The recurring cat efficiency low code saga

[xdhd350]

Replaced my cat with a Magnaflow at local shop. Lasted long enough to pass I/M readiness and get an annual inspection. However, it came back on within a week.

Took it back to the shop and they say the color of the cat shows engine is running lean and has ruined the converter.. hence, no warranty.

I recently went through and replaced a bunch of vacuum fittings, plugs, and wires. I am showing close to -10 on the boost gauge at idle.

Can anyone take a look at the picture and tell me what they think. This cat has only had 1 1/2 weeks of drive time.

I know there are many variables to create a lean mixture, but I am mainly looking for a set of eyes to see if they concur with the shop evaluation.

Thanks in advance.

[consultIII]

Cat sure looks like it got hot. Thought a rich condition was needed to do that to a cat. I know lean conditons can cause hotter combustion chamber temps but never knew it could cook a cat so far back from the heads.

The ecm maybe compensating for the perceived lean conditon and overfueling thus cooking the cat. Is the tailpipe black and is the fuel mileage bad?

Is there any other dtc? Can you view live data or freeze frame data; particularly fuel trim?
The code you have was for low oxygen storing capacity? Please post that dtc.

What model year?

What is the units for the boost gauge...mmHg. Normally aspirated engines at idle (with stock cam profiles) usually are in the 18 - 20 mmHg (Now sure what is normal for that Supercharged engine)That unit seems low which could indicate a vacuum leak and hence lean conditon.

[xdhd350]

Originally Posted by consultIII

Cat sure looks like it got hot. Thought a rich condition was needed to do that to a cat. I know lean conditons can cause hotter combustion chamber temps but never knew it could cook a cat so far back from the heads.

The ecm maybe compensating for the perceived lean conditon and overfueling thus cooking the cat. Is the tailpipe black and is the fuel mileage bad?

Is there any other dtc? Can you view live data or freeze frame data; particularly fuel trim?
The code you have was for low oxygen storing capacity? Please post that dtc.

What model year?

What is the units for the boost gauge...mmHg. Normally aspirated engines at idle (with stock cam profiles) usually are in the 18 - 20 mmHg (Now sure what is normal for that Supercharged engine)That unit seems low which could indicate a vacuum leak and hence lean conditon.

Yeah, it seems like it would be running rich. I would say gas mileage is on the low side right now. This is a 2001 SSEI.

The boost gauge on the dash goes from -10 to +10 but I don't know that it has any absolute value. I do have a real vacuum gauge to use under the hood. Will do that this weekend.

As to the fuel trims, they were actually some of the best I had seen recently. Pretty much 0 on short term, and around 7 on long term at idle.

I am using the Auto Enginuity software, but I haven't ponied up the extra 250 clams to get enhanced GM data so I am only getting basic powertrain with a bit of mode 6 data. No freeze frame on the basic version.

There is a more expensive Magnaflow that has more precious metals to scrub the exhaust better, but I elected to go with the lower priced unit, thinking it was up to the task.

The only other thing that comes to mind is that I replaced both O2 sensors a year or so ago and not knowing any better, I used the Bosch brand from my local Auto Zone. Also, the post cat sensor already has an extension on it to pull it back from the exhaust stream a bit. Just wondering if the upstream sensor is not responding fast enough for the PCM to properly manage the rich to lean transitions. But that's the other thing. I have done a data capture of the O2 sensors before and after replacing the cat and the graphs look good except for acceleration. I can attach some screenshots after I get them off the laptop.

This is the FOURTH cat I have put on this car. The original had the guts break loose at around 85K, then put in a replacement Magnaflow. I won't go into the long winded explanation just yet of why I keep replacing cats every year or so.

Oh well, this Bonny has other nagging issues for me to deal with while I figure this cat issue out. Grrrrrrr.

Thanks for the feedback.

[consultIII]

Originally Posted by xdhd350

Yeah, it seems like it would be running rich. I would say gas mileage is on the low side right now. This is a 2001 SSEI.

The boost gauge on the dash goes from -10 to +10 but I don't know that it has any absolute value. I do have a real vacuum gauge to use under the hood. Will do that this weekend.

As to the fuel trims, they were actually some of the best I had seen recently. Pretty much 0 on short term, and around 7 on long term at idle.

I am using the Auto Enginuity software, but I haven't ponied up the extra 250 clams to get enhanced GM data so I am only getting basic powertrain with a bit of mode 6 data. No freeze frame on the basic version.

There is a more expensive Magnaflow that has more precious metals to scrub the exhaust better, but I elected to go with the lower priced unit, thinking it was up to the task.

The only other thing that comes to mind is that I replaced both O2 sensors a year or so ago and not knowing any better, I used the Bosch brand from my local Auto Zone. Also, the post cat sensor already has an extension on it to pull it back from the exhaust stream a bit. Just wondering if the upstream sensor is not responding fast enough for the PCM to properly manage the rich to lean transitions. But that's the other thing. I have done a data capture of the O2 sensors before and after replacing the cat and the graphs look good except for acceleration. I can attach some screenshots after I get them off the laptop.

This is the FOURTH cat I have put on this car. The original had the guts break loose at around 85K, then put in a replacement Magnaflow. I won't go into the long winded explanation just yet of why I keep replacing cats every year or so.

Oh well, this Bonny has other nagging issues for me to deal with while I figure this cat issue out. Grrrrrrr.

Thanks for the feedback.

another way to check for vacuum leaks:
usually with a vacuum leak you will see high fuel trim numbers at idle but normal fuel trim at 2500rpm. at speed the high intake air flow makes the vacuum leak insignificant.

any other drivability issues? misfires. does it have an aftermarket alarm system; as there is a gm tsb for cat damage from this. Hard starting after a 20 or more shut down (soak)? Wheel speed sensors issues as the ebcm has a factor in misfire counters eg. rough road information.

[xdhd350]

Originally Posted by consultIII

any other drivability issues? misfires. does it have an aftermarket alarm system; as there is a gm tsb for cat damage from this. Hard starting after a 20 or more shut down (soak)? Wheel speed sensors issues as the ebcm has a factor in misfire counters eg. rough road information.

None whatsoever. Car is completely stock. Drives great, especially since I did the plug and wire change out. It's got about 140K on it and they had never been done. In fact, that's what keeps me telling the missus to keep this car.. that fact that it drives and handles like a dream. I love driving this car, but lately, it seems to be driving me (to drinking.. hehe).

Just took some pics of left and right exhaust tips... have a look and thanks for your continued suggestions.

Sorry about the missing vehicle info.. thought I had set up my signature. That's been fixed now.

[consultIII]

tips look clean. the cat may have cleaned the exhaust up. What did the old plugs look like? How many miles on the new plugs? Have a quick look at 1 or 2 plugs.

[xdhd350]

The old plugs weren't too bad. Certainly no signs of anything significantly wrong that caught my attention. A couple plug wires on bank 2 (rear) were nearly ate up from a coolant leak on the LIM which has been repaired for about a year and a half now. Practically disintegrated in my hands as I was trying to get the boot off the plug.

I'm running Autolite 605's in it right now.

It has been a busy weekend and I didn't get to do much with it. The testing will resume this week.