Exhaust: Gut Precats

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Okay, so I took on the delightful task of gutting my precats. As usual, the front precat came right out and I had it clean as a whistle in about 10 minutes. The rear was another story. So here is my humble walkthrough with photos. It's a free and simple modification.

Before we start, remember a few things:

  1. Keep your bolts (you don't want to have to go get more after you're all dirty)
  2. Keep your gaskets (again, you dont want to have to order some, or make some, just keep the ones you have)
  3. Treat all the bolts with PB Blaster the day before. Spray it on, tap the bolt, and spray it on again. This will make the removal alot easier for you.


Tools Needed

A chisel and hammer used to gut the precats
  • Simple Drill
    At least 12V with a 1 foot long 1 inch wood boring drill bit -- any ACE hardware has it
  • A Good Hammer
    I used a 2.5 pound mini sledge
  • A Good 1-Inch Wood Chisel
    Again, I used a stanley, about 9 inches total length and sharp as hell
  • A 3/8 or 1/2 Ratchet with 19mm Long Socket
  • A Long Extension
    10 inch or so
  • 12mm Socket
    And whatever size you have on your CAT/Testpipe
  • A Good Jack
  • Jack Stands
    The higher the better
  • Also Helpful:
    • 17 or Longer Breaker Bar
    • PB Blaster,
    • Good Goggles that Seal Around the Eye
    • A Broom
    • Tarp
    • Detox Suit
    • Whatever you can find... it's messy!


Questions & Answers

Q: If I gut my precats, will I fail emissions?

A: The "safe" answer is that it might, but this is not always the case. Keep in mind that your car also has the main cat (unless you've installed a testpipe), and many members have reported still being able to pass emissions if their car is running cleanly. Do this free mod at your own risk. When in doubt, or if you live in a strict emissions state, consider other exhaust modification options.

Q: Will this modification trigger my Check Engine Light (CEL)?

A: Not always. If your light turns on after gutting your precats, try unplugging your battery for some time, which will reset the code. If the light continues, consider getting the O2 Simulators from 3SX.

Q: Is it possible to just weld on new pipes in place of the precats?

A: Yes, it's possible -- but it would negate the 'free' feature of this mod. This would also pose complications in some scenarios, such as dealing with the O2 Sensors, requiring the removal of the rear precat (which is a pain), and/or keeping the stock look for inspections.


Gutting Your Precats in a Mitsubishi 3000GT/GTO or Dodge Stealth

Remove the Front Precat

  1. Remove the front heat shield. Everyone is different, but you may need to remove the heat shield from the front headers to access the bolts on the top of the precat. I had one strip... just remove the three bolts and set them off to the side.
    Remove the Front Heat Shield
  2. Detach the front O2 Sensor. It is attached near the timing belt cover to the lower-right of the last spark plug. Just press on the tab and pull, it should come right off. If you don't know which one, just follow the O2 wire from the precat up to the correct plug. In this case it's the bottom of the two.
    Detach the Front O2 Sensor
  3. Loosen the two bolts attaching the precat to the header. Either do so from the top of the engine or from underneath, either way it should be a 19mm bolt. You'll need that long extension if your doing it from underneath. This is top view (see the stripped screw hehe). You can see the O2 Sensor and wire peeking out from the right side of the precat.
    Loosen the two front precat bolts
  4. Unbolt the two bolts attaching the downpipe from the rear precat. You'll need to get underneath the car to loosen the back precat. Shown below is where the front precat attaches to the downpipe (for reference). Don't bother with these bolts yet. Further down the downpipe you will see the same setup: 2 screws, both 19mm. You'll have to use the extension to get past the bar in the way. Take these all the way off, the front precat will keep the whole downpipe suspended.
    Unbolt the downpipe from the rear precat
  5. Detach the rear 02 Sensor. Make sure to loosen the O2 sensor that's attached to the rear precat. In the photo below, I have shown where the O2 Sensor wire comes from and where it attaches (close to the transmission).
    Detach the rear 02 Sensor
  6. Unbolt and lower the downpipe. Now that both precats are loosened from the headers (remember we're keeping the front precat attached but loose, and the rear screws are totally removed). From here, we will separate the whole downpipe from the catalytic converter (cat, or the testpipe). Most of these bolts are on there pretty good, so use the PB Blaster, tap the screw, more PB, and more tapping. If needed, screw the bolt tighter, just a hare, and it will loosen the bolt -- allowing us to take it right off. Once you remove these, the downpipe will be suspended from a 12mm bolt attached to a rubber exhaust hanger. Remember to keep your gaskets!!!
    Lower the downpipe
  7. Detach the downpipe. Now you have the downpipe hanging by the bolt attached to the body, and the front precat bolts which are loose by now. Feel free to take the front precat bolts off now, letting the front of the downpipe rest on the ground. Take out the 12mm bolt thats attached to the downpipe, which is the last thing holding it on. Loosen it and let it rest on the ground. Hopefully, this is what you see in the photo below. The front precat is the first attachment followed by the downpipe.
    Detached downpipe
  8. Detach the front precat from the downpipe. It's just the two 19mm bolts holding them together. Once you have it off, set everything else aside... it's time to gut away!!


