Shown above is a photo of a dislodged plastic vacuum hose for an early MINI Cooper (2002 - 2005 R52 supercharged type). Often when a technician removes the air intake box or the plastic intake duct, this hose is overlooked. If enough tension is applied to this hose without disconnecting it from the air duct, it will break.
The job of this hose is to supply vacuum to the manifold absolute pressure or MAP sensor (shown in white circle). The MAP sensor is very important to the proper operation of the engine. Its primary function is to measure manifold pressure while the engine is running. This data is used by the DME control module to determine injection quantity and duration. Since the sensor is located below the engine thermostat and not directly fitted to the intake manifold, its correct operation requires a continuous supply of engine vacuum or pressure via the vacuum hose.
So how does the hose get broken? Good question!
Many times, when attempting to remove the air intake duct for the throttle housing, a technician may forget to disconnect the MAP sensor vacuum supply tube. Sometimes a tech may be making a thermostat repair, which requires removing the MAP sensor, and accidentally breaks the tube. Even age can play a factor in this scenario; although these tubes are made of some high strength material, they can become brittle over time. Just probing around and pushing on an old tube (item #20) could cause damage.
So how to avoid breaking the vacuum supply tube?
Going back to our first photo, you can see that there are special red connectors fitted to the air ducts. There are two red connectors shown. One is for vacuum supply to the MAP sensor, as discussed here, and the other is vacuum supply for the brake booster (equally important). If you take your time and carefully push down on the red connector, the vacuum tube is easily released. Too many time this step is overlooked or the technician doesn’t know how this connector works. To refit the tube, you simply push the vacuum supply tube back into place. If too much pressure is applied and the intake air duct tube or its connectors are damaged, replace the entire hose; the red connectors are not sold separately.
Remember, these hoses need to be disconnected if you are removing the intake air duct. They are important. They have special connectors that can be released without special tools. Use a gentle approach when working with the intake air duct and these attaching hoses. Don’t just pull on the lines to try and release them. They will break! If you find yourself with broken vacuum tubes, replace them. I have found many “repaired” hoses that come apart after a while and create vacuum leaks, causing all sorts of rough running issues.