Brakes overheating

[2016bmwi3]

Hi,

I hope to get some help. Recently, my i3 rear left and front right brakes were overheating. We took it to the dealer and they said the calipers were frozen. They also said to replace all brake pads and rotors. After 3 mobile mechanics and about $1000, they still seem to overheat. Replaced the two calipers, and all four pads and rotors. Anyone might have some suggestions?

 

[kd7iwp]

Interesting that it's those two opposite corners. Typically a brake master cylinder has two redundant outputs (for safety) so that the left front and right rear are on one channel and front right and left rear are on another. This way if you blow a brake line you will lose two brake calipers but you'll still have two left. I wonder if there's a problem inside the master cylinder such that it is not always allowing the pressure to be relieved on one of those two channels. It's not a problem I've encountered before, but it's my theory.

 

[2016bmwi3]

I just took it for another drive and there is white smoke coming out of those two sides, I checked with a thermometer and they are over 500 degrees, the rotors.

 

[2016bmwi3]

Interesting that it's those two opposite corners. Typically a brake master cylinder has two redundant outputs (for safety) so that the left front and right rear are on one channel and front right and left rear are on another. This way if you blow a brake line you will lose two brake calipers but you'll still have two left. I wonder if there's a problem inside the master cylinder such that it is not always allowing the pressure to be relieved on one of those two channels. It's not a problem I've encountered before, but it's my theory.

Do you think the master cylinder needs to be replaced?

I just did a little research on google and another BMW post said it could be the brake fluid, but it wasn't an i3.

 

[mojo]

The suggestion that kd7iwp makes seems reasonable - it would suggest that there may be a fault within the DSC (hydraulic brake control) module or (I suspect less likely) the brake master cylinder.

As an initial test, it would be interesting to jack the affected front wheel and see if it will turn by hand with the car turned off.

I guess that you could get these symptoms if the brake system was not bled properly after the calipers were replaced - but that's not a likely explanation given that the symptoms existed before the calipers were replaced.

 

[kd7iwp]

I have no reason to believe fluid is a problem, just debugging the plumbing. When you press the pedal you are moving two pistons that are end-to-end, but pushing fluid into the two parallel tracks I mentioned earlier. Those pipes go into the input of the traction control valve block where it is then distributed to the 4 calipers. I lean towards the master cylinder as when you release the pedal it should release all the pressure back into the

 

[kd7iwp]

If it wasn't bled correctly you would simply have spongy brakes and they would have trouble engaging, not releasing. Obviously the first thought any of us would have had is the calipers being stuck but since those were replaced and presumably we can rule them out, I would move up the chain to the master cylinder.

 

[kd7iwp]

I also feel like we can rule out the parking brake. Even if it were stuck on (rear wheel) I can't conceive of a way in which it would lock the front wheel.

 

[2016bmwi3]

Thank you kd7iwp and mojo

 

[kd7iwp]

ChatGPT gave a good suggestion. If you lift up one of the wheels as @mojo suggested and you verify that the wheel is stiff, then go to the master cylinder (the brake pedal should NOT be pressed) and crack open the brake line that goes to the affected wheels (I haven't looked yet to see if it's the front or rear of the two lines on the cylinder). If fluid sprays out under pressure and/or the wheel is now easy to turn that means pressure was still in the line and the master cylinder is not relieving it to the reservoir. If no pressure pushes fluid out then the problem is further downstream. I might suggest rebleeding after doing this.

 

[alohart]

Recently, my i3 rear left and front right brakes were overheating. We took it to the dealer and they said the calipers were frozen. They also said to replace all brake pads and rotors. After 3 mobile mechanics and about $1000, they still seem to overheat. Replaced the two calipers, and all four pads and rotors. Anyone might have some suggestions?

The dealer must have misdiagnosed the problem and didn't verify that their repair solved the problem. You paid $1k for their mistake and would have to buy 2 new rotors if they warped after becoming so hot. Call this dealer and demand to have the problem fixed correctly at no additional cost. The i3's brake system is similar to that on all BMW's and even all vehicles, in general, so this problem isn't something that they could blame on the i3 being an EV.

 

[drcarrera]

ChatGPT gave a good suggestion. If you lift up one of the wheels as @mojo suggested and you verify that the wheel is stiff, then go to the master cylinder (the brake pedal should NOT be pressed) and crack open the brake line that goes to the affected wheels (I haven't looked yet to see if it's the front or rear of the two lines on the cylinder). If fluid sprays out under pressure and/or the wheel is now easy to turn that means pressure was still in the line and the master cylinder is not relieving it to the reservoir. If no pressure pushes fluid out then the problem is further downstream. I might suggest rebleeding after doing this.

Be wary of spraying corrosive brake fluid everywhere, though!

 

[kd7iwp]

Be wary of spraying corrosive brake fluid everywhere, though!

Indeed. I would have a towel wrapped around the end of the pipe while you are cracking it loose. There should be be very little quantity.