Catalytic Convertor woes

[okfoz]

I have a 2007 4WD Tahoe LTZ with the 5.3 Flex Fuel engine.

Since January I have had to replace the Catalytic convertors 3 times. The first time was in January at 175,000 the second time was at 179,000 miles, and now the third time at 195,000 miles.

There is a slight, but seemingly insignificant exhaust leak from the exhaust manifold to the head, can't hear it with the windows up. The only time it is noticeable is when next to a building with windows down, or when outside the truck while it is running...

What could be killing the cats? Any ideas? I have read anything from an intake leak, a EGR valve, to a blown head gasket or something similar allowing coolant to get into the exhaust.

The o2 sensors were replaced in January with the first Cat system. (They replaced the entire Y setup).

It idles so well that sometimes I do not know if it is running.... Other times there is a slight shimmy but almost unnoticeable...

I use quality gas, Mobil, BP, Shell etc... Every now and then a cheap gas, but usually I get better MPG.... This last time I had been running Ethanol Free Premium for about 4 fill-ups when out in Yellowstone. I have not run E85 for about 4 years...

Anyone else have anything like this happen?

 

[BourbonNcigars]

Engine running rich and overpowering the cats? Maybe the o2 sensor isn't working correctly?

 

[Kraig]

What brand cats did you put on and what was the original problem that led to them being replaced?
And why not fix the exhaust leak?

 

[okfoz]

The exhaust leak was so minimal, so they said it was no big deal at the time... I am having the exhaust manifolds fixed this time... NOW they tell me that it could be the problem, even though it is barely noticeable...

The original Cats were replaced for the same reason, the same 420 (IIRC) code for low efficiency... The first and second sets were also replaced for the same reason

All O2 sensors were replaced at the same time...

Always been Bank 2.

The only other thing it could be is Injectors I guess, but the differential between metered MPG and Actual MPG has not changed in 4 years, so if they are leaking it is minimal...

 

[swathdiver]

What brand of cats did you use? Chicom cats don't last at all. I would do a compression test and leak down test on the motor, something is coming out of the chamber(s) and taking out those cats.

 

[91RS]

If you're having multiple failures, it's either junk aftermarket converters or you have something else going on. More likely, it's junk aftermarket converters.

 

[Bigkevschopshop]

You have no oil consumption issues I am guessing?

I think I would find a different exhaust shop... They may have very low quality cats and that may be their way to keep people comming back to spend more money. But I a vehicle has to be really screwed to trash cats that quick...

Its weird you are getting it on bank 2 only.... What O2 sensors did you put in there? Cheap or Expensive Bosch/delco?

Exhaust leak may cause the light to throw anyways if its leaking enough and bringing air in... I honestly think the leak that puffs out exhaust but also sucks air is throwing those 02 sensors for a loop... Its not like the old days where a small exhaust leak didn't have an effect on the controls of a motor. If the 02 senses extra air in the exhaust it will adjust the pcm to its max parameters and then throw CEL... Fix the exhaust leak, reset the computer and see what happens... You may fix it with 1 gasket....

 

[okfoz]

I would ag

You have no oil consumption issues I am guessing?

My oil consumption is typical, about 2 quarts for 3000 miles, it is unchanged for the past 4 years... Ever since I have owned the Tahoe it has always done that, and according to some that is pretty normal, 1 quart for 1500 miles...

I think I would find a different exhaust shop... They may have very low quality cats and that may be their way to keep people comming back to spend more money. But I a vehicle has to be really screwed to trash cats that quick...

I would agree, however I have a 2 year warranty, and let them screw with it, If it continues and my warranty expires, I think I will just reset the light and trade it in, as usually it takes a few days before it throws the code... The first replacement convertor the light actually turned off the day I was going to take it in, but then it came back on. Then they told me to try Premium fuel for a few tanks, which makes sense since Premium fuel burns cooler, But it still came back on...

Its weird you are getting it on bank 2 only.... What O2 sensors did you put in there? Cheap or Expensive Bosch/delco?

I know it does seem fishy... I did not chose the O2 Sensors, it was what they used, and I did not ask... In the past they have always used good parts, but I am starting to think they are using cheaper parts as time goes on...

Exhaust leak may cause the light to throw anyways if its leaking enough and bringing air in... I honestly think the leak that puffs out exhaust but also sucks air is throwing those 02 sensors for a loop... Its not like the old days where a small exhaust leak didn't have an effect on the controls of a motor. If the 02 senses extra air in the exhaust it will adjust the pcm to its max parameters and then throw CEL... Fix the exhaust leak, reset the computer and see what happens... You may fix it with 1 gasket....

I had that same thought, the extra air in theory, could tell the computer to richen that side, since the air leak would cause an indicated lean condition. Then the computer would therefore add fuel to compensate. This is where it gets sketchy, more fuel typically means a cooler engine, BUT it does cause a hotter exhaust, interestingly that is how they control the mixture on airplanes, because changes in altitude changes the amount of fuel changes, and to do that they measure the temperature of the exhaust... So the extra fuel caused the too much heat in the Cat, and then melting it...

We can only see what happens from here...

 

[Doubeleive]

I would hook up a scanner and take a look at the live data, that should point you in a better direction of what the root cause is, I drove the literal piss out of my nbs and managed to burn thru some cheap cat's about once a year for a while, switched to a different shop and cat's suddenly started lasting longer, could just be that but I would take a look at the data stream and see if anything is weird.

 

[swathdiver]

I would ag

My oil consumption is typical, about 2 quarts for 3000 miles, it is unchanged for the past 4 years... Ever since I have owned the Tahoe it has always done that, and according to some that is pretty normal, 1 quart for 1500 miles...


I would agree, however I have a 2 year warranty, and let them screw with it, If it continues and my warranty expires, I think I will just reset the light and trade it in, as usually it takes a few days before it throws the code... The first replacement convertor the light actually turned off the day I was going to take it in, but then it came back on. Then they told me to try Premium fuel for a few tanks, which makes sense since Premium fuel burns cooler, But it still came back on...



I know it does seem fishy... I did not chose the O2 Sensors, it was what they used, and I did not ask... In the past they have always used good parts, but I am starting to think they are using cheaper parts as time goes on...



I had that same thought, the extra air in theory, could tell the computer to richen that side, since the air leak would cause an indicated lean condition. Then the computer would therefore add fuel to compensate. This is where it gets sketchy, more fuel typically means a cooler engine, BUT it does cause a hotter exhaust, interestingly that is how they control the mixture on airplanes, because changes in altitude changes the amount of fuel changes, and to do that they measure the temperature of the exhaust... So the extra fuel caused the too much heat in the Cat, and then melting it...

We can only see what happens from here...

Seems to me you just don't care. Trade it in and get something you like. If wrong, replace the O2 sensors with GM OE or ACDelco Professional and get a scan tool that can see how the engine is performing. Be prepared to replace the cats, this time with something known, like a Magnaflow system. One fella did Magnaflow cats and y-pipe and had them fail after 24k miles or so. I have same system but have not reached that mileage yet. Stock probably best but it about a thousand bucks.

Sealants, adhesives can foul O2 sensors. Someone using RTV on intake gaskets, throttle body, etc. can go through the motor, out into the exhaust and mess with the O2 sensors. Leaking coolant into the exhaust, oil into the exhaust can also affect the O2 sensors and WILL destroy the cats.

Leak down and compression test or just get rid of it.