Looking for some confirmation here...catalytic converter?

[TahoeInFC]

Hey All,
Man, this is a good forum for folks like us trying to keep these OBS trucks on the road.
Recently, my truck (99 Tahoe, 5.7L, 132k miles on the counter) threw the dreaded P0430 code for a catalyst inefficiency, and I've been hunting around out here for help. I've found a ton of help in this forum, and have taken some actions according to previous advice given. (SunlitComet, I've especially appreciated your technical posts...they have helped me determine a troubleshooting direction on more than just this occasion)
I'll tell you what I've done and what I've learned, and hoping you smart folks can tell me if I'm on the right track.

I'm going to apologize in advance for the long post, but I've noticed that many OP's don't post enough info to get a diagnosis started, so I wanted to make sure you had all the details.

My truck originally threw the P0430 code about 6 months ago. I read it and cleared it to see if it would return. It didn't return until just a couple weeks ago, so I thought that first code may have been a hiccup due to bad gas, etc... So I reset the code again. No such luck this time...it came back again. Rinse, repeat, and the code returns. I guess it's time to dig in and fix it.

My first action was to read the O2 sensors with a scanner. I got the truck warmed up by driving around for 20 minutes or so, then sat in my driveway, revved to about 2000-2500 RPM and watched the sensor voltages.
Under these conditions, both upstream sensors seem to be cycling through the range of about 0.2V to about 0.8V, so appears to be going closed loop. At the time of this first reading, the downstream on bank 1 was sitting somewhere mid-range between 0.4V to 0.7V, but downstream on bank 2 was around 0.2-0.3V and just sitting there. When I backed off the throttle and let it idle, the downstream sensors started moving around more. The bank 2 downstream is capable of going to higher voltages than just 0.2V and is erratic during idle, seemingly tracking the upstream (sort of). It just drops low when the RPMs are increased. I'm thinking this is probably trimming the engine to the rich side, and is probably because of a plugged cat.
When I took this same reading today, both downstream sensors seem to be behaving this way, so I think both cats need to be changed.
All four O2 sensors were replaced about 20k miles ago, but don't know the brand. The previous owner kept the records, but I don't know if he used OEM sensors. I'm guessing he did because he kept this truck immaculate and tended to use genuine GM parts according to most of the service records he gave me.

My next action was to pull some plugs and check them out. I pulled both rear plugs, and both are fairly clean, but perhaps a little sooty down in the 'wells' of the plug. The spark elements are clean and bronze, and definitely not fouled or wet with fuel. The two front plugs look great. No white or black residue. Clean burn plugs, and no obvious issues.
The tailpipe has a very slight black soot inside, so I know it's running rich, at least at times. It runs rich at start-up for sure. My wife fired it up this morning, and it burned my eyes for a few minutes...whew! Sulfur and fuel for certain. It's not as bad after running for awhile, but still annoyingly stinky. Another sign of failing cats, or so I've read.

This truck has the improved fuel injection spider, and had some fuel metering work done just before I bought it. It runs well, but I can sometimes feel a slight miss and rough idle...not always. I'm starting to question my fuel pump, because I don't think it's ever been changed, but that's a different topic for a different thread.

So, for you experts, do you think I'm on the right track for new cats?
Do I have underlying issues that are failing the existing converters? Or are my failing cats causing issues that are affecting the engine fuel trim? Sort of a 'chicken and the egg' question, I know.
Bad O2 sensors? Usually the sensors last a long time, much more than 20k miles.

If I need new converters, I'm planning to use the direct-fit Walker cats. The are reasonably priced and for all the reading I've done, people seem generally happy. I've also read in various places that Walker supplies GM with OEM cats, so I can't be too far off the mark here. I'm not interested in 'performance cats'

Also, before anyone tells me to just "cut 'em off", I'm in an emissions area in Colorado. I wish I could just take them off, but I don't have that option. I'm not really interested in cheater pipes for the 364 days I don't need cats.

Your thoughts?

Thanks!

 

[BlackOh7on22s]

Use a temp gun and check temperatures before and after cat, this will offer you some more info on the cats. If you do post the temps before and aft cat

 

[TahoeInFC]

Use a temp gun and check temperatures before and after cat, this will offer you some more info on the cats. If you do post the temps before and aft cat

Thanks! I've seen that suggestion in other places, but I don't have a temp gun. I think I can borrow one from work tomorrow and do it over the weekend.

If I recall, if the cat is working, the output should be hotter than the input, right? By something like 100-200 degrees? If the cat is dead, then I won't see much temp difference. Is this correct?

