Tech Tip No. 11

Ned Ritchie

Originally published in the quattro quarterly

Copyright 1995

"False Air" Leaks

One reason your Audi may be hard to start, may fail to start, or simply runs rough is because air is sneaking into the engine without going through the air measuring system, or going through the system and then sneaking out.  Some mechanics call this a "false air" leak.

With fuel injected engines the correct amount of fuel is injected after measuring either the air mass or the air volume. When measuring the air mass the air to fuel ratio is about 15 parts of air to 1 part of fuel, and when measuring the air volume the air to fuel ratio would be about 11,000 gallons of air for 1 gallon gasoline.  Audi has used both systems depending on the year and model of the car.

This air measurement usually takes place near the air filter some distance from the engine.  To get from the measuring system to the engine the air may have to travel through metal pipes, rubber hoses, hose clamps, all of which have the potential for air leaks. If the intake system is connected to the crankcase all the crankcase ventilation system's valves, piping, and hoses are also things that can leak.

Let's say the system weighing the air calls ahead to the fuel injectors and says, "1500 units of air are coming."  The fuel injectors sprays in 100 units of fuel and the engine starts and runs.

When you have a leak in the air path between the air metering and the engine, the air sucked in is not measured and there is not enough fuel for the total air going into the engine. The ratio of air to fuel must fall within a certain range or the engine will not run well.  In fact, if this "false air" leak is big enough the car just won't start.

Here is an experiment for owners of 1986 to 1990 turbo Audi's: Set your hand brake, open the hood, start the engine, and then get out and pull out the oil dip stick.  The engine speed will drop and you'll possibly will kill the engine.  Why?  The small amount of air entering the hole created by removing the dip stick is enough of a "false air" leak to kill the engine!

It is possible to have a small "false air" leak that will allow starting the car in the morning but not allow a restart if shut off when still cold.  Why?  Some cars have a cold start injector to help starting when the engine is cold. This special fuel injector activates only when the engine is very cold or has not been started for some time.  Depending upon your engine and the temperature this injector will spray for only a few seconds, usually for 2 seconds or less.  The engine will start, but it is a one shot deal.  The cold start injector on most cars does not spray again if the engine is stopped and attempt is made to restart it

On turbo engines a "false air" leak can create an additional problem. Under boost the path to the engine is now under pressure.  Any leak now creates the opposite effect. Air that has be measured is squirting from the leaks, and fuel is being sprayed into the engine for the air that leaked out. The engine runs rich, rough, bucks and stumbles, black smoke pours out the exhaust pipe, and there is no power. (See Tech-Tip #  5 "Loss of Boost Pressure -- Part I")

The next few months when thinking of ways to increase power in your Audi think about this:  More fuel than needed for maximum power does not make an engine run better.

INTENDED ACCELERATION ®

© Copyright 1997-2004 Ned Ritchie