BMW Rough Idle On Startup Diagnoses and Repair – Part 1 Vacuum Leaks

As discussed at the beginning of this article, there can multiple sources of a vacuum leak in your BMW's air intake system. The only way to determine the source is by smoke testing the system. When smoke testing the intake system on your BMW, always start as far upstream as you can (as close to the air filter you can get). Remember that any leaks on the air filter side of the mass air flow sensor will not affect engine performance and do not have to be addressed (see diagram below).

Background on this test - Our 2006 530xi took a VERY hard jolt to the front end when we ran over a curb in a local parking lot. The engine immediately began to run rough and exhibited very poor acceleration. A brief visual inspection did not show any loose vacuum lines so we smoke tested using a cone adapter and plastic plug as detailed below.

1. Always begin your smoke test as far upstream as you can in the air intake system. The preferred location to begin is at the mass airflow sensor, if possible. Remember that the exact location and how you get there will differ among BMW models. These steps should serve simply as a guide.
2. Our 2006 530xi has the N52 six-cylinder engine. Since we are dealing with a BMW rough idle on startup, we will start our test at the intake muffler (air filter housing) and work our way downstream towards the cylinder head.
3. Remove the intake muffler and disconnect the mass air flow sensor so a rubber cone adapter can be inserted inside the intake boot.
4. Firmly install the rubber cone adapter into the intake boot.
5. Connect the smoke tester to the positive and negative battery terminals in the engine compartment and to an air compressor. Verify the smoke tester is on and is producing smoke. Always verify the machine is producing smoke before attaching it to the adapter!
6. Turn off the smoke and attach the nozzle to the cone adapter.
7. Make sure the cone adapter is snug and turn the smoke back on. Look for any smoke leaks in the intake boot or around the intake manifold. If there is a leak it will quickly be evident.
8. Our leak was quickly visible. Smoke began pouring out of the intake boot where it attaches to the throttle body.
9. Remove the intake boot from the throttle body and inspect for any cracks. BMW engine intake boots are infamous for drying out and cracking...especially in the flexible section.
10. After further investigation, the impact of hitting the curb caused the hose clamp on our intake boot to come loose from the throttle body allowing unmetered air to enter the engine. In fact, the hose clamp fell apart when we were removing it as shown below. We installed a new hose clamp and the rough idle was cured.

The next step in diagnosing an intake system vacuum leak is to test the system from the other end. Even if you find a leak in the intake boot in section 1 above, you should complete this step. There could be other leaks that may not be visible when testing from the intake boot.

1. Begin by removing the engine covers from the top of the engine. We do this so we can get a clean unobstructed view of the intake manifold gaskets and piping.
2. You should have a good set of diagnostic caps/plugs in your smoke test kit like the ones below. If you don't, please see our parts list above to order.
3. Remove the oil filler cap and its plastic mounting ring from the valve cover. The mounting ring must be removed to get a nice clean seal for the test cone in the next step.
4. Flip the diagnostic cone over and insert it into the oil fill opening as shown below. Insert the smoke tester nozzle into the top of the cone.
5. While using one hand to press down on the test cone, fill the system with smoke.
6. When you verify the smoke has reached the throttle body, turn the smoke machine off and install a test plug into the throttle opening as shown below.
7. Turn the smoke machine back on and look for leaks in the intake manifold gaskets, crankcase breather lines, vacuum lines, and other piping. This is a great way to also find oil leaks in the valve cover gasket.

There are multiple points for possible intake manifold vacuum leaks in a BMW rough idling engine, especially around the intake boot and throttle body. Look for any cracked tubes or pipes...especially plastic ones. The intense heat in the engine compartment combined with age causes plastic and rubber piping to become brittle and break. The intake system on your BMW must be air tight to avoid a rough idling engine.