On The Road

Taking a peek at that oil leak

I have a customer who owns a 2006 Volvo XC 90 with a 2.5-liter turbo inline five-cylinder engine. For the past year, we’ve been tracking a small engine oil seep from the firewall side of the engine near the turbo.

The leak has been difficult to find because the turbo is crammed in the engine bay. It appeared as if the turbo, upper oil pan, lower oil pan and possibly the rear main seal were leaking, but it was too difficult to see. The seep recently became more pronounced, so we took a closer look at it.

After cleaning the firewall side and bottom of the engine thoroughly, we ran the car on the hoist. What we saw next was interesting. It looked like the oil was coming from everywhere. We cleaned the engine one more time and took an even closer look. The bulk of the oil was leaking from between the engine and transmission.

To gain access to the back of the engine, one must first remove the transmission. Once the transmission is removed, the flywheel and the back of the crankshaft are visible. The purpose of the engine is to turn the crankshaft to deliver rotational power to the front pulley and the flywheel. Because the crankshaft has to stick out of the engine on both sides, it must be sealed. That’s why there is a front crank seal and a rear main crank seal.

The primary reason for these seals is to stop oil from escaping, but they also stop crankcase gases (oil vapors, unburned fuel, etc.). The reason these gases or vapors are in the crankcase is that every time the piston goes through its cycle, a small amount of compression escapes into the crankcase. This pressure can build up to as much as two to four PSI. If the pressure is not released, it will push oil past the seals or even pop the seals out.

In the early days, auto manufacturers just put a small pipe or hole on the side of the engine block to release the crankcase pressure. As they became more concerned with the environment, car companies realized they had to do better.

In the early 1970s, we started seeing the first renditions of positive crankshaft ventilation (PCV). Basically, that involved running a hose from the hole on the bottom side of the engine back up to the intake and the top of the engine. Doing this vents out the crankcase pressure and possibly some of the fuel vapors in the process. Over the years, these PCV or breather systems have become more complex. Now they can separate liquid oil, oil vapors, pressure and unburned fuel.

Diagnosing the problem

Now, back to our Volvo. Prior to inspecting the rear main oil seal leak, I told the customer we were also going to check the car’s oil trap, or crankcase breather system.

Once the transmission was out, we saw that a large amount of oil had been passing the rear main seal. We then removed the intake manifold to gain access to the oil trap. That led to our “aha” moment.

The oil trap’s lower drain was completely clogged with carbon, and the engine side of the drain was completely clogged as well. It took us several hours and also removing the lower oil pan to properly clean the oil-trap drain passage.

Over years of driving, the car’s oil-trap drain clogged with slug or carbon. Eventually, the oil trap filled with oil and was unable to vent. Pressure built up in the crankcase and tried to escape wherever it could. That’s why it seemed like oil was coming from everywhere.

We always pay attention to the PCV or breather systems for this reason. On European cars, it can be expensive to change out all of the breather-system parts. We usually want to see a symptom first, but I plan on taking a close look at all the Volvo oil-trap systems in the future.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email This email address is being protected from spambots. You need JavaScript enabled to view it. or visit sunnyvaleforeigncar.com.

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