PSA: BMW USES METRIC VACUUM HOSE NOT STANDARD

Vacuum hose isn’t always a subject that is brought up much. If it’s not something in your engine that is ramming more air down its throat for more horsepower, people think it can be a boring subject. This may so, but vacuum lines are important, and it’s important to know that BMW uses metric sizes only. What most people don't know is that your local automotive or hardware stores do not sell metric sizes, so let’s dig into this no matter how “sucky” it gets.

The Basics:

A vacuum hose can refer to a line or tube. It is a flexible rubber connection that routes vacuum pressure from your intake manifold to various other components or accessories. In the past vacuum lines were used to power many more components than today, like windshield wipers, door windows, power steering pumps, power door locks and more. In modern day times, vacuum isn’t used as much, but it is still very important for accessories like brake booster, crankcase ventilator, charcoal canister and so on. These vacuum lines can be all over the engine pay with splices, tee fittings, and other connections to route this small tube where it needs to go.


The Issues:

After years of heating up and cooling down, the rubber vacuum lines can dry out and cause cracks. When this happens, it can open up and cause a vacuum leak. When a leak occurs, the engine intake manifold can no longer pull vacuum pressure as it is trying to suck outside ambient air into the hose. This is called a vacuum leak. When this occurs, you can set off check engine lights, and cause components that run on vacuum pressure to work poorly or not at all.


Types of Vacuum Hoses:

Vacuum hoses generally come from the factory made out of rubber. This is a flexible rubber tube that works great for this application. As I stated above, over time it can crack and cause issues. To solve this, we can now buy silicone vacuum line. Silicone vacuum line has two main advantages over its rubber counterpart. 1. The silicone is chemical resistant. This is important in an engine bay, as you can spill oil, gas, brake fluid and other chemicals which could eat through rubber hose and cause leaks. 2. The silicone can resist higher temperatures, almost double to be exact. Under the hood of your car can get very hot, especially if you are on track. Being able to withstand higher temperatures means the hose won’t break down and crack.


Sizes of Vacuum Hose:

Vacuum hose comes in two flavors, standard or metric. It is most important to know that all BMWs use metric sized vacuum hose. The most common size used on BMWs is 3.5mm. What you need to know is, there is no compatible standard size hose that will fit a BMW. Local automotive and hardware stores only sell standard sizes which will end up being too big or too small by about a .5 millimeter. This may not sound like much, but if you are not using clamps on your vacuum hose, then it can slip off while driving, potentially leaving you stranded. All vacuum hose is measured by the inside diameter.

Here is a chart showing standard vacuum hose sizes.

A chart showing how vacuum hose is measured
Vacuum Hose Size Chart

The Verdict:

The good thing is, if you are not in a pinch, metric BMW vacuum hose is just as cheap or cheaper than standard sizes from your local store. Make sure to keep up with your engine maintenance. Many vacuum leaks can make you chase other problems. E46s can be a great example of this. The back of the intake manifold on the M54 engine has some block off nipples for extra vacuum ports. If these crack or fall off, you will start to receive misfire codes making you chase down spark plugs and coil packs. In another blog we will address how to properly check your engine for vacuum leaks.


If you need new vacuum hose, we offer OEM BMW, and silicone replacement hose.




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