Rev rises in neutral during hard braking.

[Robert]

Can someone help me understand why does the engine rev rises during hard braking in neutral.

[lib]

How much are the revs rising?

Are you sure you aren't hitting the accelerator by accident? It wouldn't be the first time that's happened during a sudden stop.

[Lester]

Interesting question. I can't say I have ever seen this happen. How is it that you find yourself in this situation? You should never ever be braking hard while in neutral. In fact, the car should never be in neutral when it is moving, period.

However - to answer the question more directly, chances are, your right foot is touching both the gas pedal and brake pedal at the same time. This happens to mine once in a while, the pedals are rather close together. I notice it when I am at a stop light, with the clutch in and the brake on. Then I check my foot - and sure enough it is slightly resting on the gas pedal.

Seriously - you need to stop driving with the car in neutral. In many jurisdictions it can be a moving traffic violation. You are not in control of your car.

[syenisch]

Originally Posted by Lester

Interesting question. I can't say I have ever seen this happen. How is it that you find yourself in this situation? You should never ever be braking hard while in neutral. In fact, the car should never be in neutral when it is moving, period.

However - to answer the question more directly, chances are, your right foot is touching both the gas pedal and brake pedal at the same time. This happens to mine once in a while, the pedals are rather close together. I notice it when I am at a stop light, with the clutch in and the brake on. Then I check my foot - and sure enough it is slightly resting on the gas pedal.

Seriously - you need to stop driving with the car in neutral. In many jurisdictions it can be a moving traffic violation. You are not in control of your car.

Are you doing damage to your car by driving in neutral? I like to coast up to lights... saves some gas.

[scorcherjf]

Originally Posted by syenisch

Are you doing damage to your car by driving in neutral? I like to coast up to lights... saves some gas.

I do the same... I'm pretty sure it's fine, been doing that on all my manual cars for years. If anything you'd be wearing out your clutch more by downshifting 5 gears coming to every red light.

[jeremyc74]

Originally Posted by syenisch

Are you doing damage to your car by driving in neutral? I like to coast up to lights... saves some gas.

You're not doing any damage, but you save more gas by leaving it in gear. When you're in neutral the ECU has to feed some fuel to the engine to keep it running. When you're coasting in the gear the momentum of the car takes care of that, so there's no (or very little) fuel being injected.

That may not be the case if you're trying to coast a very long distance, because obviously the engine puts some drag on the vehicle, but if you're going to stop anyway (as in coming up to a stoplight) you're better off to leave it in gear.

[jeremyc74]

Originally Posted by Robert

Can someone help me understand why does the engine rev rises during hard braking in neutral.

Is the car an auto or a manual? I've never seen it do that, but I'm not sure I've ever done any hard braking in neutral.

It's possible the ECU is kicking up the revs a little to generate more vacuum so that the brake booster works better, but I can't say that I've ever heard of that being done.

[Robert]

I have a 6 speed manual. It was wet outside last night so I wanted to see the car's behavior under hard braking from 30mph to 0. Once I reach 30 I hit the clutch and brake at the same time. I see the rev dip to 800rpm then it would bounce up to 2000rpm briefly. I recall I shifted into neutral but I could still be in 3rd gear. I expected the engine to stay at idle speed.

[Tom K.]

Originally Posted by jeremyc74

You're not doing any damage, but you save more gas by leaving it in gear. When you're in neutral the ECU has to feed some fuel to the engine to keep it running. When you're coasting in the gear the momentum of the car takes care of that, so there's no (or very little) fuel being injected.

That may not be the case if you're trying to coast a very long distance, because obviously the engine puts some drag on the vehicle, but if you're going to stop anyway (as in coming up to a stoplight) you're better off to leave it in gear.

Jeremy is correct. If you don't believe it, set the OBC to display instantaneous MPG and observe that coasting in gear gives the max 99.9 reading but in neutral, it will show about 50 mpg.

Tom

[Tom K.]

Originally Posted by Robert

I have a 6 speed manual and it was wet outside last night so I wanted to see the car's behavior under hard braking from 30mph to 0. Once I reach 30 I hit the clutch and brake at the same time. I see the rev dip to 800rpm then it would bounce up to 2000rpm briefly. I recall I shifted into neutral but I could still be in 3rd gear.

Why would you disengage the clutch to brake? Don't put the clutch pedal in until the engine is about to stall - around 10 mph in 3rd gear.

Tom

[Robert]

Originally Posted by Tom K.

Why would you disengage the clutch to brake? Don't put the clutch pedal in until the engine is about to stall - around 10 mph in 3rd gear.

