i think my cat is nearing its end.

CaptainXL

Original poster
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Dec 4, 2011
2,445
Hey guys. I just had a new symptom crop up today. When accelerating up to 70 to get on the highway I notice a slight dip or surge in performance. This only happens when flooring it. I have always thought that I might have some type of restriction. Does this sound like a good observation? I have done all the maintenance required up until now. No CEL is on and I have replaced plugs and cleaned TB. Let me know what you think. I have always had the feeling this truck was under powered a bit. The downshifting on the highway on flat road kind of alerted me that i might have an issue.
 

jimmyjam

Member
Nov 18, 2011
1,634
take it for a drive, go WOT 0-60, time it. pull out the pre-cat o2 sensor and repeat. if there is a big difference i'd say its got an issue.

what temp is your truck running at? if it is cold it will foul up the cat pretty quick
 

CaptainXL

Original poster
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Dec 4, 2011
2,445
jimmyjam said:
take it for a drive, go WOT 0-60, time it. pull out the pre-cat o2 sensor and repeat. if there is a big difference i'd say its got an issue.

what temp is your truck running at? if it is cold it will foul up the cat pretty quick

Temp is good at 200 according to scan tool. Just replaced cts and tstat a year ago so that can't be it.

So do some test runs with and without o2? Is there really gonna be a big difference in times. What am I looking for? A full second shaved off?
 

CaptainXL

Original poster
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Dec 4, 2011
2,445
That means taking off the heat shield and busting out the pipe wrench. Argh I should have just ran it back when I had the exhaust manifold off. Hindsight is 20/20.
 

jimmyjam

Member
Nov 18, 2011
1,634
well mine had gotten clogged pretty bad and it was night and day when i relieved the backpressure
 

ElAviator72

Member
Jan 11, 2012
118
Just remember, that if the vehicle is less than 8 years old and has less than 80,000 miles, the cat is protected by the EPA mandated federal emissions warranty, and you get your replacement for free :yes:
 

Voymom

Member
Feb 3, 2012
2,523
Just to increase my knowledge, but wouldn't the PCM throw a code over a clogged cat, as I would imagine it would be interfering with one of the 02 sensors?
 

Sparky

Member
Dec 4, 2011
12,927
Voymom said:
Just to increase my knowledge, but wouldn't the PCM throw a code over a clogged cat, as I would imagine it would be interfering with one of the 02 sensors?

Actually no. You'd think so but the upstream O2 sees the exhaust fresh out of the cylinders like normal, and the post-cat O2 sees "cleaned" air, so everything according to the sensors is hunky-dunky. The cat can be severely clogged, but the portion that still passes airflow still catalyzes, so it passes the O2 sensor test.

Only if the catalyst fails to work does a code get thrown.
 

Chickenhawk

Member
Dec 6, 2011
782
Interestingly enough, a clogged cat generally doesn't throw any codes because the little exhaust it lets through is still being cleaned. The rear O2 sensor is used to compare post-cat emissions to pre-cat emissions, and when the two O2 sensors start reading very similar, the computer knows the cat has stopped cleaning the exhaust properly and throws a code.

But a clogged cat is still working to clean the exhaust, albeit at a much reduced overall amount.

Clogged cat means HUGELY reduced overall performance, but rarely does it throw a code.

The OP's symptoms sound EXACTLY like a clogged cat: reduced performance on the highway; unexpected downshifts on flat roads; a 'feeling' that the truck does not accelerate like it should; a history of a bad thermostat or CTS.

My advice? Get a new cat; it will be like night and day and you will be DELIGHTED at the new-found performance.

For others reading this thread in the future, if people think they may have a bad cat, one should diagnose properly and scientifically before they spend money on parts. Unless you have every single one of the above symptoms, then have the exhaust backpressure checked. A quick and easy way is to remove the front O2 sensor to provide a place for the blocked exhaust gases to escape, but I think a better way is an exhaust backpressure test with a proper pressure gauge. This can be done inexpensively or even for free at many shops or exhaust places.

They remove the front O2 sensor and screw in a special pressure gauge. They check the pressure at idle and at 2500 RPM. Pressure at idle should be near 0 PSI. Pressure at 2500 RPM should be no more than 3 or 4 PSI.

