2008 tahoe hybrid, 80k miles, 2wd.

i have no idea what this could be, sounds like its coming from the center by the front of the prop shaft.

under accel or decel you get a LOUD thumping almost like a tire has a bad flat spot on it. the harder the accel or decel, the louder it gets. on the hwy at cruise speed or cruising anywhere at constant speed there is no noise, its smooth and nice.

yesterday i got it to about 60 mph and was slowing down rapidly and i put it into neutral. the thumping sound immediately stopped.

any ideas what this could be ? there has also been some general wierdness with the transmissino lately, when you shift from park to drive, or reverse to drive the trans 1/100 times wont engage and it sometimes revs up to 2k. you have to put it back into park, wait a sec, then put it back into drive then everythings fine again.

dont know if this is all related or not. also, the other day i was at a traffic light in drive, stopped, engine running. light turned green, i took my foot off the brake and slowly applied the gas, the engine just revved up to about 2k but the truck didnt move a bit, i let it go back to idle, then reapplied the gas and off she went like nothing was wrong. very strange...

 

Is it like stalling when accelerating from stop?

 

No it doesnt stall, but if you hammer the go pedal, it will start to accelerate, then lag for 1/4 second or so, then accelerate like it should. There is definitely a dip in the acceleration right off idle for some reason. You almost need to be careful pulling out in front of people because the motor is very torquey and responsive, but that 1/4 sec lag is unexpected. It didnt use to do that...

I dont know if that is at all related to the accel/decel thumping, or the strange transmission activity lately...

 

I had the same issue and thought it was a common behavior of a failing hybrid battery or other hybrid components. Like you said it doesn't stall just sputter and if you press the gas it will accelerate? Because I had 183k miles, and it is a hybrid, the first thing to consider is the battery? It can actually be the fuel pump. You can bring it to the dealer and have them pull the codes. Mine turned out to be a failed fuel pump.

 

Check the auxiliary transmission pump control module and auxiliary transmission pump itself. This is a known issue with the 2008's and some early 2009's. Mine exhibited some bizarre behavior before I replaced the Module which disappeared after the module replacement.

 

thanks for tips. there is no CEL so no codes to pull.

I think the 2 issues are unrelated, 1 mechanical and 1 electrical.

Any idea on the thumping noise ? its definitely mechanical, i think i read somewhere there is a TSB on rear diff pinion bearings, but this noise is definitely coming from front middle, near where prop shaft exits the trans. it literally sounds like a flat spot on a tire.

The vehicle is under a warranty still (i bought extended warranty) so hopefully it will be covered. I like to have all my facts before getting the dealer involved though.

 

Have the dealer involve so it will covered earlier.

 

OK so closure here ... my Tahoe Hybrid spent 35 days at the dealer (Jim Ellis, Atlanta).

They finally traced the noise to the entire rear diff. They replaced everything (only the pumpkin case is original now). Noise gone. It was the pinion bearings in the rear diff had come apart allowing driveshaft to wobble and make a thudding noise. Jeez, 80K on the truck never towed anything and mostly driven by my wife.

Glad I got the extended warranty, it would of cost about $4K to do all that. They also replaced the left motor mount and took the pan off trans to look for metal shavings.

The clunky start stop was caused by a transmission Aux pump that was not operating properly, so they replaced that as well (another $1K). The tech explained that since this is a hybrid vehicle, in order to pressurize the transmission with fluid and engage the clutch, they had to use an external electric pump call the aux trans pump. He said it was not reacting fast enough once vehicle entered electric mode, and was therefore not engaging the transmission clutch properly. It took about 21 days to actually order the new pump from GM, as they have since changed to a different design and wiring harness in 2010 so they had to integrate the new style pump and wiring harness, plus change/update software.

All in all, glad I had the warranty. Truck is working fine now. No thudding noises, nice smooth accel/decel, good trans engagement.

I still do have a shaking idle in gas mode, its always done that and everytime I take it to dealer I ask them to diagnose and they always tell me nothing is wrong. But sometimes at idle in gas mode, the engine will shake like its about to die, then it smooths out again. Maybe I will take it back in just before warranty ends and have them give it 1 more diagnosis.

Hey another question - are all you guys also dealing with the cracked dashboard issue ? Mine is nearly cracked in 2 above the instrument cluster now. GM is denying any help on it with me. I'll need to remove the dash cover and either repair it and recover it with vinyl, or replace it altogether. Im looking for dash removal instructions...