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Fixing Common Vacuum Leaks
 
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Pelican Technical Article:

Fixing Common Vacuum Leaks

Jared Fenton

Time:

2 hours2 hrs

Tab:

$0 to $300

Talent:

***

Tools:

Screwdrivers, T20/30/40 Torx drivers, pliers

Applicable Models:

Porsche 955 Cayenne S (2003-08)
Porsche 955 Cayenne Turbo (2003-08)

Parts Required:

New gaskets and seals

Hot Tip:

Check very carefully for leaks

Performance Gain:

Better running engine

Complementary Modification:

New spark plugs

This article specifically covers the Porsche Cayenne S, however the concepts I'll go over apply to nearly all fuel-injected gasoline engines. In a gasoline engine, the speed of the engine is controlled by the amount of air that enters through the throttle body. In a diesel engine, there is no throttle body. The speed of the engine is controlled by the amount of fuel injected into the cylinders. Diesel engines are a different animal that we will cover in a different article.

A poorly running car can often run amazingly well after an afternoon's worth of work, going through the various rubber boots and hoses around the engine. Vacuum lines, intake boots, seals and many other small gaskets around the engine can be the source of poor running and idle problems

Many times, a lean running condition can be traced back to a cracked or torn boot in the system. A good quick way to check the overall health of the system is to take the oil filler cap off with the engine running. If the system is in good shape, the idle of the engine should drop and the car will run rougher. This is due to the extra unmetered air flowing through the oil filler port. If there is no change in the vehicle's idle, then you probably have a vacuum leak somewhere in the system. Don't forget to check places where vacuum leaks may not be as obvious, such as the oil filler cap, the dipstick and also the valve cover. A quick way to check for a vacuum leak is to spray a little bit of carburetor cleaner around the engine at various places and see if the idle changes. If it does, you know you have a vacuum leak in the general area in which you sprayed. I'll now go over the various seals and gaskets on the engine that can leak.

Crankcase Breather Hoses: It's not uncommon to have these plastic lines crack after years of heat cycling in the engine compartment. This typically results in very rough running at idle. Additionally, the air-oil separator will not function due to the loss of vacuum. Over time, this can result in problems when trying to get the car to pass an emissions test. The good news is that the hoses are easy to replace, however they can be a bit expensive. See our article on Replacing Crankcase Breather Hoses for more information.

Valve Cover Gaskets: Over time, heat and engine oil can cause the valve cover gaskets to crack and start to weep oil both out the sides of the valve cover. This also has the effect of letting a small amount of air into the engine past the throttle body. This can affect the idle of the car, not to mention also spraying oil mist all over the place. I've seen one Cayenne with running problems that stumped a few mechanics. Once the gaskets were changed, the engine ran perfectly. See our article on Throttle Body Cleaning for more information.

Throttle Body: Another often overlooked place for a vacuum leak is the throttle body. Over time, the environment of the engine compartment can degrade the seal that sits in between the throttle body and the intake manifold. This seal is easily replaced and should take no more than an hour to change.

Air-Oil Separator: The engine air-oil separator is an emissions device located on the top front corner of the right side valve cover. The separator is responsible for collecting residual gases and vapors contained inside the crankcase and funneling them back into the intake manifold where they can be burned in the combustion chamber. This reduces the overall emissions of the engine.

When the separator fails, it causes an increase in the overall vacuum in the engine crankcase. In the most extreme cases, the air-oil separator fails to separate the oil from the air, and oil is then sucked into the intake manifold. Oil in the intake system is not healthy for the engine and it can foul spark plugs and destroy catalytic converters at the very least. The failure of the air-oil separator is often (but not always) accompanied by white smoke exiting the vehicle's exhaust and a generally poor running engine. You may experience a check engine light (CEL) as the oil being drawn into the intake can affect the mixture level. The oil cap may be very difficult to remove when the engine is running due to high vacuum levels. In addition, you may hear hissing from the separator when it's running. This is caused by air being sucked in past the diaphragm seal.

The good news here is that the separator can be repaired rather easily in under an hour. The most common cause of failure is a diaphragm in the top of the separator that cracks, causing an air leak. In this article, I'll go over the steps involved with replacing this diaphragm. The seal itself is part number 948-107-437-50 and should not cost more than $5-10. See our article on Repairing the Air-Oil Separator for more information.

Shown here are the upper crankcase breather hoses on your Cayenne.
Figure 1

Shown here are the upper crankcase breather hoses on your Cayenne. Pull the front engine cover off the top of the engine and inspect the hose between both valve covers (green arrow) as well as the hose connecting the air-oil separator to the intake manifold (yellow arrow) Also listen around the area of the air-oil separator (blue arrow) for any leaks. See our articles on Replacing Crankcase Breather Hoses for more information.

Also be sure to check the hoses (green arrow) on both sides leading from the air filter housings to the throttle body.
Figure 2

Also be sure to check the hoses (green arrow) on both sides leading from the air filter housings to the throttle body. These can develop cracks that can affect the fuel injection system. See our article on MAF Sensor Replacement for more information.

One other often-overlooked place that can develop vacuum leaks is the small seal on the inside of the oil filler cap (green arrow).
Figure 3

One other often-overlooked place that can develop vacuum leaks is the small seal on the inside of the oil filler cap (green arrow). Replacement is easy, simply pull the old seal out and place the new one in its place.

Shown here is the throttle body seal on your Porsche Cayenne.
Figure 4

Shown here is the throttle body seal on your Porsche Cayenne. This is another seal that can lose its effectiveness over time. See our article on Throttle Body Cleaning for more information.

Shown here is one of the two large gaskets that fit on the underside of the intake manifold.
Figure 5

Shown here is one of the two large gaskets that fit on the underside of the intake manifold. While not as common, I have seen one gasket that did fail. See our article on Intake Manifold Removal for more information.

The valve cover gaskets can also be a source of vacuum leaks.
Figure 6

The valve cover gaskets can also be a source of vacuum leaks. Shown here is the engine with the left side valve cover removed. Please see our article on Valve Cover Gasket Replacement for more information.

Here is the air-oil separator that sits on top of the right side valve cover.
Figure 7

Here is the air-oil separator that sits on top of the right side valve cover. This can also leak internally and cause running problems.

Here is the gasket that sits inside the air-oil separator.
Figure 8

Here is the gasket that sits inside the air-oil separator. As you can see, the one in our project vehicle had been leaking (green arrow) because of this crack just above the seal. See our article on Repairing the Air-Oil Separator for more information.

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