(I even bought my own exhaust backpressure test kit because it was under a $100 and we have two Trailblazers in the family. If anyone in or near Winnipeg wants to borrow, let me know.)

(Oops. Thanks Sparky! Beat me by 1 minute! I gotta learn to type with more than 2 fingers some day!)
 

Voymom

Member
Feb 3, 2012
2,523
Sparky said:
Actually no. You'd think so but the upstream O2 sees the exhaust fresh out of the cylinders like normal, and the post-cat O2 sees "cleaned" air, so everything according to the sensors is hunky-dunky. The cat can be severely clogged, but the portion that still passes airflow still catalyzes, so it passes the O2 sensor test.

Only if the catalyst fails to work does a code get thrown.

That's pretty interesting. I know a clogged cat shows many symptoms, but to someone who doesn't know any better could be extremely clueless probably resulting in some major damage? It amazes me that we have low tire pressure sensors, but nothing for stuff like the cat converter. Tire inflation is apart of a safety feature so it makes sense to have a sensor, but I'd imagine since it is in our visual surroundings we'd be able to see an issue. :undecided: compared to something that is not in our immediate visual surroundings....Very nice to know!
 

ElAviator72

Member
Jan 11, 2012
118
Voymom said:
Tire inflation is apart of a safety feature so it makes sense to have a sensor, but I'd imagine since it is in our visual surroundings we'd be able to see an issue. :undecided: QUOTE]

'07 and up have TPMS, it is kind of a love-hate feature in the nation :wink: If you have a nicer rig with the DIC, you get real time tire pressure readings up in the cab. If you have an LS (like my wife's truck), you get an idiot light when something is wrong, and the onus is on you to figure it out :redface: 90% of the time my wife gets a TPMS warning, it is bogus. Driving by the local airport seems to cause one of the TPMS sensors to drop offline on occasion :undecided:
 

jimmyjam

Member
Nov 18, 2011
1,634
when my cat was clogged i intermittently got a MAF code thrown FWIW
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
ElAviator72 said:
Just remember, that if the vehicle is less than 8 years old and has less than 80,000 miles, the cat is protected by the EPA mandated federal emissions warranty, and you get your replacement for free :yes:

Unfortunately I have 120k on the odo. But this is worth repeating for anyone that needs to know.

jimmyjam said:
when my cat was clogged i intermittently got a MAF code thrown FWIW

Sounds about right. The mass airflow coming in definitely would not jive with the throttle position.

Will an exhaust shop let you run without the cat for testing purposes? I know the EPA doesn't like test pipes but I wont be running with a test pipe for testing purposes. Just a short jog up and down the highway.
 

Chickenhawk

Member
Dec 6, 2011
782
It depends on the rules of the road where you live. That being said, it should be no problem for a short test ride, and any good exhaust shop will do this for you.

Quite frankly, based on your symptoms though, I think you are just wasting your time. Get a new cat installed and it will be like night and day.

Today's cats are very sensitive to what used to be only minor variations in mixture, and running with a bad thermostat for a month or two, or running with a spark plug misfire for even an hour or so is enough to cause potential damage.
 

CaptainXL

Original poster
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Dec 4, 2011
2,445
Been searching. Having a really hard time finding a quality converter for the LWB 129".
 

Sparky

Member
Dec 4, 2011
12,927
I'd personally get a universal the right size and have the old one cut out and the new welded in. Probably cheaper that way too.
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
Sparky said:
I'd personally get a universal the right size and have the old one cut out and the new welded in. Probably cheaper that way too.

Right. I would like to know who has done this without taking the pipe out. There isn't alot of room up in there for a welder. Heck how would you even fit a saw to cut it out? Has it been done?
 

DocBrown

Member
Dec 8, 2011
501
Yes, its been done. You may have to search the OS for someone who has more detail. It is cheaper to the universal, but IMHO, not any easier. Check NAPA, that's where I got my OEM style CAT. RockAuto also has a nice selection. I kind of know the guys at my local NAPA and they applied my cost to a "business account" to knock a bit off. IIRC it came up well under $200. If you have AAA you can get something like a 10% discount at NAPA also. The funny thing is that the CAT for the EXT is generally cheaper than a SWB.

