Emissions Devices

Disclaimer: The information contained within this article is intended for educational purposes only. It may be illegal to remove, modify, or tamper with any emissions device on your vehicle, depending on the state in which you live. Please check your local rules and regulations to ensure that you abide by them.

Before we discuss how these systems operate, I would like to comment on a general misconception associated with fuel injection in general. I have heard on numerous occassions that fuel injection in general is too complicated and restrictive due to emissions devices. In reality, the problem lies with people assuming that the emissions components are somehow required as part of the whole fuel injection platform, and that these devices pose a significant restriction in performance. Just because you are running a fuel injection system, does not mean you will have emissions devices. In addition, having emissions equipment does not prevent someone from running whatever engine modifications they want. The loss in power from these emission devices is quite small.

As many of you already know, Tuned Port Injection engines were factory installed on vehicles that were required to be emissions compliant. To ensure that emissions requirements were met, GM used several devices to lower emissions output. Basically, these can be broken down into three main systems : EGR, A.I.R., and EECS. None of these are absolutely necessary to run a Tuned Port Injection intake, but may be legally necessary if you want to be street legal in the state where you live.

Exhaust Gas Recirculation System (EGR)

The sole purpose of the EGR system is to reduce the formation of Oxides of Nitrogen (NOx). These are formed when the temperature in the combustion chamber reaches very high levels. To avoid this situation, the EGR system is used to lower combustion chamber temperatures by admitting small amounts of exhaust gas back into the combustion chamber. As you can imagine, exhaust gas does not burn, and does not help the combustion process. If the EGR system would admit exhaust gas into the combustion chamber at idle, it would cause a rough idle, or stalling. As a result, EGR does not allow exhaust gas into the chamber at idle (nor at wide open throttle, more on that later...). To accomplish this, there has to be some way of controlling EGR flow.

The way this is accomplished is actually quite simple. First of all, there is a round, flying saucer - looking part that mounts on the intake manifold. This is called the EGR valve. There is an opening in the intake manifold from the cylinder head that allows exhaust gas to move up to the valve. When vacuum is applied to the valve, it allows the exhaust gases to pass. To control when the valve receives vacuum, an EGR solenoid is used. This part mounts on the passenger side near the back of the intake manifold. It has a vacuum line which supples ported manifold vacuum, and another vacuum line running to the EGR valve. When the ECM requests EGR to be ON, it sends a signal the the EGR solenoid, which then allows vacuum to be applied to the EGR valve. When the ECM wishes EGR to be off, the solenoid will cease to apply vacuum to the EGR valve.

The ECM will turn on the EGR solenoid by grounding it. It does this via pulse width modulation (PWM). This means that the ECM will turn on and off the solenoid many times a second. Just how many times this occurs will affect the amount of vacuum applied to the EGR valve, and therefore the amount of exhaust gas admitted into the combustion chamber. During the time that EGR is requested by the ECM, fuel output and spark advance are also altered. More timing is added, and the fuel mixture is leaned a bit.

During WOT (wide open throttle), the ECM shuts down EGR. Since EGR will play no role whatsoever under WOT, it will not impede the engine's ability to generate horsepower.

How does the ECM know if there is a problem with the EGR system? Well, the answer depends on what year TPI setup you have. If you have an 85-89 setup, then there will be a single wire that goes to the base of the EGR valve. This is basically a temperature switch. When the ECM requests EGR, it will check this wire to see if there is a change in temperature. If no change is detected, the ECM will think there is a problem with the system, and will throw a code 32. Most of the time, the problem is the temperature switch that is bad. However, you should first check for vacuum leaks, check that all vacuum lines are properly routed, check the harness connector at the EGR solenoid with a voltmeter, and make sure that the EGR passages are not clogged with deposits.

The 90-92 TPI setups did not use a temperature switch on the EGR base. Instead, the ECM monitors the MAP voltage to determine if an EGR request was successful or not. When EGR is turned on, engine vacuum will lower a little. The ECM will throw a code 32 if it suspects that the EGR request was not successful.

So what would someone gain by disabling EGR? Just about the only thing would be less parts under the hood. Basically, removing EGR gives you more space (although not much). To correctly disable EGR from your vehicle, you MUST at the very least, disable it in the prom. This is done by setting the minimum temperature to enable EGR to 151 degrees Celsius (maximum allowable temperature), and setting the minimum vehicle speed to enable EGR to 255 mph (maximum allowable speed). Since the engine will never reach either of these conditions, EGR will never be requested by the ECM. Since EGR will never be requested, it does not matter if you leave the EGR system all installed in its original place, or if you remove it from the car. If you decide to remove it, you will need an EGR block off plate to cover the hole in the intake manifold.

If you simply remove the EGR system from the car, but do not disable it in the ECM, you will run into significant problems. You will likely run into detonation, the engine will run very poorly, lack power, and will probably run on the hot side as well. In addition, you will get a code 32 before running very long. If you recall from before, the ECM alters fuel and spark advance when it thinks EGR should be on. If no EGR flow is possible because you removed it, you will have a lean condition which will be further aggravated by advanced timing.

I have not had a chance to discuss A.I.R or EECS (canister purge). I will finish the article as soon as I have some spare time, but I wanted to post the EGR section for the moment atleast.