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Safely Modding Your 3800 For Dummies digitaloutsider - W-body.com - April 18, 2017 Overview: This guide is intended for 3800 (particularly the Supercharged Series II [L67] and Series III [L32]) owners who are new to modifying their car for performance. This document is not intended to be an all-encompassing how-to for running 11's. Within this guide, I will discuss how to safely, and rather inexpensively, modify your supercharged 3800 to pick up a few ponies as well as dispel some myths about the platform. Acknowledgements: The idea for this guide originated on GrandPrixForums in 2009 by Nick (blueguy), however, some parts of that guide have become outdated by losing various vendors, conducting better research, and finding betters ways of doing things. Much of Nick's guide is still relevant today and it's certainly worth the read when you have the time, OR if you want to go faster than the modifications in this guide allow for. Introduction: So you've decided to steal cable modify your 3800 car. Great choice! Whether you're looking to learn more about engines and performance or just want to make your way to the grocery store faster, the GM 3800 platform offers an easy and cost-effective starting point. When it debuted in 1996, the Series II 3800 offered class-leading supercharged performance. Fortunately or unfortunately, the rest of the world kept moving, and in the days of 300 horsepower minivans, 240 ponies doesn't seem as impressive anymore, especially considering we have a supercharger bolted on. Good thing we're only a few clicks away from a lot of extra power being delivered right to our door. I would like to layout a very specific caveat: The transmission. The GM 4T65E was used in all Supercharged 3800 applications from 1997 to 2007. It was also bolted to everything from 170 HP 3100s all the way up to 300 HP LS4 V8s with very little overall differences. This, of course, means that GM's bean counters were in full effect in regards to the design of this transmission, and there were cost cutting measures taken. The internals of the 4T65E bolted to your 3800 are largely no different than the ones in a base Buick Century. Towards that end, it's best to just accept now that the more you modify your car, the greater your risk of breaking your transmission are. Fortunately, if you're serious about going fast, high performance transmission builds can be purchased from vendors like Triple Edge Performance. I would recommend that one of your first modifications be a transmission cooler. I would also recommend the medium shift kit as outlined in this guide, or the TransGo shift kit. Before you mod: You need to fix everything wrong with your car before you modify it. You need to fix everything wrong with your car before you modify it. You need to fix everything wrong with your car before you modify it. You'd think that this guidance would be self-evident, but after being on the various W-body/3800 forums and Facebook groups over the last 15 years, I'm sorry to tell you that it is not. Before you change ANYTHING to alter the performance of your car, you need to be sure that it's in good working order. Let's be honest, the newest supercharged 3800 car is 10 years old now, the oldest is over 20.. most of them are going to be in various states of disrepair. You need to, at the very least, perform a basic tune-up and catch up on years of deferred maintenance. Nick actually outlined this very well in his original guide, so I will copy it mostly verbatim here. Start here: Lower intake manifold gaskets and coolant elbows if they haven't been done (if you aren't 100% sure, just do them). New spark plugs and wires. Oil change, preferably switch to synthetic and a quality filter. Fuel filter. Coolant Flush. There is no compelling reason to move away from Dexcool, don't bother doing it. Transmission fluid