The DISA valve is a component situated in many BMW intake manifolds, and is vital to the smooth operation of the engine. DISA stands for the German “Differenzierte Sauganlage” which basically means Differentiated Intake, and the valve for it is often called (at least in our catalog) an Intake Manifold Adjusting Unit. Rough driving, loss of power, and increased fuel consumption can all be caused by a malfunctioning DISA valve, which is a known failure point. It is, afterall, situated right on top of the engine, plugged straight into the intake manifold. There were a few variations of the system, but to truly diagnose if your DISA valve is bad, it helps to understand a little bit of the background theory. I will go over that for the tech geeks out there at the end of this post. The nitty gritty of it is that this is a car part that will cause problems eventually, and it’s important to know of it before you begin to encounter the first signs of it breaking.
A Failing DISA Valve – Shop DISA Valves
The valve can fail in a few different ways, but realizing when it’s about to fail is crucial. This is because they can literally fall apart and scatter debris into your engine. You do not want this. The failure starts with an annoying rattle as the valve is unable to stay closed. When you unplug the valve, the airflow will simply push it open and the rattling will go away. Here’s a great video illustrating this on an M50 family engine, specifically an E39. You can test the diaphragm that actuates the valve by putting vacuum on it. If it holds vacuum but is still being loud and rattly, you may be looking at a failure of the flap itself.
DISA valves generally last around 70-100k miles depending on what version it is, driving style, and environment. Since the valve’s primary job is to divert airflow into the cylinders, the engine will not run properly if the air flap is not making a proper seal. Generally speaking, the result is problems with fuel mixture, resulting in poor fuel economy. Occasionally the air seals will go bad without the actual valve failing. The main culprit is the O-Ring that connects right to the manifold. If this seal goes bad, you will end up with a vacuum leak, resulting in un-metered air getting into the cylinders. On an M52, you can purchase just the seal, part number 11617504543. In most other cars you must buy a new unit or sketchily attempt to RTV the bad o-ring.
The Air Mass Meter (Or MAF Mass Air Flow sensor) tells the engine ECU exactly how much air is entering the engine. If any of the above issues allow in air that the ECU is not expecting, it will not be able to add more fuel to balance the mixture, and you will end up with a dangerous lean condition. A fully failed or leaking DISA valve will usually signal a check engine light, as if it wasn’t signal enough that the engine runs poopy. Now of course there are other places in the intake system where leaks can develop after the AMM, so keep an open mind when diagnosing.
On many cars, the valve is located right on the end of the intake manifold. On later cars, BMW installed a 3 stage DISA system that uses two valves. The second is harder to get to, installed on the bottom of the manifold.
How intake tuning works, and why this all exists
Think of your engine’s intake manifold like a brass horn. To get the sound you want, you lengthen or shorten the horn to achieve a resonance (tone). As your intake valves close on a particular cylinder, the air that was charging in abruptly encounters the back of the closed valves and bounces off. Eventually that pressure wave will travel back up the intake, hit the back side of it, bounce again, and charge straight back towards the cylinder (all at roughly the speed of sound). Still with me?
The idea on intake tuning is that you want that pressure wave to arrive back at the cylinder at precisely the moment the intake valves open again, allowing the air sitting there to whoosh right in like Japanese Soccer team getting onto a Shinjuku train with a running start. This principle also works with exhausts on sort of the same principle, except on a header you want the pressure wave to pull the exhaust out of the freshly open exhaust valves (this is called “Scavenging”).
So, visualizing this, you can understand that at low RPM, it’s beneficial to have a longer length tube so that as the engine spins slowly, the pressure wave has a little more time to get to the back of the plenum and arrive to the cylinder as the valves open. At high RPM, shorter runners are beneficial to get this ram air effect with a very short frequency.
This effect only works for a narrow RPM sweet spot. When tuning an intake for a race car, you want to position this ram air effect at your target rpm range that you spent most of the time at, usually towards the top of the rev range. The downside is that when you aren’t at that RPM, you will not gain these benefits and the car will be down on power. For awhile this was just a given, but at some point car manufacturers started adding in variable runner length manifolds, so that the extra power lump can occur at two separate stages. A set of long tubes for cruising-around-town torque, and a way to switch to short tubes when you get on the go-pedal and want to rev it up a little.
Yea Ok, but what does a DISA valve actually do?
Mercedes Benz handles this two stage intake scenario with a very complicated set of plastic folding valves and flaps, which tend to go bad and require a very expensive service. BMW developed a pretty ingenious solution that uses only this small valve to block off flow across various portions of the intake. The intake is actually two separate loops folded over itself, which is why the throttle body is positioned at the bottom. At low RPM, the valve stays closed.
This forces the pressure wave to take the long way around the manifold through the “resonance tubes”, resulting in a longer runner and ultimately better low end torque.
When the engine transfers over to a high rpm setting, the valve opens, allowing the pressure wave to take a short cut and act more like a short length intake.
Both resonance tubes are then open to allow the maximum amount of air to be sucked into the engine as an added bonus.
In later engines, a third circuit is added to fill out the midrange RPM spectrum, along with a second DISA valve. The principle is the same, with only the length of the shortcut changed in order to achieve a medium range power boost.
When merged with Vanos or Double Vanos, BMW has achieved a highly variable engine with a much broader torque curve without adding substantial complexity. Yes, Vanos Solenoids do go bad, and DISA valves also break, but they are not hard to replace and generally are inexpensive to repair. If you have any questions as to what DISA valve fits your car, contact customer service today, or simply put your car into our vehicle selector and check out our categories yourself. I hope this made sense, and let me know if you have any feedback by commenting below!