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2004 chevy trailblazer_ls
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Discussion Starter · #1 ·
Hey guys, I need some expert opinions on what has proven to be a hard issue to resolve. I bought a 2004 Trailblazer 4.2L about 2 months ago. About a week or so after I owned it, it started to seem quite sluggish to me, and it also shifted hard, especially in the lower gears. Then the check engine light went on, which ended up being the thermostat stuck open. I had it replaced, and since then the engine light hasn't come back on, but the truck continues to feel "sluggish", and it also shifts hard, and sort of lunges from one gear to the next. To try and fix the problem, I replaced the air filter, had a new fuel filter put in, changed the spark plugs with AC Delco iridiums, cleaned the throttle body, cleaned the electric terminals and grounds for the battery, bought a new battery, and I also use injector cleaner with every gas fill-up. Everything I just listed seemed to help give the truck slightly more pep, and the hard shifting would lighten, but it is still very much there. I took the truck to a mechanic for an "engine performance test", and he said that everything looked good, and he did nothing but reflash the PCM because I asked him to do that as well. This still did not help, but since he said it was OK, I took the vehicle on a 3 hour trip for a vacation. This is when the transmission went out, and I had a local transmission shop rebuild the hold thing, including the torque converter. After I got the truck back, the shifting lunge is still present, and the truck still seems sluggish to me. For instance, when I'm on the highway, my truck has a hard time accelerating from say 50 to 60 mph. When I say hard time accelerating, I mean I step on the gas and it seems like nothing is happening. I can feel more gas being injected, but the RPMs barely go up, and it barely goes faster. In addition to the sluggishness, it seems like the shift points are sometimes at odd places as well. The only other thing I can think of is that the CAT converter is bad, but I'm skeptic to that idea since my truck passed its emissions test while all of these problems were going on. Does anyone have any idea as to what the problem may be???
 

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2003 gmc envoy_slt
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:tiphat Welcome :tiphat

Sounds like it could be a plugged catalytic converter.

The shift points wouldn't be the same because you're pushing the pedal down more than you would if the speed was where it would be if the cat was good. Hope that makes sense.

You can remove the upstream O2 sensor and take it for a drive. See if it runs better.

Or take it to a mechanic and have a pressure gauge hooked up to test for excess back pressure.

As for the emissions test, as long as there are no CEL's or pending codes, it will pass.
 

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Discussion Starter · #3 ·
Thanks for the reply wooluf. I'll test the cat this coming tuesday or wednesday. Unfortunately I work every day until then. I guess I kind of ruled out the cat after it passed the emissions test, but what you said makes sense since the truck isn't throwing any codes. I'll post the outcome after I get it tested.
 

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Discussion Starter · #4 ·
Well, I took my TB in to a mechanic this morning, and he spent all day looking at it, but when I told him I wanted the back pressure tested to see if the CAT was bad, he said that it couldn't be the CAT since there were no codes being thrown by the vehicle, and that the truck wouldn't go over 10-15 mph if the CAT was indeed plugged. He took the truck for a drive, and said he couldn't feel anything too much out of the ordinary. For this reason, he said he wouldn't do a back pressure test since it would be a waste of his time and my money.
I drive this truck every day, and I know that it is running sluggish and shifting harder and more of a lunging motion than normal. I think it might be time to find a new mechanic. Is what he told me about the CAT converter true, or should I take it to a different mechanic somewhere else to get the back pressure tested? Any ideas?
 

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A cat can be partially plugged, and our platform, throw no codes. He doesn't know our platform as much as we do, and he doesn't know as well as you do how it should drive. :hopeless
 

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2004 chevy trailblazer_lt
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Time for a new mechanic. Any good muffler shop should be able to do a simple backpressure test. Your mechanic is old-school, and the way they used to test them was to drill a hole in the exhaust pipe upstream of the cat, screw in a fitting to test it and then weld up the hole.

However, the 21st century just called and your mechanic was sleeping. It is a simple process. (I bought my own test tool and it cost me $100. It is only a standard pressure gauge that reads 0 to 20 PSI instead of the usual 0 to 200 PSI.)

The cat is tested by unscrewing the front oxygen sensor - which is easily accessed on our platforms - and replacing it with the test gauge. It is tested at idle and at 2500 RPM. Pressure should read 0 PSI at idle and 3 PSI at 2500. I would be willing to bet yours is upwards of 2 or 3 PSI at idle and 6 to 8 at 2500 RPM, based on your symptoms.

Yes, it is almost definitely your cat that is plugged. The thing to realize is that a partially plugged cat will not throw a code because even though the flow is restricted, it still cleans the exhaust sufficiently to not throw a code. There is no code for a plugged cat; only for a poorly performing cat.

Some shops test the cat by measuring the temperature because plugged cats will be almost red hot, but again this is not definitive because they can be quite plugged, affecting performance, and not be so hot it is detectable yet.