Tom

During emergency hard braking you would hit both pedal at the same time because there's no time to wait till last minute to hit clutch. You are expecting to stop suddenly and now. It takes maybe a second or two max to stop from that speed. I don't have that good of a reaction.

Also I understand the fuel saving part but I always end up braking too little and use some of the engine's drag to slow down. Then goes the debate which is cheaper to replace brake pads vs. engine, that's a separate issue.

[Lester]

OK, what if some idiot swerves, cuts you off in traffic, runs a red light etc. - now you are not in control of your car if it is in neutral. If the car is in neutral, you have to take that extra second and 1/2 to put it back into gear, then make your evasive manuever. Too late, game over....hopefully nobody gets hurt.

In an emergency situation, you will need to have both hands on the wheel, you can't afford to be shifting back into gear and trying to emergency steer at the same time. No vehicle should ever be in left neutral while it is moving. There is no wiggle room here. It is plain and simple bad driving practice. In fact - it is a ticketable offense, although rarely written up.

[Lester]

Originally Posted by Robert

I have a 6 speed manual. It was wet outside last night so I wanted to see the car's behavior under hard braking from 30mph to 0. Once I reach 30 I hit the clutch and brake at the same time. I see the rev dip to 800rpm then it would bounce up to 2000rpm briefly. I recall I shifted into neutral but I could still be in 3rd gear. I expected the engine to stay at idle speed.

Your foot touched the gas pedal duruing the hard brake.

[Robert]

Guys, I understand your goodwill to help me understand the danger of stopping in neutral. That's not the question. First, I don't coast to stop in neutral. Second, I conducted the test at 12am in an deserted area. Please don't turn this into a safety discussion. The question is why does the engine rev up if it's suppose to be in neutral or disengaged from driven train.

Please stay on topic!

I could have accidently hit gas but I observed this twice. The only reason I can imagine why it would pop up.

[Tom K.]

Originally Posted by Robert

During emergency hard braking you would hit both pedal at the same time because there's no time to wait till last minute to hit clutch. You are expecting to stop suddenly and now. It takes maybe a second or two max to stop from that speed. I don't have that good of a reaction.

Also I understand the fuel saving part but I always end up braking too little and use some of the engine's drag to slow down. Then goes the debate which is cheaper to replace brake pads vs. engine, that's a separate issue.

You initially said "hard", not emergency braking. I agree I'd probably react in an emergency by disengaging the clutch along with full braking - but theoretically, wouldn't you be eliminating some braking force by "freewheeling" the engine? And in an emergency, stalling the engine once stopped would probably be a secondary concern.

Re the pad vs. engine wear issue, does engine braking promote engine wear? (I'm assuming you aren't downshifting into 2nd at 80 mph!!) Also, my experience with BMW's has been that the rotors are quite thin and generally have to be replaced with the pads - a total $500+ per axle expense once out of warranty.

Tom

[Robert]

Originally Posted by Tom K.

You initially said "hard", not emergency braking. I agree I'd probably react in an emergency by disengaging the clutch along with full braking
Tom

Tom, sorry for the confusion. I use the term interchangeably because I normally would not brake hard during a non-emergency situation. I would instead anticipated the stop and slowed down slowly. Only time I found myself hard braking is in an emergency.

[Robert]

Originally Posted by Tom K.

Also, my experience with BMW's has been that the rotors are quite thin and generally have to be replaced with the pads - a total $500+ per axle expense once out of warranty.

Tom

Great that's something to look forward to. I hope that's the front brake set as oppose to the smaller, generally cheaper, rear set.

[wnta135]

Could this be related to that fancy brake energy recovery system that's supposed to be on the 135i? A quick search did not find any details.

[Tom K.]

Originally Posted by Robert

Great that's something to look forward to. I hope that's the front brake set as oppose to the smaller, generally cheaper, rear set.

Dealer price for my 328i (SP) was about $540 front and $515 rear - not too much of a savings for the rear. Of course, for the first 4 years/50k miles, it's on BMW's dime.
Tom

[richsnuc]

Yer foot hits the gas...mine does too. One of the many curses of big feet!

[jotapeh]

In an automatic, shifting to neutral slightly raises revs by about 100rpm. Noticed this while parked at a red light that was taking ages.

I like to pretend I know why this happens and I have some guesses, but to be honest they're probably completely wrong and I'd rather not put my stupidity into writing here on this board.

[Robert]

Lester has made me believe that side of my foot probably hit the gas pedal. I like my driving shoes those are long and narrow but not much grip when weather is wet.