Intake Air Leaks

Air leaks in the manifold and intake plenum seems to be a common nuisance with fuel injected Z cars.

Fortunately these air leaks are easy to diagnose and resolve.

There are two types of air leaks:

• Internal leak that bypasses a valve or sensor
• External air enters plenum or manifold (often referred to as a vacuum leak)
  

Internal leaks are 99% limited to the throttle body area though an AFM leak can occur. When this type of leak bypasses the throttle body (TB), a fast idle will result. Here are the primary TB leaks and their resolution:

1. Aux Air Regulator (AAR) stuck open (mechanically). This valve's purpose is to cause a controlled leak when car is cold. As the car warms up, the valve closes.

2. Boost Controlled Deceleration Device stuck open (electrically). This valve's purpose is to allow more air into combustion chamber when you pull foot off gas quickly.

3. Idle Set screw improperly set. This valve's purpose is to adjust air bypassing the TB so that the idle speed can be set.

4. Throttle Body valve gummed up and not closing fully. Since this valve is mechanical and controlled with a complex linkage from the gas pedal. A weak return spring on the TB assembly or a binding link could be the culprit.

External leaks (Vacuum Leaks) can occur in many parts of the engine. Since the engine functions by pumping air through it, air leaks in the system can be very disruptive.

External leaks in the intake manifold occur downstream from the AFM and are "unmetered". This extra air sneaks unnoticed past the AFM thus the ECU does not know about it. Since the ECU is unable to add the extra gas to mix with the extra air, the air/fuel mixture becomes lean. This can result in a faster idle and hotter engine. If the leak is big, the engine may stumble or stall out.

Common areas to check for manifold air leaks are:

1. Rubber boots between AFM and Throttle Body
   
   
   Ripped AFM to TB boot (note the silicone applied by the previous owner)
   
   
2. Vacuum hoses to manifold (brake booster, evaporative fuel canister, pollution devices, fuel pressure regulator, distributor)
3. Hoses to/from AAR and valve cover breather
4. manifold to head gasket
5. injectors, cold start valve seals

External air leaks can also occur at the head or engine block. These typically cause the engine to stumble or even stall.

Common areas to check for engine air leaks are:

1. Valve cover
2. Oil cap
3. Oil pan
4. Dip stick seal
5. front engine seal

 

Note: If the air to fuel mixture is set too rich at idle, a vacuum leak will be difficult to detect. Typically, a weak AFM spring will cause a richer mix and a typical check such as removing the oil cap to make the engine die will not do so.