Trust us; when you get a new cat on this truck and step on it at 60 MPH ... the acceleration will blow you away. (These are fast trucks, and there is a reason why a lot of police departments use them for undercover and surveillance units, even though they are not a police-package vehicle. They are fast, roomy, handle well and are ubiquitously nondescript. Don't tell anyone I told you this.)
 

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Time for a new mechanic. Any good muffler shop should be able to do a simple backpressure test. Your mechanic is old-school, and the way they used to test them was to drill a hole in the exhaust pipe upstream of the cat, screw in a fitting to test it and then weld up the hole.

However, the 21st century just called and your mechanic was sleeping. It is a simple process. (I bought my own test tool and it cost me $100. It is only a standard pressure gauge that reads 0 to 20 PSI instead of the usual 0 to 200 PSI.)

The cat is tested by unscrewing the front oxygen sensor - which is easily accessed on our platforms - and replacing it with the test gauge. It is tested at idle and at 2500 RPM. Pressure should read 0 PSI at idle and 3 PSI at 2500. I would be willing to bet yours is upwards of 2 or 3 PSI at idle and 6 to 8 at 2500 RPM, based on your symptoms.

Yes, it is almost definitely your cat that is plugged. The thing to realize is that a partially plugged cat will not throw a code because even though the flow is restricted, it still cleans the exhaust sufficiently to not throw a code. There is no code for a plugged cat; only for a poorly performing cat.

Some shops test the cat by measuring the temperature because plugged cats will be almost red hot, but again this is not definitive because they can be quite plugged, affecting performance, and not be so hot it is detectable yet.

Trust us; when you get a new cat on this truck and step on it at 60 MPH ... the acceleration will blow you away. (These are fast trucks, and there is a reason why a lot of police departments use them for undercover and surveillance units, even though they are not a police-package vehicle. They are fast, roomy, handle well and are ubiquitously nondescript. Don't tell anyone I told you this.)
I've had similar issues with mine. I took it to a shop and had the CAT tested by the old drill a hole method, does that work at all or is it just not the right way? What mileage do our CAT's usually begin to restrict flow? I'm at 103k and my truck has always ran pretty sluggish compared to my buddy's '04, his will blow mine away in performance and mileage alike.
 

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A cat should last the lifetime of the vehicle if the truck is maintained properly.

What shortens the life of the cat is a rich mixture. Today's cats, as opposed to ones made even 10 years ago, is that they run on a VERY narrow range of stoichiometric combustion ratios. These reflect the higher emissions standards of today. What older cats could shake off will plug a more modern design in a hurry.

This is usually related to a rich mixture from running a vehicle too long with a bad thermostat. This will plug a cat in a few months. Also, running a vehicle with a misfire indication - a flashing check engine light - can destroy a cat in only minutes or hours.

The absolutely most common problem on our trucks that is plugging more and more cats as they age is a thermostat not closing properly. This is indicated by the temperature gauge being slightly left of straight up and down when normal.

This is why we advise folks to fix their thermostat/coolant temp sensor first, then look at getting their cat tested.

Yes, you can do it the old school way and drill and hole and reweld it ... but why. It is easy to pop out the front oxygen sensor and plug in the tester. If they don't do it this way, then they either don't have the proper $8 oxygen sensor socket or the knowledge to know how to do this. Either way, I would consider a new mechanic.

There is no way your truck should feel sluggish. But keep in mind there is no way any of us in here can tell you how you maintained your truck, how much you might have driven it with a bad thermostat or how long it was driven with a flashing check engine light.

So, bottom line: at 100k, you should be changing the plugs (use ONLY AC Delco 41-103 plugs) and the front oxygen sensor. (Use ONLY AC Delco sensors.) You should also check your thermostat. If your gauge is even one or two ticks to the left while running warm, it probably needs replacing.

Once you have done this, have your cat tested. Let us know the pressure readings.

A good cat should run 0 PSI at idle and 3 or 4 PSI at 2500 RPM.

(You know, the funny thing is I have said this many times: "test your cat and let us know the readings." No one ever does. This can be frustrating when you give someone what the readings are on a good cat but no one ever follows your suggestions. We want to increase the information in our own database to help future members!)

Trust me; this topic will come up more and more in the next few years as these vehicles age.
 

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Discussion Starter · #9 ·
I really appreciate everyone's insight so far, it has been extremely helpful. And just to let everyone know, I already called a new mechanic, and he said that a back pressure test should be no problem at all. I will definitely post the pressure readings on this forum once I get the results, most likely early next week.
As far as my '04 TB goes, I forgot to mention that it has about 88K miles on it. I bought the truck only 2 1/2 months ago, and only one week after being owned a light went on, which ended up being the thermostat. After what chickenhawk posted it makes perfect sense that the CAT is most likely plugged due to the thermostat being stuck open. As for maintenance of the truck by the previous owner, I have no clue...I wish there was a way to find out what the past owner did, or didn't, do to maintain the trucks condition.
 

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2003 gmc envoy_slt
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I also use injector cleaner with every gas fill-up. QUOTE]


Just re-read your first post. Injector cleaner every fill-up is a waste of money, IMHO.

