Over-revving low speed

[ro250]

I have a 2017 BWM440i, auto.

Recently I've noticed at low speed, 1st or 2nd gear, when I accelerate on pull away it's as if it's wheelspinning. At first, I thought it was because it's been cold but I've now realised it isn't (there's no traction control light flashing).

This isn't hard acceleration, just normal, and the revs seem to spike up by around 1000rpm for a second, without the car responding, then they drop back and it seems normal. Otherwise it drives fine and when I put my foot down it pulls fine and seems normal.

It's due a service next month but would like to go into that with some knowledge!

[E-bmw]

If it is the revs rising rather than the wheels spinning then you may well have a transmission issue.

Torque converter is obviously a possibility, as is low fluid.

[ro250]

E-bmw said:

If it is the revs rising rather than the wheels spinning then you may well have a transmission issue.

Torque converter is obviously a possibility, as is low fluid.

Thanks, yes, if it were a manual I'd probably describe it as the clutch slipping. The car's only done 21,000 miles and fully serviced so hoping nothing serious. It doesn't do it all the time and generally drives fine!

[buggalugs]

It is somewhat normal for the TC clutch not to lock up straight away at low speed / 1st gear especially when cold, if that's what it is then it's by design not a fault. If it locks up smoothly and is OK the rest of the time I wouldn't worry about it.

[GiantCardboardPlato]

My understanding is the (if it’s a zf auto) BMW actually intend the transmission to work like this, especially for diesels. The high level of initial slip lets the revs build quickly, which spins up the turbo, which generates more power and gets you faster acceleration overall compared to locking up the transmission (or directly connecting to wheels like with clutch ideal up in a manual) at lower RPM.

I also hate the initial rev flare because it leads to a less involved feel with the driving, but it is faster.

I have a zf something in a petrol BMW and I do notice differences between cold/hot, but its behaviour is so variable/ context dependent due to the way it’s programmed I’ve always found it really hard to say what they are in a systematic way.

[ro250]

Thanks for these replies. It's odd how I've not noticed this before (had car nearly 2 years although don't do many miles). I do think I'm now pulling away with this in mind which isn't helping as I'm probably not driving normally!

It did occur to me that if there was a problem then it would more likely be when I'm pressing on, and when I do that in higher gears, it behaves fine either using the paddles or auto shifting.

[MB140]

buggalugs said:

It is somewhat normal for the TC clutch not to lock up straight away at low speed / 1st gear especially when cold, if that's what it is then it's by design not a fault. If it locks up smoothly and is OK the rest of the time I wouldn't worry about it.

My M135i, the first change of the day pottling out the drive/estate when its cold is always slightly unsmooth, its almost like its re-calibrating itself, its also slightly higher up the rev range 2.5k-3k rpm when it changes. After that its smooth as can be. It sounds like clutch slip a little i suppose as the revs are higher than it normally changes at 2k rpm just pottling about.