and filter. Dexron VI or Alison TES-389 compliant ONLY. Generally need ~7 quarts for a pan drop. 100k+ mile engines: Change the supercharger oil. Do not use ANYTHING other than the proper S/C oil from the dealer. S/C and Serpentine belts. Once all of that is taken care of, you can start to move into the next major step.. Scanning: The basis for everything we do with performance mods will center around scanning the car and knowing what's going on inside. If you don't have a scantool that can read Knock Retard, do not modify your car. It's as simple as that. I don't care what ZZP told you, I don't care what your buddy says, I don't care what some guy on Facebook posted; if you don't have a scantool that can read Knock Retard, do not modify your car. Knock Retard, or KR, happens when the PCM detects predetonation (knock) and pulls timing to prevent engine damage. Your goal is to have 0 degrees of KR all the time. People will tell you that there's a "safe level" of KR, and on a stock car, sure, there might be. But as you start increasing boost and power, even small amounts of KR can bring our old pal Chippy the Piston to visit. Like most things about this platform, it's cheap and easy to scan. If you own an Android phone or tablet, the lowest cost of entry is to buy a BAFX Bluetooth OBD-II adapter and Torque Pro. iOS users can purchase OBD Fusion, and the website lists all compatible adapters. Cheap Android devices are getting so inexpensive that you could probably pick one up from eBay and just keep it in the car all the time to use with Torque. If you've scanned and everything looks kosher, it's probably safe to start throwing some mods at the car. The basics: Intake. The first mod most owners do is an intake. And why wouldn't you? It's a great first step because it frees up a little power, but more importantly it sounds awesome. Supercharger whine is cooler than going fast, anyway. Really though, I'd start here. It's cheap. You don't need to spend hundreds of dollars on an intake, but I also don't recommend just plopping a cone on the end of the throttle body. For 99-05 L67, go buy 4" PVC pipe from Home Depot along with a cone air filter (for 97-98 L67, go buy the newer F-duct and do all the following stuff). Remove the factory airbox. Cut the PVC pipe to the proper length. You want to slide the pipe far enough in the F-duct (the rubber coupler that attaches to the throttle body) to bypass the accordion folds. Have the filter as far into the corner of the engine bay as you can so it gets the coolest air possible. You can get a PCM tray out of a 90's S-Series Blazer/Jimmy/whatever if you want to secure it to the car. I have used foam pipe wrap (like what goes around residential HVAC lines) to put around and under the PCM to prevent it from being banged around. For 04-07 L32 Grand Prix, the same pretty much applies, but you'll be attaching the pipe to the MAF tube. If you like to spend a lot of money, CAIs are available from various manufacturers including ZZP, Cold Air Inductions, and others. Exhaust. What goes in must come out, right? Unfortunately the stock exhaust, especially on the L67 cars, is awful. The front manifolds have casting artifacts throughout and the ports are often uneven in diameter. Things get worse once you leave the manifolds as the factory downpipe is double walled, which reduces the ID down to 2.5" and there's an insanely stupid "U-bend" in the pipe where the rear O2 sensor installs. GM removed their cranium from their anus and the L32 cars lack the U-bend, but still have the same 2.5" piping. Before ANY pulley drop, the factory downpipe should be removed. There are two ways to go about doing this: ZZP bolt-in downpipe. If you're keeping the stock catback (which is generally good to 300 WHP), select the 3" DP and 2.5" flange exit. If you're having an exhaust shop weld up a full 3" exhaust, select the 3" DP with 3" exit or