First off, this is a 2 man job. The hardest part about changing the the OEM cat is getting the bolts off the exhaust manifold. They will be rusted on. Take the right wheel off for easier access and use a torch (acetylene, not propane. Propane won't get it hot enough) to heat the bolts red hot. You'll also need an air impact wrench and long extensions to access the bolts from the bottom. With an EXT you will also have to take off the cross member with the transmission mount on it. Use a floor jack to support the transmission while you're working on the CAT. Take off the hangers on the rest of the exhaust so you have room to wedge the CAT out. Replace the down stream O2 sensor, and put it on BEFORE mounting the new CAT. There is almost no room to put it on after the fact, not impossible, but very hard.
 

CaptainXL

Original poster
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Dec 4, 2011
2,445
Ok. Thanks for the info Doc. It helps. But now im not so certain I have a problem. My Torque app says my peak HP is 267. But my 0-60 time is 32 seconds?:confused:
 

tblazerdude

Member
Dec 4, 2011
321
Your loss of power @ WOT on the highway can also be caused by a vacuum leak. Also, when was the last time you changed your fuel filter?
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
tblazerdude said:
Your loss of power @ WOT on the highway can also be caused by a vacuum leak. Also, when was the last time you changed your fuel filter?

There is no fuel filter on 05-09 models nor an external fpr. They are both integral to the in-tank fuel pump. But now that I am thinking about it my pump is making some strange noises as of late. I will check the fuel pressure again. Will a fuel pump thats about to go start and then stop making its typical whirring sound?
 

Voymom

Member
Feb 3, 2012
2,523
CaptainXL said:
There is no fuel filter on 05-09 models nor an external fpr. They are both integral to the in-tank fuel pump. But now that I am thinking about it my pump is making some strange noises as of late. I will check the fuel pressure again. Will a fuel pump thats about to go start and then stop making its typical whirring sound?

Have you checked your water pump? Or power steering pump? Both of those on mine made a whirring sound when they were about to go out.
 

Chickenhawk

Member
Dec 6, 2011
782
Before you worry about the fuel pump, have you eliminated the cat as the problem? It is far easier to diagnose a bad cat than it is to find an intermittent fault in a fuel pump.

While not comprehensive, many fuel pumps tend to start to fail at low-draw levels. What happens is that they wear, heat up a bit and start to exhibit symptoms during steady highway cruising after running for a while. They can also show early symptoms of wear when you stop for gas after running for a while and it is suddenly hard to start afterwards. Except for random catastrophic failure, it is more rare for a fuel pump to start to exhibit symptoms at high-draw volumes.

But, again, don't take my word on this. Start with the easiest, most likely and cheapest diagnosis first, then go on from there. Get the cat tested. (Don't rely on a muffler shop to just do a temperature test. Get the backpressure tested and post the readings at idle and 2500 RPM here in this forum so we have a form of reference for backpressure readings for our own knowledge base.)
 

DocBrown

Member
Dec 8, 2011
501
CaptainXL said:
Ok. Thanks for the info Doc. It helps. But now im not so certain I have a problem. My Torque app says my peak HP is 267. But my 0-60 time is 32 seconds?:confused:

I'm not sure you'd see less HP/torque. The engine itself is/should be still operating the way it's supposed to. As Chicken suggests, get the back pressure tested. I'm still betting you have a plugged CAT.
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
Chickenhawk said:
Before you worry about the fuel pump, have you eliminated the cat as the problem? )

Im trying. Every shop ive checked with in town is afraid to test it. They keep telling me to wait until the CEL illuminates.
 

Chickenhawk

Member
Dec 6, 2011
782
Well, they have a poor understanding of the CEL.

It measures only inputs relating to emissions and checks them against an acceptable range of readings. If it falls outside the range for a number of cycles, depending on the sensor, it illuminates a CEL.

A bad cat will illuminate a CEL when it fails to clean the emissions sufficiently. A PLUGGED cat, on the other hand, may be perfectly clean and may never illuminate a CEL, even though the exhaust is 99% plugged and your 0-to-60 time can be recorded on a calendar instead of a stopwatch.

