EGR and EVAP Emissions Systems nonsense

I am sure by now everyone has heard from the old racer down the street that you need to yank all of that emissions "crap" off of your car because it hurts performance and does not have any benefits.  Nothing could be further from the truth.  First off, when functioning properly, both the EGR and EVAP systems are SHUT OFF at Wide Open Throttle.  So exactly how are these systems degrading performance?  The short answer is they don't hurt performance and removing them can cause annoying or damaging results.

The EVAP system is designed to recycle or burn unused gasoline vapors instead of allowing them to escape into the atmosphere from the fuel system.  Gasoline fuels vaporize at room temperature.  At higher temperatures, more gasoline vaporizes and can build high pressures and in greater concentrations.  This can occur even with the vehicle sitting in the hot sun on a summer day!  On most fuel injected cars, the fuel is constantly circulated thru the fuel rails which are mounted on the top of the engine.  As a direct result, engine heat is constantly transferred to the fuel system.  This increased heat causes more fuel to vaporize in the tank which results in a pressure buildup.  All of these vapors have to go somewhere.  If allowed to escape to the atmosphere, at the very least they can present an annoying and smelly problem -- raw gas fumes.  The charcoal canister is designed to store these gas vapors until the engine is operated.  When the engine is operating at preset parameters, the canister is vented to the intake manifold and thus is "purged" of its stored vapors.  Most 96-up OBD-2 equipped vehicles have what is known as an Advanced EVAP system.  Here, a second canister is installed back by the gas tank and its function is to store gas vapors accumulated during refueling.

The EGR system is a little more involved in its function.  The EGR valve and system are designed to introduce INERT exhaust gases into the intake manifold and engine during preset circumstances.  The primary reason for doing this is to cool the combustion event which results in less NOx gas emissions from forming.  There are a few side-effects to having a functioning EGR system present.  First, because the EGR system is cooling the combustion event, less heat is transferred to the engine resulting in less cooling load on the cooling system.  Second, the inert gas present takes up space that would otherwise need to be occupied by more fuel and air.  Therefore, in affect, the engine effectively becomes "smaller" (breathing less) and more efficient using less fuel when the EGR is operating.  A properly functioning EGR system has been shown to cause slightly better gas mileage compared to an identical engine and vehicle without a functioning EGR system.  Now there is a potentially damaging drawback to removing or disabling an EGR system on a vehicle.  If the computer does not know that the EGR is not functioning, it may be commanding more ignition timing (as it usually does when it commands the EGR system ON) which can result in hotter combustion temps and even detonation.  If you remove or disable your EGR system, you should have the computer reprogrammed to disable the EGR spark advance parameters or significant engine damage could result.  Shown below are some of the EGR adjusting tables from a 1987 Fiero 2.8L stock chip program that show the adjustments the ECM makes to fuel and spark when it thinks the EGR is operating...

As you can see in the left table above, the Fiero ECM will add as much as 9.5 degrees of spark advance to the existing timing advance when it thinks the EGR is operating at 43.75% Duty Cycle or more.  The table on the right illustrates the amount of reduction in fuel delivery the ECM is giving the engine while it thinks the EGR is operating (the BPC number = base injector pulse constant; the lower the number, the leaner the fuel delivery).  As with all GM ECMs, the ability to detect a fault with the EGR system is built in by the factory.  However, in order for the ECM to detect a fault with the EGR system, the engine must be run for a predetermined amount of time and operated under certain conditions before these "self-checks" of the EGR system are run.  Once a fault is detected, the ECM can set a trouble code for the EGR system which will suspend all EGR operating commands (including changes to the fuel and spark).  The trouble is, if you have disabled your EGR without reprogramming the computer; until a fault is detected, the ECM could be giving your engine more timing and less fuel than it needs which can lead to damage.  On most GM computers, once the engine is shut off, these self-checks may be run again by the ECM on the next key cycle.  Some engines, such as the Fiero application being discussed, uses no knock (detonation) sensor.  So damage occurring to the engine by disabling the EGR system without reprogramming the computer is a real threat.  The CORRECT way of disabling your EGR system is to have the computer reprogrammed to disable it which will disable the adjustments made to the fuel and spark delivery.


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