exit flange depending on how you want it. Do not buy the 2.5" downpipe for a supercharged application. It's up to you whether or not you want to buy a catted version, but be aware that not having a cat will trigger a check engine lamp. The ZZP downpipe can be used standalone or mated to a.. ZZP Powerlog. ZZP offers two Powerlogs, one for the front, and one for the rear. Think of these as the most extreme form of ported exhaust manifolds you can buy, but better. If you're not going totally insane, a front Powerlog and downpipe is a great place to start. It removes both the restrictive front manifold and U-bend. You can reasonably expect to drop a couple of degrees in KR from these two alone. If you want to free the exhaust up more, ZZP also offers a rear Powerlog, and with both logs combined with the downpipe, you get a very reasonable facsimile to headers.. without the fitment issues. Yes, it's kind of pricy to do it this way, but it also just bolts together and works. Headers. Once upon a time, there were a lot of vendors peddling their own version of 3800 headers. You had SLP, Thrasher/TOGs, Pacesetters.. today the market is a lot smaller. Your primary players are ZZP, Speed Daddy, and OBX. First up, the ZZP headers. Downside: they're very expensive. Upside: they "just work" and fit, but for that price they ought to. The most popular headers available for the W-body/3800 are Speed Daddy headers off of eBay. The big upside? They're cheap and they're based on (ripped off) SLP's design. $210 for a full set of headers shipped right to your door. The downside is that they don't fit perfect for everybody. Some people, particularly Regal owners, get stuck having to find creative ways to bash the firewall into make the O2 sensor fit. Others run these with no problem at all. If you don't want to spend tons of cash on exhaust and don't mind working a little to make them perfect, the SD's are a great choice. Be sure to throw away every gasket and clamp in the box. Buy a better flange gasket and use band clamps. It's also likely that the flex section will blow out at some point. Replace it and it's good to go. The last major manufacturer is OBX. They're ripoff Pacesetters. Real Pacesetters were bad enough.. don't buy these. Thermostat. You're going to see a lot of people with a lot of opinions when it comes to thermostats. In my opinion, 160 degree t-stats are just too cold. You don't get any heat in the winter, and 2001+ cars will throw a check engine lamp for coolant temp under regulated temperature. Your best bet is a quality 180 degree thermostat, with a small hole drilled in it to make bleeding easier. Tune. One of the best ways to wake a car up and eliminate KR is to tune it. Factory tunes are conservative and are intended to make the car work in a wide variety of driving situations, no matter what weather, what time of year, what elevation, etc. Performance tunes are meant for going fast. Like most things we've talked about, you have a few options here: Canned Tune. Canned tunes, or "mail order" tunes are generic performance tunes that are provided to the customer via shipping a PCM (Powertrain Control Module) back and forth to the vendor. These are generally not tuned specifically for you and will never be as good as an in-car street or dyno tune. That being said, they can be a good place to start and might even make sense for someone who is going to stick with only minor mods. You can buy tunes from Overkill, INTENSE, and ZZP. Keep in mind that the PCM is really at the heart of your performance build.. I would certainly do your own research before you decide to buy from a vendor. Google is your friend and these cars are VERY well documented. Tuner. You've got two choices here, HPTuners or DHP. HPTuners is a full vehicle tuning and scanning suite that works on a LOT of different GM, Ford, and Chrysler vehicles, not just our 3800 cars. The base