolling radius for it to not upset the TPM.

I pumped them all up to my usual PSI then let one of them down to around 25psi and went for a drive. I got the usual low pressure warning when doing this after a couple of miles.

[Andystobbs]

It is a frustrating system sometimes. The TPMS system can be fitted instead but its a faff.

[ekuLUKEkul]

Originally Posted by Andystobbs

It is a frustrating system sometimes. The TPMS system can be fitted instead but its a faff.

Yeah it isn't the greatest system in the world. Especially when you look on the Idrive and it shows the car with all 4 wheels green or amber, it looks as if it is going to be a lot more detailed but is actually very primitive.

Not to worry though it does it's job mostly and I just wanted to ensure it wasn't broken as I get irritated by things not working, even if I don't use them often.

[Andystobbs]

I'm particularly annoyed at mine just now. A 650 mile old Bridgestone RFT is completely ruined after getting a puncture. The system only alerts at approx 20psi by which time the inner sidewall has crumbled internally and the tyre is scrap.

[Silver Streek]

I guess the moral of the story is to check your tyre pressures regularly yourself. You are more likely to pick up a slow puncture that way and not ruin a good tyre.

[motornuts91]

i thought that the tyre monitor only works with run flats?

[ekuLUKEkul]

Originally Posted by motornuts91

i thought that the tyre monitor only works with run flats?

No it definitely works without as I run non runflats and it works. Tbh thinking about it, surely it is less reliable with runflats?

As established in this thread it relies on the rolling radius of the wheel, the reason mine "wasn't working" was because both front tyres were going down at a close enough rate that the radius was never so out on one to the other that the car didn't realise that either were getting flatter.

With ruflats having a reinforced sidewall does this not mean that the radius is never off from the fully inflated tyre and therefore the sensor will never alert the driver?

Surely it must work so I am wrong but it is interesting to ponder?

[Silver Streek]

Originally Posted by ekuLUKEkul

With ruflats having a reinforced sidewall does this not mean that the radius is never off from the fully inflated tyre and therefore the sensor will never alert the driver?

Surely it must work so I am wrong but it is interesting to ponder?

No runflats still roll with a smaller radius when they loose air so the ABS sensor still picks up a soft tyre. I run non runflats now but had runflats when I bought the car. One lost pressure and it picked it up fine. The fact the car has runflats and no spare tyre I think is the reason BMW added the tyre sensing to let you know you had a flat before you destroyed the tyre by by driving too far on a flat tyre.

[ekuLUKEkul]

Originally Posted by Silver Streek

No runflats still roll with a smaller radius when they loose air so the ABS sensor still picks up a soft tyre. I run non runflats now but had runflats when I bought the car. One lost pressure and it picked it up fine. The fact the car has runflats and no spare tyre I think is the reason BMW added the tyre sensing to let you know you had a flat before you destroyed the tyre by by driving too far on a flat tyre.

I figured it had to work! Just wasn't sure how!