Nobody likes it when their cars have mechanical issues. BMW repairs are often expensive and they frequently require the dealership’s attention. One of the most terrifying potential problems that you can experience is the prospect of your car accelerating on its own. So what should you do if your BMW is revving on its own?
What you should do when your BMW revvs on its own depends on when the vehicle is revving. If it revvs on its own during a cold start idle, you should let the car sit for 1-2 minutes. If the revving stops, you can drive the vehicle. If the revving continues, turn off the vehicle and diagnose the issue or have a mechanic inspect it. If it revvs on its own while in drive, stop the car immediately and turn the vehicle off.
Lets take a deeper dive into why it matters when your BMW is revving and how that can help you diagnose the problem. Lets also look into the most common causes of a BMW revving on its own and how to diagnose them.
What it means when your BMW is revving on its own
When someone says that there vehicle is “revving” what that means is the engine’s revolutions per minute (RPMs) are rising. When a vehicle is revving on its own that means that the engine is turning sporadically. The speed at which the engine is turning is measured by RPMs and is displayed in your instrument cluster.
High RPM during a cold start
Sometimes it is normal for your BMW to revv higher on its own than it otherwise would. A typical example is when you start your BMW on a cold winter morning and you notice that the car revvs up to 2000 RPMs for the first minute.
This is completely normal and as long as your car drops down to around 700-800 RPMs afterwards you do not have a problem. Initially high RPMs on a cold start is just caused by the fact that fuel is likely to thicken in cold temperatures and your BMW will compensate for this by producing a higher RPM initially.
BMW owners in South Florida likely never experience this, but for BMW owners in Boston this is a common occurrence during the Fall and Winter months. It used to be common knowledge that you should wait a few minutes for your “engine to warm up” on a cold start. This bit of knowledge was true for old-fashioned carburetor engines but is no longer necessary or even recommended. Just wait a few seconds until your RPMs have normalized and you are good to go!
But what if your high RPM does not go away or it fluctuates sporadically? If your BMW is suffering from a sporadic RPM at idle it is often due to an air leak somewhere in your vacuum lines. Old vacuum lines often crack overtime and even relatively small cracks in vacuum lines will create an erratic idle.
You can certainly drive your vehicle with these symptoms, although I would not recommend it because it creates a risk that the vehicle will suddenly die at idle (happened to me once).
High spikes in RPM
If on the other hand your vehicle is experiencing massive spikes in RPMs at idle and/or in drive, your issue is likely a little more complex than a cracked vacuum line. When I say massive spikes, I mean fluctuations of 1,500 RPMs or greater. This could mean that your RPMs are spiking so high that you are getting close to redlining (in which case you should shut the car off immediately).
There are several different common causes associated with spikes in RPM. Since RPM spikes can create a potentially dangerous situation if it occurs while in drive I strongly suggest that you park it until you get it repaired.
The most common causes of a BMW revving on its own
The good news about a BMW revving on its own is that the causes are almost always relatively simple to fix and inexpensive compared to many other repairs. I have experienced revving issues a number of times on my BMW E36 and E46. Although, the initial problem is certainly frustrating, the cause can often be diagnosed within an hour.
The most common causes of a BMW revving on its own are: 1) a bad idle control valve, 2) sticky throttle cable, 3) bad throttle position sensor/throttle plate issue, 4) floormat pushing the accelerator, or 5) cracked vacuum line.
That is a lot of different possible issues, but the good news is that every one of these problems are easy to diagnose, easy-moderate to fix, and relatively inexpensive. Lets dive into how you can diagnose your revving problem.
So how can you diagnose your BMW’s revving problem?
The very first thing you want to do is determine whether your 1) BMW is surging or 2) if it is rough idling, running like crap, and the RPMs are bouncing up and down in a given range. As I have previously highlighted, surging usually indicates that the problem is not as simple as a cracked vacuum line. Just a rough idle along with a bouncing RPM often indicates you have an air leak.
Check for any engine codes
The very first thing you should do whenever you have a significant issue with your BMW is check the engine codes. Although a minor vacuum leak frequently will not throw any check engine codes, a faulty idle control valve or bad throttle position sensor will. Hopefully, you get an immediate and specific result out of checking the check engine codes and you do not have to look any further into diagnosis.
Check the floor mat
If you don’t get any codes then the next lowest hanging fruit would be to check your floor mat. Yes I know, checking the floor mat sounds ridiculous and simple, but I have seen a number of people complain that their BMW’s idle was mysteriously surging, only to discover that it was because the floor mat had been pressed against the accelerator.
You can remedy a sliding floor mat by installing some double sided carpet tape underneath the mat or by installing some Velcro on the floor mat and on the floor.
Check for vacuum leaks
Vacuum leaks are an extremely common cause of poor idling and RPM bouncing. Everyone who has ever owned a vehicle with some mileage on it has likely experienced the effects of a vacuum leak. Vacuum leaks are pretty rare on vehicles with low mileage but old rubber and plastic vacuum lines commonly crack.
One of the easiest indicators of a vacuum leak is a high pitched hissing noise. You will not hear a high pitched hissing noise with every vacuum leak, but oftentimes you will.
Method #1: Cigar Smoke
There are a number of methods that mechanics and DIYers use to diagnose vacuum. My own personal favorite was illustrated on YouTube by Scotty Kilmer, which consisted of him smoking a cigar into some tubing hooked into the intake. This is an awesome and inexpensive method for diagnosing vacuum leaks for casual smokers.
Method #2: Carburetor Cleaner
For those of us who are non-smokers there are several other methods. One old school method is with carburetor cleaner. ErictheCarGuy as a great how to video. I generally don’t recommend this for the DIY beginner because there is a risk of starting a fire and the risk is substantially higher if the engine is hot. Personally, I ONLY USE this method when the engine is cold. I also insist that if you do try this method that you have a fire extinguisher near by just in case.
This method calls for spraying around the intake method and hose connections until you hear a change in the idle. Sometimes the spray will even stall the engine and then you will have identified a clear source of your problem.
Method #3: Smoke Machine
The last method I am going to mention is to use a smoke machine. The good news is that smoke machines have become significantly more economical for most DIYers. I picked up the one that I use for BMWs for around $100 and it works like a charm. Smoke machines require a little investment but they are an excellent tool that you will be able to use for years to come. Smoke machines can be used for vacuum leaks, EVAP leaks and even exhaust leaks.
You can pick a smoke machine up here.
In conclusion, if you have a BMW that is revving on its own there are a number of potential causes. If the vehicle revvs high during a cold start and goes down after a few second then this is normal. If the vehicle surges in drive, you should immediately stop the car and have a mechanic fix the problem. If you have an idling problem, you likely can drive on it for a while, however, be aware that this creates a risk of stalling.
Diagnosis is usually simple with revving. Check the engine codes, floor matt, and then look for a high pitched whistling noise coming from the engine bay. The noise indicates the existence of a vacuum leak, which you can then diagnose.