VCM Suite offers 8 vehicle credits, which is enough for 4 different 3800 cars. If you're serious about tuning, you probably can't go wrong with HPT.. unfortunately it's pricey: $500 for the standard version, $650 for the "professional" version. Supports up to 2007. DHP or Digital Horsepower/PowrTuner is a now-depreciated tuning and scanning suite. DHP debuted in the mid 2000's and was abandoned by 2011 or so. While that means there's no official support, it also means it has a relatively low cost of entry. You can usually find used DHP interfaces for around $250 in the various 3800/W-body classifieds. Combined with TinyTuner, DHP still offers a compelling selection of options and great 3800 tuning capabilities. 97-04 cars only. Pulley. There's a reason why this section is at the end. A pulley drop should never be your first modification. Yes, a smaller pulley means more boost, but more boost out of an M90 also means more heat. More heat means more knock. If you haven't done the proper supporting mods first, Chippy the Piston will be making a visit to your engine. Assuming you've got a strong running stock car, you've freed up the intake and exhaust, and you've scanned throughly ensuring you have no KR, you're safe to drop a pulley. Avoid the thought process where smaller pulley = moar better. That isn't true. With the right combination of tuning and supporting mods, a car with a bigger pulley can outperform one with a smaller pulley and no/poorly executed mods. You have two options for changing pulley sizes. You can buy either a press-on pulley, which requires a special tool remove AND install or an MPS hub. An MPS hub allows you to bolt on different pulleys with just the use of an allen wrench. If you plan on doing more mods in the future, an MPS hub is probably the best deal. You can buy them new from INTENSE and ZZP for about $100. Each MPS kit comes with one pulley. No matter whether you go with an MPS or Press-on, a SUPERCHARGER PULLEY REMOVAL TOOL MUST BE USED. DO NOT USE ANY OTHER TYPE OF PULLEY PULLER ON YOUR SUPERCHARGER. The following is a list of pulleys that are "generally regarded as safe" with the appropriate mods: Generation 3 M90 (L67): 3.6" pulley - A great starting point for a car with minimal mods, such as maybe just a downpipe and intake. 3.5" pulley - Good choice for an intake, front powerlog, and downpipe. 3.4" pulley - Consider the 3.4" if you're running an intake, full headers, and a tune. You should also consider a cooler spark plug, such as the Autolite 104. Generation V M90 (L32, 04+ GP): 3.6" pulley - The defacto "safe" L32 pulley if you don't have lots of modifications. You should at least have a 3" downpipe. Preferably a front Powerlog. 3.5" pulley - If you have headers (or F&R Plog + DP) and a tune, a 3.5" pulley can be a good fit for the L32 cars. 3.4" pulley - All of the above, plus 1.9 rockers. Advanced Modding considerations: As stated earlier in this guide, my primary goal is to give novice 3800 owners a good starting point to begin modifying their car. In my honest opinion, once you start dropping below 3.4" pulleys and headers, you really need to have a good mental grasp on the work that you're doing. At that point, you should NOT be relying on a guide to tell you how to mod your car. You need to understand tuning and be able to change your own PCM around without relying on canned tunes. That being said, I will BRIEFLY talk about a few of the more advanced modifications available to you once you feel more confident in your abilities. E85. E85 is race fuel on a budget. Not only are you getting 105 octane out of the box, you're getting the added benefit of ethanol cooling the intake charge. If you live in an area with easy availability to E85 and you're comfortable with tuning, you might want to move this modification up a little in your list. E85 suppresses knock and allows for much smaller pulleys and more advanced timing. When it comes to bang-for-the-buck, I'm not sure you can do any better. You will NEED a fuel pump rewire kit, bigger injectors, and a tune. Some people will tell you that you can get away with just dropping in 42.5 lbs injectors and throwing in a tank of E85 on the stock tune. That's partially right.. E85 requires 30% more fuel, and those injectors are approximately 30% bigger. Unfortunately, that does not account for other tuning that is required to prevent hard cold starts and other drivability issues. 