I'll post back when I have some temperatures. In the meantime, does everything else add up?

 

[TahoeInFC]

Alright, I borrowed a Fluke temp gun from work. Nice toy...I might have to look for one of these.

Got it good and warm driving around town, then brought it home and had my 14 year old daughter hold the pedal at around 2300RPM while I climbed around underneath. They're never too young to learn about cars (Also gave her a good opportunity to run over her old man, but she let me live another day ).

Bank 1:
Front of cat - ~200 - 220 degrees
Rear of cat - ~430 - 450 degrees

Bank 2:
Front of cat - ~330 degrees
Rear of cat - ~409 degrees

I wasn't sure about this, so took a second reading on Bank 2. It was jumping around all over the place and was hard to get a good temperature reading. Bank 1 was much more steady.

Front: 269 - 300 degrees
Rear: ~ 390 - 417 degrees


I'm failing on bank 2, which seems to agree with the readings. Bank 2 is only partially working, thus the smaller temperature delta across the cat. Bank 1 seems to be working OK, and has not yet thrown a code. All codes have been on bank 2.

Also, I ran it back up to around 2300 RPM with the scanner connected. Bank 1 began to work with the upstream sensor cycling and the downstream sensor sitting between 0.7V and 0.4V. The downstream sensor would sometimes have deviations that agree with the upstream spikes, so I think Bank 1 is also on its way out, but just hasn't failed yet.
Bank 2 cycles the upstream sensor, but the downstream sensor just sticks around 0.2 - 0.3V and doesn't get higher until I let off the gas. When it goes down to idle, the downstream sensor jumps up a little and is erratic after that.

Sooooo...I'm tempted to just change both cats, then get some good readings on the sensors. If they start working as expected with the new converters, then great. If not, I'll just replace all four sensors with Delcos. I think Bank 2 converter is getting plugged, so the downstream bank 2 sensor is getting leaned out all the way at high RPM's. Not getting the flow it needs to get a good measurement.


Your thoughts on the catalytic converters?

Thanks!


One more tidbit...I might have found another reason it was running rich. A couple of the clamps downstream of the MAF were pretty loose...I might have had some leakage at the air input which can throw off the A/F ratio calculations. I tightened them, so will see if it starts to run better.

 

[TigerEyz3]

I've always been one to replace things in entirety when something fails... don't replace just one converter as the other is likely to fail shortly after you've replaced the one. Might as well deal with both sides at the same time and be done with it. Also, replace the O2 sensors while you're at it. Other than that, my last suggestion is to go with MagnaFlow over Walker for the converter replacement. I've read lots of "less than pleased" info on the Walker (and other cheaper brands) duration/longevity.

I've personally replaced my cats (shortly after replacing my factory motor at ~326k miles...now have 381k miles) and have been very happy with the MagnaFlow quality.

 

[TahoeInFC]

I've always been one to replace things in entirety when something fails... don't replace just one converter as the other is likely to fail shortly after you've replaced the one. Might as well deal with both sides at the same time and be done with it. Also, replace the O2 sensors while you're at it. Other than that, my last suggestion is to go with MagnaFlow over Walker for the converter replacement. I've read lots of "less than pleased" info on the Walker (and other cheaper brands) duration/longevity.

I've personally replaced my cats (shortly after replacing my factory motor at ~326k miles...now have 381k miles) and have been very happy with the MagnaFlow quality.

Thanks for the reply!
Yeah, I'm in total agreement. Even though one cat still seems to be doing it's thing, they're the same age, so both are coming out for new ones at the same time. Just need an opportunity to buy the parts and put 'em in.
About the O2 sensors...the ones in there now only have about 20k on them...aren't they supposed to last 60-80k? I was going to put the new cats in and then check the system with a scantool to make sure both banks agree and I'm getting believable signals from the O2 sensors. If all look good, should be OK for awhile. If not, then I'll go back and replace them, and probably all of them since they are also the same age as one another. If they are behaving, is there a reason to replace them? Can a failing cat cause a sensor to somehow fail also? Just curious.
It just occurred to me that I was going to swap downstream sensors and see if the problem followed the sensor to the other bank... forgot to try that. I'll do that before changing anything.

Do you have a particular MagnaFlow converter in mind? I did a lot of reading on converters and the MagnaFlow cats tend to generally make people happy, however I've read a lot of comments on MagnaFlows that their direct fit setups for my specific Tahoe don't fit well and often have to be modified either with bending, tweaking or filing. Then if things don't work out (fails, throws a code, leaky weld, etc..), MagnaFlow won't do anything about it because of the mods just to get them to fit, even within warranty. Perhaps the way to go with MagnaFlow is to buy a couple of individual converters and just have them welded in place of the old ones, and stay away from the kits. Don't know. I'm glad to hear you've had good luck with them. Did you use the direct fit kit?

