@WeaponX … The ONLY SIN anyone can commit regarding Automotive Mechanical Repairs here at GMTN… is One of WILLFUL Ignorance. You will never be chastised because you lack the long exposure and experience that comes from the experiences of MANY GMT Nation Members who have been working on the GM Atlas 4.2L Engine Platform for years. It is impossible to have an innate, intimate knowledge of these Motors based strictly upon ‘Mechanical Intuition’.
We all come from the same point of origin… But by using the Proper Tools, Proper Diagnostic Methods and the use of GM OEM Service Manuals (courtesy
@Mooseman) along with many On Topic Threads that are available here… will make all of the difference in Helping You to come up to speed on figuring things out over time. You can Download the GM Shop Service Manual for your particular SUV at THIS Link:
********* Please note that the FireFox browser has issues with the password handling and that these downloads are not under HTTPS, flagging them as security risks. Chrome and Edge work fine once you allow the download. ********* ATSG 4L60E Manual 4L60E Rebuild Manual 2005 GMC Envoy XUV...
gmtnation.com
The ‘SAIS’ Acronym stands for “SECONDARY AIR INJECTION SYSTEM” and the 'SAIS' Pump Solenoid is mounted on the 2005 and later model Trailblazers and Envoys Upper Passenger Side of the Engine Head… but NOT on the 2002-2004 SUVs. That port is blocked off in the Early Model Trailblazers and Envoys via a small Metal Plate placed over a High Temp Graphite Impregnated Gasket placed over the ‘SAIS’ opening in the Head and held in place by Two Studs and Two Nuts. If that Plate were to come loose… un-metered Air would enter the Exhaust Stream and cause the O2 Sensor(s) to pick up and read TOO MUCH OXYGEN. The resulting affect would cause the Air-Fuel “STOICH” to get out of whack.
Its purpose is to pump Fresh Air into the Exhaust System Stream just after starting up a "Cold Engine" to temporarily provide additional Oxygen to the Catalytic Converter and speed up the time it takes the CAT to Heat Up and allow the Upstream Oxygen Sensor to enter a “Closed Loop” Condition. This allows the PCM to begin sensing the Short Term and Long Term Fuel Trim Conditions more quickly and allow the Engine to run as close to the Magical “Stoichiometric Perfection” Air to Fuel Ratio of (15) Parts Oxygen to (1) Part Fuel. This is one of quite a few EPA Regulatory Mechanical Devices Federally Mandated under the EPA “Clean Air Act”.
I'd like to encourage you to visit my “Flickr-Bucket” via this link and familiarize yourself with the Thousands of Available Images covering many aspects of the Atlas Engine. It is FREE and always available for Ready Reference. For example… Here are Two images of the 'SAIS' Solenoid from one such Album: One WITH the 'SAIS' Device Bolted to the Engine Head... and one WITHOUT and instead, using a Flat Plate Bolted to the Engine Head to close off that 'SAIS' Air Inlet Port:
www.flickr.com
View attachment 91887View attachment 91888