42.5s will likely be too small of an injector if you plan on seriously modding (sub 3.2" pullies, cam, etc). If you're moving to E85 for performance, start with the 60 lbs Siemens/Deka injectors. Keep in mind that your gas mileage WILL suffer. If you can't get E85 easily, your next best bet is an.. Intercooler. The next bang-for-the-buck advanced mod would be an intercooler. Let's face it, the M90s just aren't efficient superchargers when you start spinning them faster than they were originally designed for. The incredibly hot intake charge promotes knock and kills power. If you're serious about performance, and you can't get E85, you need an intercooler. Intercoolers are pricey, but give you the ultimate flexibility with pulleys, tunes, and fuel options. You don't need to worry about straying too far from an E85 station when you're running an IC. You can buy intercooler kits or piece your own together. Ed makes some great phenolic core ICs, which are available here. If you're going to piece a kit together, start with his core. Edit: GrandPrixForums is down for now. Ed's store eBay store is available here. Rockers. High-ratio rockers can be an easier alternative to a mild cam. You won't get as big of gains out of them (since you're only altering the cam lift, not duration, overlap, etc.). Stock rockers are 1.6:1 ratio. The higher the ratio, the more valve lift the rockers add. The ratio of the rockers determines that lift. For example, with a set of 1.9 ratio rockers, the rocker will move 1.9 times the lift of the cam. Adding higher ratio rockers not only increases power out of the box, you're also generally able to run smaller SC pullies and add timing as the engine becomes more efficient at moving air in and out. Deciding on rockers really depends on your future modding considerations. If you plan on a cam, don't bother with rockers; unless you're running a VERY small ratio (like 1.65:1 or 1.7:1), you're not going to be able to pair different ratio rockers and a cam. You will need to upgrade your valve springs when running higher ratio rockers (and let's be honest, your engine probably has well over 100k miles. It's time to do it anyway). It's recommended that you use GM L76/L92 valve springs (GM PN#12589774) and retainers (GM PN#10166344). While you have the springs out, go ahead and change the valve stem seals. You can reuse your stock keepers/locks. Camshaft upgrades are well beyond the scope of this guide. Fortunately, these cars are so well documented it's usually pretty easy to find what you need by searching Google. Keep in mind that mild cams such as the VS and S1X need much less work to get running than more wild cams like the XPZ or S3. Conclusion: Hopefully I've helped to remove a lot of the mystery around modding these cars. The 3800 platform is simple, reliable, and cheap. It's a great way for novice tuners and builders to cut their teeth before moving onto bigger and better things. If you can think of any additional information you would like here, or if I missed something, or if you think I'm a moron, feel free to comment below. I'd like to hear your feedback!
