I’ve been in this industry for so long that it always amazes me when I see something new – like I did last month.
A customer called to say he saw fluid leaking from the bottom of his 2013 BMW X3 28i 2 liter.
He said the leak was on the back-left side of the car. He told me there were approximately 20 drops of a darker fluid on his garage floor. Obviously, I envisioned a leak from the back of the engine.
Once the car arrived at the shop, we put it on a lift. I then took the bottom engine cover off and started looking for the leak. Usually you see small seepages here and there, but this engine was spotless. The engine was spotless, the transmission was spotless, brake hydraulics were spotless and there were no leaks from the cooling system.
That’s when I called the customer to ask him exactly where he saw the leak. He said the leak was on the driver’s side – behind the left rear wheel. So, I immediately walked back out to the car and, sure enough, there was engine oil leaking from the rear muffler noise-reduction flap diaphragm.
Just to be clear: There should never be oil leaking from an external component on the exhaust. Oil could get inside the exhaust due to a worn engine or worn turbo, but that was not happening in this case.
Before I go further, let me explain where the vacuum comes from to control the exhaust-flap diaphragm. There are many components controlled by the vacuum, including the exhaust gas recirculation valve, secondary air valve, vacuum advance, turbo control valve, brake booster and exhaust flap. All of these components used to get their vacuum power from the intake manifold, but as cars became more advanced, they needed a more precise vacuum source.
Air/fuel mixture is partially controlled in the intake manifold, so if you rob vacuum power from that, it will impact the overall fuel trim. Basically, if you were to apply the brakes, you would be impeding the fuel-trim process. Adding a mechanical vacuum pump to the cylinder head can create as much vacuum power as needed without affecting the engine’s fuel-trim operations. Vacuum pumps on German cars are nearly always attached to the cylinder head and are driven by the camshaft. The vacuum pump is lubricated internally through the cylinder head from the engine.
Diagnosing the problem
Back to our BMW, which has a vacuum diaphragm connected to a rod and flap. The noise-reduction diaphragm is part of the rear muffler. Its main purpose is to quiet the exhaust under certain circumstances. It also can be used to speed up the warming of the catalytic converter to help with emissions.
The noise-reduction flap diaphragm is controlled by vacuum. There is a vacuum hose that runs from the rear muffler to the front of the engine. Once the vacuum hose is in the engine bay, it connects to a vacuum reservoir that is part of the upper engine cover. The vacuum reservoir is fed by the vacuum pump on the back of the engine.
This is where it got interesting. The vacuum pump is lubricated with engine oil, but that oil and vacuum never mix. Because the vacuum pump is always pulling down the vacuum, it should not let air or oil out of it.
So, what happened? There is a seal in the vacuum pump that keeps the oil and air separated. There is also a vacuum check valve that allows vacuum to be pulled down but stops air from going in the opposite direction. The vacuum pump seal went bad and allowed oil to get in, destroying the check valve. When the engine was turned off, oil flowed out of the vacuum pump and into the vacuum reservoir. From there, it migrated outward and eventually reached the rear muffler noise-reduction flap diaphragm. That diaphragm is not designed for oil, so it leaked out.
The repair involved cleaning the vacuum lines and the vacuum diaphragm, removing the oil from the vacuum reservoir and replacing the vacuum pump. Because the vacuum line is so long, we cleaned it several times to ensure we removed most of the oil.
After performing all of these tasks, we tested the system and returned the car to the owner.
Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.
