Power Loss, Bad Mpg, Weird Shifting

[shaunt23]

Hi All!
I have a 2003 Tahoe Z71 currently with 140k miles. Around 3 years ago at approximately 115k miles, the performance changed. (I have always used synthetic oil and 91 octane fuel) Every part of the car is factory stock (except for the muffler, flowmaster)

Car feels sluggish, likes is towing 5 tons or binding... (checked brakes, no binding, no issues)
MPG went from 14-18 down to 12 and is always at 11.9 or 12 now...
can still tow a boat, but not really on an incline and its too much for the car now...
wants to upshift very quickly
sometimes after a full stop, when you start again, it feels like the trans didn't go back to first gear and then kicks and starts...


NO CODES!!

since then I have repaired / replaced the following and still no change:

***everything OE***
spark plugs
spark plug wires
air filter
air mass flow sensor
coolant temp sensor
thermostat
fuel pressure regulator
fuel filter
battery
O2 sensors front and back
Cat and Muffler is completely clean
throttle body (improved response a bit)
injectors
rebuilt trans
put shift kit
rebuilt transfer case

I have taken it to the dealer and a private transmission shop to diagnose the trans, they say it runs fine... i think they are just checking the scanner and since no codes, they say its fine...
I had a compression test done, and the compression was excellent.
The only existing issue there is a small exhaust manifold leak.

any help or ideas with this???

 

[treehan77]

knock sensors?

 

[mizzouguy]

e-brake cable froze up?

 

[00YukonSLT5.3Rwd]

I am having same problem with my 00 Yukon 5.3. It's driving me crazy trying to find. Also I noticed a whirring sound in the engine but have not found a vacuum leak. I just started getting a code for the knock sensor. I bought both sensors, harness, and all gaskets for the intake since it has to be pulled in order to change them. Pulling a small trailer the truck kicks back the gears like I'm pulling a semi over 60mph. Also under hard acceleration it shifts weird. I don't know what's up with it, I replaced the cats thinking it was restricted. It went from running like a racer to the power of a Volvo overnight.

 

[mizzouguy]

fan clutch bad/always locked at full speed?

 

[shaunt23]

definitely no e-brake... I checked that its fine.

the knock sensors i am thinking might be the culprit!! can the knock sensors be bad and me not get codes? is that possible?

 

[shaunt23]

I am having same problem with my 00 Yukon 5.3. It's driving me crazy trying to find. Also I noticed a whirring sound in the engine but have not found a vacuum leak. I just started getting a code for the knock sensor. I bought both sensors, harness, and all gaskets for the intake since it has to be pulled in order to change them. Pulling a small trailer the truck kicks back the gears like I'm pulling a semi over 60mph. Also under hard acceleration it shifts weird. I don't know what's up with it, I replaced the cats thinking it was restricted. It went from running like a racer to the power of a Volvo overnight.

yup! overnight... that sucks... I miss the power that my car had... I am sure you do too....

 

[00YukonSLT5.3Rwd]

Well I just did the knock sensors, sensor harness, thermostat, and intake gaskets. Took it for a drive and it's better but not what I remember. It may just need to settle in because I disconnected the battery. The idle dropped so low I thought it was going to stall. The weird shifting seems to have cleared up. I think either the egr or air pump is going, it makes a weird noise. Also I'm wondering if the throttle body is going since it's fly by wire. I'm going to drive it and see what happens.

 

[00YukonSLT5.3Rwd]

definitely no e-brake... I checked that its fine.

the knock sensors i am thinking might be the culprit!! can the knock sensors be bad and me not get codes? is that possible?

I think that my intake gaskets were leaking causing some of my problem. The idle has really dropped since I replaced intake gaskets. My rear knock sensor boot was off and the sensor was totally corroded.

 

[2004DenaliXL]

My 2004 Yukon Denali XL had sensors go bad, it was traction control, or wheel speed sensors. When pulling out from a stop sign or stoplight when you would accelerate it would bog the motor down and downshift sometimes. After I had the sensors replaced it was fine.