Overview: This guide covers removal and reinstallation of both the Harmonic Balancer and Crankshaft Position Sensor on a Series II/Series III 3800. Tools Required: 24mm socket for the crank bolt, 19mm socket for the lug nuts, 18mm socket for the subframe bolts, 15mm/belt tool for the belts, 13mm socket for the crank position sensor bolts, 10mm socket for the flexplate inspection cover, harmonic balancer puller, OEM 27296 GM 3800 bolt kit, screwdriver, various extensions, torque wrench, breaker bar/impact/both, vice grips, ratchet. Instructions: 1. Remove the drive belt(s) from the engine. 2. Lift the passenger side of the car and remove the right front wheel. Always use jackstands, NEVER trust jacks, especially not scissor jacks. Only place jackstands on a solid surface like concrete or asphalt. 3. Remove the flexplate inspection cover: 4. Put vice grips on the flexplate to prevent it from rotating. There is probably a J-tool for this, but this how I've always done it: 5. Remove the undercarriage passenger side splash shield. 6. Lower the subframe/engine cradle on the passenger side by approximately one inch. You should not be totally removing the bolts and insulators. 7. Remove the harmonic balancer bolt. 8. Insert the adapter pin from the OEM kit into the hole in the balancer where the bolt came out of. 9. Attach the balancer remover. You should be using washers on the heads of the bolts, unlike my picture: 10. Tighten the center bolt on the puller until the balancer comes loose. Remove the puller and remove the balancer. If you're only replacing the balancer, you are done with removal. Proceed to step 16. 11. With the balancer removed, unplug the connector from the CKPS. Looks like I have a small front main seal leak. I'll worry about it later: 12. Remove the black plastic debris shield. 13. Remove the two 13mm bolt/studs that hold the CKPS. Remove the CKPS. 14. Install new CKPS. Bolt torque is 22 lb ft. 15. Reinstall debris shield and plug sensor in. 16. Reinstall harmonic balancer and bolt. Torque is 111 lb ft, plus 76Â°. 17. Reinstall splash shield. 18. Remove vice grips and reinstall dust cover. 19. Reattach wheel and torque lug nuts to 100 lb ft. 20. Reinstall drive belts.
Accelerator Control Cable Bracket Bolt 16 NÂ·m 12 lb ft Balance Shaft Driven Gear Bolt 22 + 70Â° NÂ·m 16 + 70Â° lb ft Balance Shaft Retainer Bolt 30 NÂ·m 22 lb ft Camshaft Position Sensor Bolt 10 NÂ·m 89 lb in Camshaft Sprocket Bolt 100 + 90Â° NÂ·m 74 + 90Â° lb ft Camshaft Thrust Plate Bolt 15 NÂ·m 11 lb ft Canister Purge Solenoid Valve Bracket Bolt (VIN K) 50 NÂ·m 37 lb ft Canister Purge Vacuum Switch Bolt (VIN K) 50 NÂ·m 37 lb ft Connecting Rod Cap Bolt 27 + 50Â° NÂ·m 20 + 50Â° lb ft Crankshaft Balancer Bolt 150 + 76Â° NÂ·m 111 + 76Â° lb ft Crankshaft Main Bearing Cap Bolt 40 + 110Â° NÂ·m 30 + 110Â° lb ft Crankshaft Main Bearing Cap Bolt (Side) 15 + 45Â° NÂ·m 11 + 45Â° lb ft Crankshaft Position Sensor Stud 30 NÂ·m 22 lb ft Crankshaft Rear Oil Seal Housing Bolt 15 + 50Â° NÂ·m 11 + 50Â° lb ft Cylinder Head Bolt 50 + 120Â° NÂ·m 37 + 120Â° lb ft Drive Belt Tensioner Bracket Bolt/Nut 50 NÂ·m 37 lb ft Drive Belt Tensioner Bracket Stud 6 NÂ·m 53 lb in EGR Valve Adapter Bolt 50 NÂ·m 37 lb ft EGR Valve Inlet Pipe Bolt 29 NÂ·m 21 lb ft EGR Valve Nut 29 NÂ·m 21 lb ft EGR Valve Outlet Pipe Bolt/Nut 29 NÂ·m 21 lb ft Engine Block Coolant Drain Plug (VIN 1) 18 NÂ·m 13 lb ft Engine Block Coolant Drain Plug (VIN K) 30 NÂ·m 22 lb ft Engine Block Oil Gallery Plug 30 NÂ·m 22 lb ft Engine Coolant Temperature Sensor 25 NÂ·m 18 lb ft Engine Flywheel Bolt 15 + 50Â° NÂ·m 11 + 50Â° lb ft Engine Front Cover Bolt/Stud 20 + 40Â° NÂ·m 15 + 40Â° lb ft Engine Lift Bracket Bolt/Nut/Stud 30 NÂ·m 22 lb ft Engine Mount Nut, Lower 78 NÂ·m 58 lb ft Engine Mount Nut, Upper 78 NÂ·m 58 lb ft Engine Oil Pressure Indicator Switch 16 NÂ·m 12 lb ft Exhaust Crossover Heat Shield Nut 20 NÂ·m 15 lb ft Exhaust Crossover Pipe Bolt/Stud 18 NÂ·m 13 lb ft Exhaust Manifold Bolt/Nut 30 NÂ·m 22 lb ft Exhaust Manifold Heat Shield Nut 20 NÂ·m 15 lb ft Exhaust Manifold Stud (Inner) 30 NÂ·m 22 lb ft Exhaust Manifold Stud (Outer) 10 NÂ·m 89 lb in Fuel Injector Rail