| I know this is a technical post and should be posted in tech, but I also know that a large number of folks dont even bother to read the technical forum (you know who you are) and I feel that this writeup needs to reach as many folks as possible. Over the years I have had numerous incidences where I have been contacted by people having issues with their cars as well as cars coming to me with issues and in many cases, it has boiled down to problems with the EGR's vacuum lines being either cracked, broken, or improperly connected. I cannot stress enough how CRITICAL the configuration of this component is in vehicles that still have it, but I will at least show you why. For those of you that have a factory service manual, refer to bible page EF&EC-11 and 12. This is a hose routing diagram for just about everything connected to the plenum for both NA and TT models. VIEW B is what you want to focus your attention on. EF&EC page 8 and 9 give another view of all of the connections as well. Be sure to take a look at this as it shows you what exactly the connections go to. Once you have looked at these pages, turn to page EF&EC-18. I will begin there. 
There is no further information about this device in the FSM, but I will fill you in.
The EGR valve has a single control vacuum line that is attached to the top of the device, which controls the valve's position pneumatically. This control hose is routed to the EGR control solenoid valve. This solenoid has three ports on it and it is controlled via the ECU. These three ports connect to the following three points:
1) THe EGR valve itself.
2) The passenger throttlebody nipple located at the bottom of the throttlebody.
3) The turbocharger inlet pipe (on the plastic pipe with the accordion flex-joint)
3a) For NA guys, this third point is on the throttlebody inlet pipe (also the plastic pipe with the accordion flex joint). Here is an image of the EGR solenoid and its connections:
 In the event the EGR connections aren't properly hooked up on the non-turbo Z, it is of significantly less consequence. On the TT, however, it can lead to catastrophic engine failure. Theory
The EGR system serves to route exhaust gases, which are inert to the combustion process (because they have already been 'combusted'), back into the intake manifold. This is done to 'dilute' the intake air into the cylinders so that there is less available oxygen for the combustion process. This lowers combustion temperatures, which, thereby also lowers the formation of nitrogen compounds (NOx). The EGR valve's position is controlled by several factors:
1) EGR solenoid position
2) Throttle position
3) Engine load
Connections
Looking back at the EGR control solenoid, you will see that it connects to two points on the engine (the third being the EGR valve itself).
1) Throttlebody: The nipple under the passenger throttlebody is routed by a small passage into the throttlebody chamber, JUST before the butterfly. You can see this if you remove the throttlebody intake pipe. When the throttle is closed (idle) the small port feels no vacuum and it feels no more pressure than atmospheric. However, when you crack the throttle partially open (such as when cruising), this orifice feels a vacuum. The intensity of the vacuum is dependent on how far open the throttle plate is. 2) Turbo inlet pipe(TT)/throttlebody inlet pipe(NA)
The turbo inlet pipe and the throttlebody inlet pipe connection is solely for a neutral pressure source. Twinturbo
 Non-Turbo
 Operation
The ECU controls where the source of either vacuum or pressure(in the TT only) is coming from that is routed to the EGR valve itself. When a vacuum is applied to the EGR valve, it opens. When no vacuum is applied to the EGR valve, it remains in its static, closed state position. When positive pressure is applied to the EGR valve, there is an increase in closing pressure of the EGR valve, which allows it to resist the passage of exhaust gas from the exhaust manifold with greater ability. When you are at idle or cruising down the highway, the ECU turns on the EGR solenoid such that any vacuum created at the throttlebody port will cause the EGR to open in varying amounts dependent on throttle position. This is to reduce the emission of NOx gases. In the non-turbo, there is no positive intake manifold pressure at wide-open-throttle, however, the EGR valve's internal spring is more than strong enough to hold the valvebody closed against the pressure in the exhaust manifold. This prevents any exhaust gases from being recirculated back into the intake manifold, which would reduce engine performance. In the twinturbo it is a different story. When boosting, the exhaust manifold is typically under greater pressure than the intake manifold. The EGR valve's internal spring is