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Possible Trailblazer cat issue

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Jul 21, 2019, 8:44 AM

Post #1 of 3 (721 views)
Possible Trailblazer cat issue Sign In

I recently purchased a 2005 Trailblazer (4.2 l) with less than 80k miles from family. So I'm pretty familiar with the history. Unfortunately that history involves a misfire code last year and my dad taking it to the local guy he knew in town. He changed the spark plugs and cleared the code. It came back. So my dad took it back and the guy poured a quart of ATF in the gas tank, cleared the code again and told him to see if the code comes back. It did, of course and now it had been being driven with misfires for weeks. So I checked the code and it was a cylinder 3 misfiring. So for $60 and 10 minutes, I replaced the coil and all was well. Well, until recently when I started noticing power loss on highway inclines. Sometimes I can push it to the floor without it going any faster. Other times on incline, it just seems lacking in response/sluggish. Otherwise it seems to drive fine but I've also started noticing a low rumble coming from underneath the mid-rear of the vehicle. Not super loud but I've noticed it. No rattle but it does get louder and can be heard in the car when accelerating on the highway.

I'm limited in experience, knowledge and tools but have been tackling increasingly harder projects with my cars with success (not that any have been all that hard). I did the only test I had tools for and I'm not even sure I did i correctly but I checked inlet/oulet temp of the cat with an infrared gun. I drove the car around the block, then parked in my driveway and had my wife keep it a 3k rpms while I checked the temps. I got 500 inlet and 400 outlet. Checked it twice. From reading online, I understand this indicates a pretty obvious problem. But then why don't I have an engine light or code? I've been reading and watching a ton on this the last few days and it lead me down a path towards the cat and these temps seemed to confirm a problem. Does it sound like that's the case even without an engine light/code?

I researched the procedure for replacing the cat. It seems I have multiple options but I'm seeing some issue with each one.

I considered the lacquer thinner in the gas tank just as something cheap to try first but I'm figuring if the cat clogging is from the misfires, wouldn't it most likely be pretty baked/welded on? I'd love to remove it and see if I could clean it by soaking but from my research, it sounds like I'd need to cut the pipe or remove the transmission crossmember to get it out. Cutting the pipe means I can't put it back (or does it?) and removing the cross member makes me nervous, being a novice.

I see there are universal cats that say they work with my vehicle and not very expensive. I tried watching youtube videos on these installs but all the ones I could find skipped the actual installing moments. Is it really just as easy as cutting out the old cat and clamping this one in with a piece of make-up pipe?

Then there's the direct fit. But now I'm back to the issue of needing to remove the crossmember. This will also cost about 2.5x as much as the universal instal and approaching more than I want to spend on the vehicle. If I knew it would take care of the problem and I could accomplish it, then I'd do it. But neither is guaranteed. I'm also concerned about needing to remove the 15 year old flange bolts that are way up in an awkward place so I'm nervous about breaking one.

Thanks in advance for any insight/help anyone is willing to offer. I'm pretty set on attempting something, I just haven't decided what.

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Jul 21, 2019, 9:48 AM

Post #2 of 3 (711 views)
Re: Possible Trailblazer cat issue Sign In

There is a very simple solution to replacing a Cat.

Take the car to a specialty exhaust shop, they can cut out and weld in a new cat, usually cheaper than you can even buy the cat. That's what repair shops do now.

In the meantime there is another inexpensive test to determine if the cat is plugged.

Get an accurate vacuum gauge and attach it to a direct manifold vacuum port.

A good running engine will produce 18-20" of vacuum at idle. If you bring the RPM up to 2,000 and hold it steady, the vacuum should remain close to that. If you have a plugged cat, the vacuum will drop substantially at that RPM.

You may have to get the car good and hot first as that is when a plugged cat will be at it's worst.

Another way is to measure exhaust pressure at the upstream 0/2 sensor. Should stay under 1 PSI.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jul 21, 2019, 9:52 AM

Post #3 of 3 (709 views)
Re: Possible Trailblazer cat issue Sign In

OMG - Do people really do that stuff? Surprised it runs at all about now!
I doubt things are going to go well removing or loosening cat for back pressure check. Other way is vacuum - must "T" in a line that's actual manifold vacuum. If you can find the spot and do that it's old but real info.
Vacuum reading unless real high altitude should be about 18ish Hg at idle and be the same at 2,000 RPMs or there's restriction, no surprise with a quart of ATF in the tank that would kill almost anything.
Driven with a misfire noised and more? What do you want to do with this? Seems pretty well destroyed and tricks like lacquer thinner aren't some answer IDK where you got those ideas or think so just don't do that crap.
You really could use a REAL tech (find one) and have this diagnosed with your observations including the rusted flare what it's going to take or if this engine has had it already? Rust alone on assorted things can just make wild work of dumb stuff to do otherwise think hard about how hard you want to work on it to make right no fakes really making it right,


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