Gut the Front Precat

  1. Chisel and/or drill out the front precat. There are many methods to gut. Once you see what you'll be dealing with, you may want to change your tactics. The front precat took me, literally, about 10 minutes. So take that sucker out to your front or back lawn, secure it with your legs or on a table in a vise, and go at it. One thing to note: there is a wire mesh around the honeycomb and a mesh circle both before and after the honeycomb. Getting this stuck in the drill is not cool. So chisel, dump, chisel, dump and once you get down far enough, grab the mesh with some needlenose pliers and yank it out. If it won't come out, chisel around it. It's not hard, you'll get it easy enough. Do whatever you find works best.
    • I used the chisel at first. Extremely effective! Chisel around the edges first, and then make an "X" through the middle. Take the time to tip it over and dump the debris into a box. Here are the chisel marks:
      Front precat gutted with a chisel
    • Alternatively, you can use a drill. Note: A 12" x 1/4" masonry bit followed up by a 12" x 1" masonry bit really cleaned out the precats with ease. The masonry bit tip will not easily penetrate steel as it is not porous enough for it to bite.
      Front precat gutted with a drill
  2. Remove the big wire mesh, rings, and debris. This is all of the crap that will come out of the precat. Take note of the 2 rings, the big wire mesh "sponge" and the debris. However, it was found that on the first generation (1991-1993) turbo models, there is no wire mesh in the precats. If you chip at it with the chisel, it will be in chunks. No mess, no fuss, nice and easy -- definetly the easiest way to do it (and cleanest). If you don't think they are that restrictive test it. Blow through the precat before you gut it, and after. There's a huge difference. Besides, look at all that crap you just took out!!! Multiply it by two.. and yeah, its alot of restriction...
    Wire mesh and rings in front precat
    Debris from the front precat

  3. Clean the debris mess. When we're done, it's as clean as a whistle!!! However, see the O2 Sensor at the bottom? Don't chisel or drill it. It's important so be careful when chiseling. It is at the bottom, before the bend, so you should be good to go. Don't get crazy with it. When it's clean, its done. Blow it out and make sure everything is gone.
    Gutted front precat (with 02 Sensor exposed)


Gut the Rear Precat

Now get under the car. Suit up!!! You are going to hate this stuff. After the first one, you feel confident, you think you're almost done, you think its a piece of cake. Nope!!! The rear precat likely has the O2 Sensor on the front, not the back, so you have to work around it and make sure not to damage it. That and you're trapped under your car. That and everything you clean out of it is going to fall right back on your face. So, suit up and get ready to get dirty.

Wanna see why its such a messy job????

Gutting the Rear Precat - The Nastiest Job Ever Video
  1. Chisel and/or drill out the rear precat. Below is a photo under the car of the rear precat. One is untouched and the other is after 2 drills with the 1 inch boring bit.
    The rear precat (nongutted)
    The rear precat with two drill holes in it

  2. Remove the big wire mesh, rings, and debris. It will fight back.. lol. It will spit, spew, blow crap all over you. So, get ready. Some use a fan to blow the stuff away from them as they drill. On the other hand, chiseling is tough unless you get a long enough chisel and even then it's not nearly as hard as the first precat. Drilling away works, but it's messy and can take a while. You will get stuff in you eye, just accept it. lol. You will get dirty and you will hate it.. but the end is coming soon!!!
    Remove the wire mesh and debris from the rear precat
  3. Empty out the precat. And it's clean! OMG! It's possible!
    Gutted rear precat
  4. Clean the debris mess. Look at all that debris!!! A big difference between what comes outta the front precat and the rear one, eh?? No chunks here... just dust, and lots of it (due to the drilling).
    Debris from the rear precat


Reassemble Downpipe and Precats

Now we're done. Just for fun you can fire up the car without the downpipe in the way. It sounds mean! And yes, you might get flames out of the bottom so don't get carried away with it. Have your fun and get back to putting it together so you can enjoy it...

  1. Re-attach the front precat to the downpipe assembly. Put anti-seize on all the bolts if you like. It makes it easier if you have to do it all again for some reason. So attach the front precat.. and clean a bit under the car.
  2. Re-attach the back part of the downpipe to the exhaust hanger near the testpipe/cat. It's a simple bolt and will keep it suspended like you need it to.
  3. Replace your gasket and re-attach the rest of the downpipe and front precat. You still have the gasket, right?? Attach the front of the downpipe (front precat) to the front header from underneath. You might need some small hands or some patience. Don't forget your gasket and just lightly screw on the bolts. Make sure everything matches up on the rear precat. The testpipe will move around so you should be okay. However, don't tighten the screws just yet, just keep the downpipe from falling on you.
  4. Re-attach the downpipe to the rear precat. Again, put them on there and screw them on a bit, but don't tighten them up completely yet.
  5. Tighten all bolts. After that we move to the test pipe. On all connections, do not forget your gaskets!!! Once all three connections are bolted up, go back and tighten them all on. Use the anti-seize if want, and make sure they are tight.
  6. Re-attach 02 Sensors and heat shield. Double check all of your gaskets and connections to make sure they are tight. Replace the heat shield if you took it off. Re-attach both O2 sensors, and smile!!! You're done!!

Closing Notes

This is probably one of the messiest jobs you can do. It sucks, and it's time consuming. However, if you have all the right tools, it really is not very hard and it's FREE (minus the trips to the cancer center for lung cancer, hehe). I hope this helps everyone out. You will notice a more breathy and airy sound when you hit the gas. Be forewarned: you don't have a loose connection, it's just the air whooshing through your now wide open precats. That, with a test pipe, is a tad bit smelly at startup, but not to bad once your car warms up.

Okay, I'm done! SLURPY TIME!!!!

Hope you enjoyed it. Original guide coutesty of Jstrodda79, feel free to PM if you have any questions. --Jstodda79 19:36, 4 April 2006 (EDT)

This article originated from a thread or post on 3000GT/Stealth International. (1864)