sufficient to resist the exhaust manifold pressure, but only at stock boost levels. When you turn up the boost, it WILL partially blow open and allow some exhaust gases to enter the intake manifold. However, if the EGR valve, hoses, and solenoid are properly connected, when you are under boost, the boost pressure in the intake manifold is routed to the EGR valve to increase its closing pressure. As long as the valve itself isn't defective, it will not allow any exhaust gases to blow through into the intake manifold. And finally, when the ECU turns off the EGR solenoid, the EGR valve is then feeling the neutral pressure in the turbo intake pipe(TT)/throttlebody inlet pipe(NA)the EGR valve feels no vacuum, therefore the EGR remains in a closed position. Catastrophe
If the EGR is not properly connected (or it is defective) and allows exhaust gas to make its way into the intake manifold, this creates a couple of serious problems. 1) The ECU relies heavily on accurate mass-airflow-sensor information in order to determine fuel delivery and ignition timing (along with a slew of other control sequences). The typical trend is that as airflow increases, the fuel delivery increases and the ignition timing decreases. In the event that the EGR valve is allowing exhaust gases to enter the intake manifold, this recirculation of air reduces the amount of air drawn through the mass airflow sensor. This will cause the fuel delivery to drop, and the timing to advance. 2) Exhaust gas is hot, REALLY hot. The more boost you are making, the hotter the exhaust gases are. Typical exhaust temps on a twinturbo are from around 1100F up to 1550F (max safe). 93 octane fuel has an auto-ignition temperature of around 850F - this is the temperature at which it will spontaneously ignite, even without a spark or flame. There aren't really any concerns with blowing up the intake manifold since there is a large quantity of cooler air coming through the throttlebodies, not to mention, the injectors are waaay down at the intake valves. However, when amounts of super-hot exhaust gases are making their way into the intake manifold and on into the combustion chamber, you are significantly increasing the chances of detonation. Mix this with the fact that a leaky EGR is going to cause a leaner mixture and more timing advance, and you have a mix for catastrophe on your hands. ------------------------------------------------------------ I am sure that many of the vets out there already know this and have seen this problem before, but this is intended for the rest of you guys out there. I know it is a big fad to get in the engine bay and replace all the vacuum lines with fancy colored silicon counterparts and I also know how shotty the work of other mechanics can be, so do yourself a favor and spend a half-hour of your time going out to the car and checking that this system is working as it should. As for those that currently have their engines out of the car, or are planning to replace/rebuild/build-up an engine, consider completely removing this item. Eventually the EGR valve itself gets all gummed up or carbonated to a point where it will leak to some degree no-matter-what, so if you have the opportunity to ditch it, ditch it and pat yourself on the back. There have been numerous vehicles that I am fully aware of that live in California that are using my ECU programing, JWT programming, etc, and do not have EGR in their cars. Guess what - they have all passed emissions without it. Unless you get a really nit-picky inspector that goes digging around in the back of your engine looking for the EGR valve itself, you aren't going to fail an emissions test due to the lack of this component. You also aren't going to blow your engine up without it - you are far more likely to blow the engine BECAUSE of the EGR being in there and leaking. 300-Degree sells an EGR Elimination kit for $45 which makes the elimintation of this sytem VERY quick and easy. It comes with the plenum block-off plates as well as the exhaust manifold plug (for OEM manifolds).
[ http://www.300degree.com/hard_parts/egrkit/ ]
 Even though everything may be hooked up properly, there is still a small chance that your EGR valve is defective; either the vacuum control diaphragm has ruptured, the spring has failed or lost its temper, the valve/seat have become too carbonized to seal, or the valve is gummed up and sticking. This is a part that isn't a common failure item, but if the car incessently detonates at conservative boost levels and you have eliminated all other possibilities, this guy would be the next thing to consider. Good luck to all of you!
[ ashspecz.com ]
[ agpowers@bellsouth.net ] Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein
|
OK Folks, Time for the EGR101 class to begin. - 