A bottle of Chevron Techron at every oil change is more than enough.

Also, if you're worried about past maintenance, assume it wasn't done.

Especially the transfer case fluid change, due every 50k miles.:m2:
 

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2005 gmc envoy_sle_xl
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These are fast trucks, and there is a reason why a lot of police departments use them for undercover and surveillance units
Interesting to note that I can't smoke the tires in my XL at all. They will chirp a little though. However in my 2007 GMC Canyon with the Atlas I5 at 235HP I could smoke em all I wanted. Granted the Canyon weighs 1500lbs less but... I dunno. Do you see any issues here?
 

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LOL! Well, the ability to smoke tires will indicate much faster wear on the rear tires, but not much else.

Basically, it means not much weight is on the tires during acceleration. This is why lots of pickup trucks can smoke their tires, but that doesn't make them "fast" trucks.

A spinning tire is not at its optimal traction capability for obvious reasons, and this is why police pursuit training spends a lot of time talking about the concepts of contact patch, weight transfer and traction circles. The idea is to optimize traction, not artificially destroy it by smoking the rears.

(But I should point out that some police agencies use Trailblazers as undercover vehicles because they are fast and ubiquitous, but they do not use them as marked or unmarked vehicles. They are not tested or rated as a police-pursuit vehicle, and police agencies would not allow one to engage in pursuits. On the other hand, I drove the 2011 Dodge Charger Police Enforcer on the track a few months ago ... and it was a huge improvement over the previous version. You are going to see a lot of these vehicles patrolling the streets in the next few years!)
 

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Discussion Starter · #14 ·
I finally got in to the mechanic yesterday (a different shop), and this guy actually tested the back pressure like I asked him to. To my surprise, it had a pressure of 0 psi at idle, and jumped up to almost 3 psi from idle to 2500 rpm, then leveled back to around 1 psi when held at 2500 rpm. He then took the truck for a drive with a diagnostic computer plugged in to the PCM, and nothing looked out of whack. However, he did say that the truck did seem somewhat restricted, but he didn't know what else to check. So he actually did the back pressure test again, and same results. After that, he said the only other thing he could possibly think of was something gummed up in the fuel system somewhere. He told me to pour a can of Seafoam in the fuel tank and see if that clears anything up.
I'm not totally convinced that it's a problem in the fuel system, but something is definitely causing my truck to be sluggish. What else could it possibly be? I'm stumped...
 

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88k maybe a weak Fuel pump or weak ignition? Have you had the plug gaps checked? An engine is a big air pump sucks it in pumps it out its either air or fuel. Are you using 5w30? Check all fluids and filled correctly? Is the parking brake dragging or stuck caliper? The higher the rpm's the harder these engines push you in your seat.
 

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It might be some kind of driveline restriction. You said the trans was rebuilt after it was sluggish so I would rule that out. Maybe something in the t-case or rear end?
 

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Discussion Starter · #18 ·
I just took the truck on a 2 1/2 hour trip up north to go deer hunting, and the sluggishness doesn't feel like it's coming from the rear of the truck, however I shouldn't rule it out. If it could possibly be something in the transfer case, what do you suggest I do, just a fluid change?
Another thing to note is that the truck got what seemed to me to be poor gas mileage. I wasn't towing anything, but I did have a decent amount of gear in the truck, but it still only got a little over 16 mpg, most of the driving being on the freeway. (normally I get about 17 1/2 mpg)
Also, when I was driving on the freeway, the truck was difficult to keep at a speed of around 70+ mph, and I really had to hammer on her to get her up to speed. It just seemed like it was working very hard at the high speed, and whenever I would let off the gas or slow down a tiny bit, the truck would shift to a lower gear and ride constant around 1500 rpm. It just doesn't seem normal to me because I would think the truck would ride at around 2000-2200 rpm at high speeds. Basically, I think the truck was up-and-down shifting to frequently on the freeway, just didn't seem natural to me...
 

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well ask your mechanic if he has a welder and ask him to take out the 02 sensor or cut the cat and test drive it and if it is not the cat he can just weld it back and rule that out!:undecided
 

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Discussion Starter · #20 ·
Well, it's been a while since the last post, and I apologize for that, but I have some new news about my truck that I think some of you may be interested in. About 2 months ago, I had a new CAT installed (thank goodness for tax returns :D), which unfortunately did not fix the sluggish problem. I also replaced the CPAS, which didn't seem to help the problem at all either.
I did recently get a service engine light, so I took it in again to get diagnostic tested. It ended up throwing codes for P0014 and P0016. I took the truck to the local Chevy dealer, and they seem to think they found the problem. They found a lag from the internal phase actuator gear, and they also said it's possible that the timing chains may have stretched.
Now I know that both of these are expensive fixes, as I was quoted $752 for a new phase gear, and around $2000 for timing chain replacement. Do these prices seem fair, or should I be able to get a better price (and trust the work) at a different mechanic?
I really appreciate everyone's input and patience! :hail: This is a great site, and I hope my truck's problems can help others with similar problems in the future!!
:thx
 
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