Nut 10 NÂ·m 89 lb in Fuel Injector Rail Stud 25 NÂ·m 18 lb ft Fuel Injector Sight Shield Bracket Nut 30 NÂ·m 22 lb ft Generator Brace Bracket Bolt 50 NÂ·m 37 lb ft Heated Oxygen Sensor 42 NÂ·m 31 lb ft ICM Bolt (VIN 1) 30 NÂ·m 22 lb ft ICM Nut 50 NÂ·m 37 lb ft ICM Stud (VIN K) 6 NÂ·m 53 lb in ICM Wiring Harness Bolt 2 NÂ·m 18 lb in Idler Pulley Bolt 50 NÂ·m 37 lb ft Idler Pulley Bracket Bolt 30 NÂ·m 22 lb ft Lower Intake Manifold Bolt 15 NÂ·m 11 lb ft MAP Sensor Bolt (VIN 1) 3 NÂ·m 27 lb in MAP Sensor Bracket Bolt (VIN 1) 30 NÂ·m 22 lb ft Oil Cooler Adapter Connector 50 NÂ·m 37 lb ft Oil Filter 30 NÂ·m 22 lb ft Oil Filter Adapter Bolt 15 + 50Â° NÂ·m 11 + 50Â° lb ft Oil Level Indicator Switch 20 NÂ·m 15 lb ft Oil Level Indicator Tube Nut 19 NÂ·m 14 lb ft Oil Pan Bolt 14 NÂ·m 125 lb in Oil Pan Drain Plug 30 NÂ·m 22 lb ft Oil Pump Cover Screw 11 NÂ·m 98 lb in Oil Pump Screen Bolt 15 NÂ·m 11 lb ft Power Brake Booster Heat Shield Nut 20 NÂ·m 15 lb ft Spark Plug 27 NÂ·m 20 lb ft Starter Motor Heat Shield Bolt 30 NÂ·m 22 lb ft Supercharger Bolt/Stud (VIN 1) 23 NÂ·m 17 lb ft Supercharger Bypass Valve and Fuel Pressure Regulator Valve Harness Bolt (VIN 1) 10 NÂ·m 89 lb in Supercharger Bypass Valve Nut (VIN 1) 8 NÂ·m 71 lb in Throttle Body Nut 10 NÂ·m 89 lb in Throttle Body Support Bolt (VIN K) 10 NÂ·m 89 lb in Timing Chain Dampener Bolt 22 NÂ·m 16 lb ft Upper Intake Manifold Bolt (VIN K) 10 NÂ·m 89 lb in Valve Lifter Guide Retainer Bolt 30 NÂ·m 22 lb ft Valve Rocker Arm Bolt 15 + 90Â° NÂ·m 11 + 90Â° lb ft Valve Rocker Arm Cover Bolt 10 NÂ·m 89 lb in Water Outlet Bolt 27 NÂ·m 20 lb ft Water Pump Bolt 15 + 80Â° NÂ·m 11 + 80Â° lb ft Water Pump Pulley Bolt 13 NÂ·m 116 lb in Wiring Harness Shield Bolt/Nut 10 NÂ·m 89 lb in
In an effort to preserve information about these cars and tuning, I figure I'd write this up. This information was figured out by the members of GrandPrixForums, but the thread that it's in is very long and full of off-topic posting. Overview: Owners may wish to delete the rear oxygen sensor in order to install a wideband in the factory bung location. In 2001+ cars, this causes a new cat diagnostic to run repeatedly, skewing fuel trims and negatively impacting fuel economy. Cause: Beginning in MY 2001 (and possibly some prior MY California emissions cars), GM added a cat diagnostic test in order to determine the efficiency and operation of both the catalyst and rear O2 sensor. This test works by richening the fuel trims to an extreme degree in order to saturate the catalyst, then waiting for the O2 to respond appropriately. It will run this test over and over during cruise until it is satisfied. Without a rear O2, the test never passes and fuel economy suffers. LTFT readings will also skew rich (high double digit LTFTs), causing tuning difficulties. O2 Simulators do not correct the issue because the PCM is expecting a specific response from the amount of fuel being dumped. Fix: Utilizing Tiny Tuner, this diagnostic can be disabled. As of April 2017, HPTuners still does not offer this ability. By and large, this patch is available for most 2011-2014 L36, L67, L32, L82, and LA1 OSIDs, but some may be missing. To disable the test, open your .bin in Tiny Tuner and navigate to the following: Diagnostics > Main > CAT Diagnostic Enable Coolant Temp Stock is 20. Change to 285 (positive, not negative like the screenshot). Navigate to Diagnostics > Main > CAT Diagnostic Tests per Trip Stock is 18. Change to 0. This should disable the O2 diagnostic test going forward. Errata: The original version of this document (and the screenshots) suggest changing the enable temp to -285. This value is too low and would allow the diagnostic to run anyway. The diagnostic would run every time the car is driven until interrupted. After interrupted, the test would be suspended. Setting it to a high value that the car would never see prevents it from running at all.