Bad Cats on 99 Sierra after Dual Exhaust Install?

rutherfj · June 29, 2002

I had my 99 Sierra Z71 Ext dualed about 2 months ago. The muffler shop cut off the OEM mufler and pipe and basically installed a custom catback with FloMaster 30 series.

About a week ago my truck started hesitating at constant throttle and detinates ( I think?) at anything over 3500 RPM. Top end on the truck is a sad 70 mph now. I thought I got some bad fuel so I took it to a service center and had the fuel system cleaned with a Wynn's 3 part kit. By the way the check engine light has been lit this whole time and sometimes flashing.

The fuel cleaning didn't work so I went for broke and too it to the dealer. This truck has 38,400 miles on it with no extended warranty. After about 6 hours the dealer said the computer was flashing a enigne misfire code and it was because my cats were stopped up. He says my welds for my new exhaust pipe were made to close to the cat and wasn't allowing the cat to flow breath properly and in return ruined them.

Is this possible? Has anyone experinced anything like this. Also the dealer says new cats and labor will be about a cool $1,000. Also, GM covers all emmisions related equipment for 80,000 miles, so theres a chance they may replace them and fix this problem for free if the shop manager will approve on Monday.

Do you think this problem will reoccur if new cats are installed? Should I just cut the cats off and forget them? Can you just cut holes in new head pipe in place of the old cats and stick the o2 sensors back in exhaust flow with no problems? Should I fault the Muffler shop guy?

Thanks a ton guys........Jason

CMNTMXR81 · July 1, 2002

Welds causing the cats to back up?? I find that hard to believe unless the melted metal inside the pipe is two inches thick or so which would physically be impossible as all the melted weld would be on the outside of the joint. So I don't buy that. A cat-back is nothing more than a noise abatement device and would have no effect on catalytic converter operation unless the cats were physically part of the muffler.

What code were you getting? To me it sounds like something else is the problem. Are you getting a slow heating response from either bank of O2 sensors? If so, that could overrichen the fuel mixture, engaging the A.I.R. pump to soon in which case it wouldn't be able to do it's job and then you'd get a secondary A.I.R. injection failure code.

CityTruck · July 1, 2002

Yeah, I'd say it's more likely the O2 sensors got damaged as well. The CAT's are designed to get very hot. For $1000 you could get a new Y pipe (perhaps aftermarket though I don't know where) and aftermarket bolt on exhaust from many aftermarket exhaust makers. How was the dealership going to deal with the "welded on" setup anyway? They would probably replace the Y pipe and exhaust with stock. Do it yourself and get a nicer setup. If you're going to pursue the muffler shop, get the GM shop to give you the old Y pipe and prove to you that the cat's are bad due to the welding.

rutherfj · July 1, 2002

The code it was flashing was "engine misfire" I can't remember the number. Chevy says they will give me new cats and original pipe to the muffler. I have to find someone (muffler shop) to install them. Do you think if the 02 sensors were bad it would have shown up on the diagnostic test they ran? They had the truck for over 6 hours running diags trying to determine the problem and this is the only thing they could come up with. Is it possible for a cat to just go bad all of a sudden?

CMNTMXR81 · July 2, 2002

A slow heating O2 would also show up as P0130 - P0139. These codes vary from vehicle to vehicle and PCM to PCM parameter variances.

Your misfire code would've been somewhere between P0300 to 301-309 for individual cylinders.

With the fact that it is misfiring, I would somewhat rule out the slow heating O2's at this point especially since you didn't get the accompaning code (P0130 - P0139). Something else is causing the engine to misfire. Bad wire, bad plug, fouling out fuel injector, insufficient response from IAT and MAF sensors, etc, etc...

Running rich as one would when misfiring would clog the cats if left to run in that condition too long, especially of the cats never get hot enough to "light off" the unspent hydrocarbons. If you drive for extended periods of time, the cats will get hot enough to "light off" the catalyst and function as near normal as possible despite the conditions.

Replacing the cats won't cure the problem, something else is causing a miss and eventually the new cats will get clogged too.

The cat-back you installed should have nothing to do with this as it flows freer than the stock "muffler of death" anyway. If anything, it's probably helping your problem from being worse.

rutherfj · July 9, 2002

It looks as if everything is working properly now. Chevy replaced my factory head pipe with new cats and I had a aftermarket bolt on cat back installed. I went with a Ravin 3" single in and 2 1/2" dual out muffler, I think it was a Z55 series and kept the 2 1/2" duals the rest of the way having them cut and turned down just behind the tires.

Chevy and the new muffler shop seem to think that by cutting the factory flanged pipe close to the passenger side cat and replacing it with a smaller diameter pipe will not allow that cat to breath/flow properly and will in time fail. It took mine about 2 months to start noticeably degrading. It does kinda make sense due to the fact that the factory dual from the collectors join into one (larger diameter) single pipe before connecting to the factory muffler. Taking this down to a 2 1/4" pipe could be restriciting enough.

Anyway, Chevy put my truck back on the computer and everything cleared and has remained that way so far.

The truck runs far better then when I had the Flomasters and I got my low end torque back. I would definately recommend the Ravin Z55 muffler. It has a lower consistant rumble than the flomaster 30 series. The flomaster had a higher pitched tone. The guy who installed the flomasters refunded the mufflers (can't complain there)!

Thanks for all of your help and comments. I hope this is the last time I come accross any exhaust related troubles.

CityTruck · July 9, 2002

Chevy and the new muffler shop seem to think that by cutting the factory flanged pipe close to the passenger side cat and replacing it with a smaller diameter pipe ...

Ahh, that would do it. I didn't realize the welded pipe was smaller.

CMNTMXR81 · July 9, 2002

Glad to hear that all worked out. Yea, a bottleneck like that could cause problems albeit, I didn't expect it to be so severe.

I almost bought those Ravin's when I was shopping around. How do they sound?

rutherfj · July 10, 2002

The Raven sounds great. I was a little hesitant about leaving the FloMaster realm. I have had FloMasters on my last 5 vehicles (3 of em 5.0 Mustangs) and have loved them.

Its a little louder than the 2 chamber Flo, but has a much lower pitched tone. This of course means a little more cab resononance(sp), but I think it sound mean.

About the only thing I can compare it to is a friends Pontiac Firebird WS6 with Borla catback. It has a very similar rumble and tone.