 

[TigerEyz3]

Typically, when converters (or O2 sensors) fail, there's something else causing the problem - oil or fuel getting to the converter is most notable for converter failure. Your reference for O2 sensor life doesn't sound out of line, but I'm certain that environment and driving habits play a role in their life span as well. That said, given that you've only got 20k on them and assuming everything else is good, they should still be good. Downstream O2 sensors typically get replaced more often than the upstream sensors (more activity and more heat variation probably play a part). It's a good idea you suggested about changing the O2 sensor position to see if just the sensor is bad.

As far as MagnaFlow standing behind their products, the things you listed don't fall under manufacturer warranty claims, so not really there issue to resolve. I've always been "that guy" when it comes to researching products before purchase - almost to the point of "analysis paralysis". I've been known to spend more than originally intended because I want to get the best quality that I can fit into my budget - "I would rather pay a little more now than a lot more later".

I didn't go with a direct fit kit as there's nothing about my exhaust that is stock any more... Converters were the last thing to go of my stock exhaust setup. That said, I've got 2.5" piping at my converters - my signature "mods list" link has the correct part number for my converters listed. MagnaFlow makes converters with smaller diameter piping for stock systems...the part number you'd reference assuming you want a stock configuration converter is 99004HM (HM stands for "Heavy Metal").

 

[TahoeInFC]

Thanks for the advice! Yeah, I definitely know the 'paralysis by analysis' trap well. It drives my wife crazy when I research a purchase...can't help it, I guess. If that's all that irritates her about me on any given day, I'm in good shape.

I'm also in the camp of spending a little more if it will serve me well. Definitely not a fan of spending money and doing work, only to have to do the same expensive job in another year or so.

Your advice about engine function has my interest for sure. This Tahoe has about 133k on it, and runs very well, but I know there's the possibility that it's burning a little more oil than it used to or that the fuel system isn't running the correct ratios. I know it runs a little rich at startup, and seems to pull back like it should when it goes closed loop, but I can still smell a tinge of sulfury fuel even after running awhile. It doesn't belch out any blue or black smoke that might indicate an oil burning problem, although it will be about 1/2 - 3/4 quart low by the time of the next oil change. It has used Mobil1 synthetic since new, so I just kept up that tradition at oil change time.

I don't get any 'damp' spark plugs or fuel smell that might indicate a bad fuel regulator, but I should probably check again. This truck already has the improved fuel injector spider, so I don't have the old issue of bad poppet injectors. I DO think my fuel pump is on its way out because it's humming progressively louder when I drop below 1/2 tank of gas. I don't think it's ever been changed. I've heard that fuel pressure issues from a failing pump can cause odd fuel mixture issues, so may have something else to check with that. Perhaps the engine is running rich and hurting the cat. No additional CEL codes than the converter at this time that might indicate a fuel or spark problem.

But one thing is for certain...I'd be PISSED if I ruin a new set of converters with an engine problem I haven't detected.

I'll try switching the O2 sensors this weekend and see if it jacks up the readings on the other bank.

Any other ideas?

Thanks!

 

[TigerEyz3]

I had ~326k on my factory motor before I pulled it and dropped in the HT383E. Chances are your motor still has many more miles left in it. Check the compression and the tuning (since you're not running factory tuning) to make sure that everything is within spec. Assuming so, fueling issues can then be addressed:

• easiest thing to replace is the fuel filter
• fuel pumps go out over time, often without any warning (working one day, dead the next)
  
• the fuel pressure regulator is in the spider (should be good based on mileage/replacement time frame)
• yes, fuel pressure issues can cause fueling problems... I think 66psi is the magic number, with 50psi being minimum required for start.

Take a look at the following:

• • fuel pump (physical electrical connections are common issues)
  • assuming the fuel pump is working as it should, check the fuel lines
    • fuel filter clogs are not uncommon
    • check the fuel pressure port (located at top rear of motor) and see what pressure you get when:
      • not running (determines if there's a pressure drop/leak after system off)
        
      • key on
      • running at idle
      • running under acceleration

 

[east302]

I'll try switching the O2 sensors this weekend and see if it jacks up the readings on the other bank.

Swapping the sensors is a reasonable approach.

Here is the factory backpressure test to check for an exhaust restriction by removing an upstream oxygen sensor...







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