You should stay well away from any shop who tells you to wait for a CEL. They are not mechanics or diagnosticians; they are simply parts changers who wouldn't have a clue that CELs are merely symptoms and not causes.

I bought my own backpressure gauge for less than $100.
 

DocBrown

Member
Dec 8, 2011
501
Chickenhawk said:
Well, they have a poor understanding of the CEL.

It measures only inputs relating to emissions and checks them against an acceptable range of readings. If it falls outside the range for a number of cycles, depending on the sensor, it illuminates a CEL.

A bad cat will illuminate a CEL when it fails to clean the emissions sufficiently. A PLUGGED cat, on the other hand, may be perfectly clean and may never illuminate a CEL, even though the exhaust is 99% plugged and your 0-to-60 time can be recorded on a calendar instead of a stopwatch.

You should stay well away from any shop who tells you to wait for a CEL. They are not mechanics or diagnosticians; they are simply parts changers who wouldn't have a clue that CELs are merely symptoms and not causes.

I bought my own backpressure gauge for less than $100.

:yes: Exactly! That shop clearly has no clue how things work.

I never had a CEL when my CAT took a dump. If you survey everyone on GMT that had to replace their CAT, you'll find that very few had a CEL to indicate a problem. It was the symptoms that led us to the solution.
 

jimmyjam

Member
Nov 18, 2011
1,634
lol and like i said my clogged cat was throwing an intermittent MAF code. they'd probably charge you for a new MAF and send you out the door
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
I went to a tuffy muffler in town that performed a pressure test before the cat and also a gas analysis. They said the pressure is normal and there are no restrictions. They also said that if your cat was even partially blocked you would notice a constant loss of power. I dont have that. Looks like I need to keep looking elsewhere.
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
Well more good news. I put the scan tool on the truck today and drove it around town to watch the misfire data. Cylinder 6 was showing over 116 misfires driving just a couple miles. So now I am looking at a valve issue, fuel injector or coil. Going to ohm out the injector and swap coils tonight.
 

CaptainXL

Original poster
Member
Dec 4, 2011
2,445
Update. The other day before a 500 mile road trip to Detroit for a Tigers baseball game I swapped #6 and #4 coils and I haven't had any misfires. I inspected both spark plugs and didn't see any issues. This was an odd experience. Case closed I guess.
 

Menthol

Member
Dec 8, 2011
177
CaptainXL said:
Update. The other day before a 500 mile road trip to Detroit for a Tigers baseball game I swapped #6 and #4 coils and I haven't had any misfires. I inspected both spark plugs and didn't see any issues. This was an odd experience. Case closed I guess.

Why only #4 and #6?
 

Menthol

Member
Dec 8, 2011
177
I have a similar issue which THINK is a bad cat converter also but need help diagnosing it for sure. History in last six months. Vehicle hardly driven, maybe 30kms week. Just before winter I noticed the thermostat was bad, not sure how long it was like that because the vehicle was driven by the wife like once a week. Thermostat and sensor was change about 2 months back and all is good with temps. Occassionally driven once or twice a week no issues,

On Sunday took it for a 15 min drive and all was good, when vehicle was restarted it stated bogging down during acceleration, code was P0300 (Random misfire). Started it yesterday, 5 minute drive was ok then started bogging down again. There is also a rattling in the exhaust.
Today took all 6 plugs out and checked them; there was a bit of water in well #6, also water in boot between coil and boot. I have not finish putting everything back yet to try it again but I’m not sure this is the cause of the bogging down because the vehicle starts up and revs smooth, only after about 5 mins it starts to bog down but once it bogs down the revs are not smooth anymore.
Tomorrow after putting everything back together, I will know if it was the water in #6 well.
Anyone experience anything similar.
 

Sparky

Member
Dec 4, 2011
12,927
Mine started out ok then ran like crap when I had water in my #6 well.
 

Menthol

Member
Dec 8, 2011
177
Update.

Drove all afternoon, vehicle ran fine. It seem that, that bit of water in spark plug well #6 was the culprit. No, more misfire.
 

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