Transmission advice/insight needed

[dna59]

Ok guys let's see if you all can help me out...that is if my issue is even a problem. From taking ownership of this vehicle to me my '05 Envoy 6cyl seems lazy from takeoff. Now I don't know if it's just how these vehicles drive but it's not something I'd expect. I could also almost swear that at some very rare times the vehicle does not have this issue. I don't think I'm getting crazy. I know one other member here described pretty much the same thing I'm experiencing. Nothing jumps out as being wrong with the vehicle. Below I will list everything I've done relating to the vehcile below and see what you guys think. Thanks.

Engine replaced with a lower mileage unit (original engine was blown)
Transmission replaced with a lower mileage unit (no difference)
Torque converter replaced with a brand new ACDelco until (no difference)
Transmission filter and fluid replaced (helped shift points)
Brake pads replaced and calipers serviced (no noticeable difference)
Tires kept at correct PSI (no noticeable difference)
Throttle body cleaned multiple times (no noticeable difference)
Spark plugs replaced twice with ACDelco Iridium (no noticeable difference)
PCMofNC tune (helped a little)
Removed 20" SS repos (helped a little)
Switched to Shell gas as this seems to be the best locally (only mpg seems to be affected)

*I do not keep unnecessary added weight and 98% of the time I'm the only occupant of the vehicle*

 

[Mooseman]

When you had the tune done by PCMofNC, did you have torque management turned off?

And what gearing does your truck have? I know my '02 and the '07 are both dogs with the 3.42.

 

[Chickenhawk]

Where is the needle in the coolant temp gauge when it is fully warm? Center, left of center or right of center?

 

[dna59]

@Mooseman yes the torque management was turned off and yes it's the 3.42. @Chickenhawk it's always at pretty much center. If I read it through the OBDII it would be around the 190-200 range.

 

[Mooseman]

How's the cat? Done a backpressure test or pull the O2 sensor? Checking the fuel trims would be a good idea. What about other maintenance?

 

[dna59]

@Mooseman no I haven't checked the cat. Could there me something wrong with it despite not getting fowl smells? I can have a look at fuel trims. When the best time to check them? Once the vehicle has reached operating temp? Beside what has been listed I have only really touched the cooling system by replacing everything. I have no issues with the engine fan staying on high. I had to replace two fans because of this and that surely affected take off until I put in the Behr unit

 

[Mooseman]

If it ever ran at a low temp for a while, it could have plugged the cat. Smells, present or not, are not necessarily indicative of its proper function. Mine was partially plugged pretty badly without a CEL or smell.

Fuel trims you should check after warmup at idle and while driving (not wide open throttle). Ideally, they shouldn't go higher or lower than +10/-10.

 

[dna59]

@Mooseman I checked the fule trims and they are well within the range you suggested. You'd advise I check my cat? Do our trucks have any sensors on or after the cat or is it something that can be done to without any ill consequences?

 

[Mooseman]

A quick and dirty check would be to pull the upstream O2 sensor and see if the power improves. Best way would be a backpressure test.

 

[dna59]

Might have to do the quick and dirty way. Don't know anyone that has the equipment required for a back pressure test and the places that probably have it will want a million dollars to do it.

 

[Chickenhawk]

Most muffler places will do them for free (if you buy the new cat off them.)

Personally, I would just cut off the wire to the sensor and use a deep socket. It will be in there good. We recommend a new O2 sensor with every plug change, if for nothing more than to prevent it freezing solidly in the manifold. You will need the special socket to install the sensor, but if you can get a muffler shop to do it cheap, ask for a backpressure test and get them to install the new one instead of the old one. It has to come out anyway to do the test.

By the way, with our exhaust system, there should be very little backpressure. It should read near 0 PSI at idle and only 1 or 2 PSI at 2500 RPM.

 

[dna59]

If I ask a muffler shop here to do that they'll look at me like I'm crazy. Their idea of a back pressure test it putting their hand on the end of the tail pipe to "feel" the pressure and check for leaks.

 

[Chickenhawk]

That's why I bought my own tester. Not cheap, but with two Trailblazers in the family and as they age, the cat will start to clog almost inevitably some day.
https://www.otctools.com/products/exhaust-back-pressure-gauge