Replacing Catalytic Converters in 1999 Silverado LS Z71 Ex Cab SB

TwoRiversCO · June 6, 2017

Hello! Long time lurker, first time poster here.

I've been dealing with an annoying metallic rattling sound under light acceleration and when coming to a stop. Based on some information I found on this form, I got under the truck and started knocking things around a bit. The catalytic converter on the driver's side made all kinds of noise when I hit it; so did the pipe running between the driver and passenger side converters. The passenger side converter made a similar noise, but not nearly as exaggerated.

I drove my truck to a local muffler shop, and they quoted me $870 to replace the exhaust system from the manifold back to the muffler (basically both converters back to the "Y" before the muffler). I don't have the funds for that sort of repair, so ended up buying an Evan Fischer REPG960301 Silver Powder-Coated Catalytic Converter with Heat Shield (https://www.amazon.com/gp/product/B007VZWFQ4/ref=od_aui_detailpages00?ie=UTF8&psc=1) from Amazon. I have a Haynes manual for my truck and try to do most of my own work; however, I'll be the first to admit I AM NO MECHANIC!

The Haynes manual basically says to disconnect the O2 sensors (4 total); remove the three manifold bolts on each side; remove the two bolts by the muffler; and remove the converters. Installation is the same thing, just in reverse order. This sounds pretty easy and I feel like I could have it done in a couple of hours. I would spray some liquid wrench on the bolts the night before, and get to work early the next morning while it's cool out.

Some of the information I've read online, as well as some YouTube videos I've seen, mention removing the cross member supporting the transmission to get the old converters out and the new ones in. If this is necessary, I don't see why my Haynes manual omits this step? If it is necessary, it sounds like it complicates things a bit, and it's not something I've done before. If I need to do this, where would be the best place to put my jack/jack stand to support the transmission/transfer case?

Again, my truck is a 1999 Silverado LS Z71, extended cab (3 door), short bed, 5.3L. It may also help to mention that the part I bought is one solid piece. I saw some that come in two pieces, and you connect them with a bracket, but I wanted to avoid any potential need to weld anything (never done it and don't have the equipment). Thanks in advance for any help, and I hope what is discussed here will benefit others in the future!

Edited June 6, 2017 by TwoRiversCO

TwoRiversCO · June 13, 2017

Got the "Y" pipe replaced today, and thought I'd post here again in case anyone ever searches for information about this repair. It was NOT necessary to remove the cross member supporting the transmission. The whole process took me about two hours, and went as follows:

1. Disconnect all four O2 sensors and position the wires in such a way that they won't get hung up when moving the exhaust around. The sensor connection before the cat on the driver's side is a bit of a pain to get to.

2. Loosen the nuts above each cat where it connects to the exhaust manifold.

3. Loosen the nuts at the back of the "Y" pipe, where the assembly bolts to the muffler.

4. Now that you know you can remove all of the nuts, begin to completely remove them. I removed the ones above the cats first, and the ones by the muffler last.

5. Once everything is removed, work the flanges to disconnect the old "Y" pipe from the exhaust manifold and the muffler pipe.

6. You need to be aware of the location and position of the O2 sensors, and manipulate the exhaust back and forth to get it to drop down. When mine dropped I was able to slide it out toward the front of the truck.

7. Remove the O2 sensors from the old "Y" pipe and put them in the new "Y" pipe. I put them in the same positions on the new assembly and used some anti-seize. Probably not necessary, but couldn't hurt.

8. Crawl back under there with your new "Y" pipe assembly and get it into position. Again, this requires some time and patience to avoid damaging the O2 sensors.

9. Start the nuts on all the three flanges by hand, just to make sure everything lines up.

10. I couldn't find any torque specs for the nuts, so I just went around tightening them all up a bit at a time until everything was as tight as I could get it by hand.

If anyone knows where to find torque specs for this, or can confirm there aren't any, I would greatly appreciate it! All of the original nuts are 15mm, and you'll need a deep socket to get over the studs. I had to use a breaker bar to loosen them initially, and I sprayed penetrating oil on them the night before. The nuts that thread onto the studs by the muffler were 14mm on the aftermarket part I purchased. Also, the gasket for the flange above the passenger side cat was a different style. The original was solid, and the aftermarket one was a metal mesh material, like maybe it crushed or something as I tightened the flange down. Has anyone else experienced this?

Anyhow, this did fix my "metallic rattling" problem at low RPM's. The exhaust is nice and quiet, and I'm hopeful it will last a long time. Now to figure out my "whining" noise (maybe power steering pump?), and get my oil leak(s) fixed. I'm dropping a nickel sized drop or two a day. I've cleaned it all up several times and I'm pretty sure it's my oil pan gasket. Also worried about my rear main seal, but based on what I'm seeing I think the oil pan is the place to start.

donbhooks · August 18, 2019

Good description - Thanks!

I've ordered Walker cats for my 06 crew and will be putting them on next week IF I can get the 13 year old nuts off. The PBBlaster starts this evening. How much did you have to jack up the front end to get the Y pipe to clear? Two of the O2 sensors (bad heaters) will have to be replaced along with the computer (the cats melted pulling a travel trailer 300 miles with the computer calling for more fuel all the way - I only got 9 mpg).