2011 Sierra 1500 SLT what would you do?

[kyleag89]

I have a 2011 Sierra with 210k miles on it now and is in excellent physical condition inside and out with the exception of rust just beginning to form above the rear passenger wheel well.  Mechanically it's been better than most vehicles I've owned until recently the transmission started flaring shifts sometimes but not always and it seems to have lost power.  At the same time this happened I noticed my oil pan started leaking around 3/4 of the perimeter!  I just bought this truck a little over 2 years ago with 150k on it and spent $13,500 on it cash.  I really can't afford to have the transmission rebuilt without going into personal reasons just know it's not an option in the time frame I need it done.  I am a competent mechanic but have never torn a trans down only remove and replace.  I'm looking for suggestions on what you guys would do in my situation?

 

I'm seriously considering attempting the trans rebuild myself.  I know the clutches will need replaced and seems to be the problem with it.  

 

Edit: Truck has the 5.3 and 6180 with towing package and heavy duty cooling 

Edited July 2, 2025 by kyleag89

[Jworks]

The rust issue:  If you live above the "salt line" you can likely expect more of this.  By "salt line" I mean the northern states that use salt on the roads a lot in the winter.  If you live south of the salt line the question is did this truck come from those northern states (and how long it was up there) before you bought it.  A carfax report would probably tell you the history of where this truck has been.  If its from the north I'd probably sell it.  Or....you could ask a local body shop [one that will give you a straight and honest answer]  for their recommendation just to see if there is something they recommend.  I have a 2011 also with no rust issues.  Bought it used but it was in Alabama its whole life. 

 

Oil pan :  It probably needs a new gasket, not a big expense here.

 

Transmission:  I'm not sure what you mean by flaring.  Any kind of shifting issue needs to be addressed.  This will likely be expensive, as you know.  Has the transmission been serviced (changing fluid) ?  I change mine every 30k miles regardless of anything, which has been the standard as long as I know.  You bought it used, so there's no way of knowing if the prior owner did this.  I bought mine from a dealer and the trans fluid was bright pink so I knew they'd just changed it before putting it on the lot to sell.  Try to find a COMPETENT independent transmission guy and tell him the issue.  Expensive?  Yes, even with an independent trans guy.  But not as much as at a dealer.  I use a local guy -- small ratty-looking shop, but does really good work and uses good materials.  But it still costs. 

 

Maybe consider selling it or trading it in.  There are some people on here that know a lot more than me.  Hopefully one of them will give you more professional advice.                                                                         

[kyleag89]

I am always hesitant to change transmission fluid on a vehicle with this many miles.  I've always heard that it's a bad idea due to clutch friction material being in the fluid and changing it can cause the trans to slip.  I actually had an 89 firebird formula and after changing the fluid the transmission got worse.  Coincidence? Maybe but Im not willing to take that risk.  Besides the fluid in it now doesn't look or smell bad.  It's not fresh pink but a clean red color and smells new.

 

As for the rust I live on the PA/MD line and regularly drive in both states.  The area of PA I drive in uses silt on the roads but MD uses salt and a liquid chemical they pre treat the roads with before a storm is coming.  I think my rust issues are just a matter of luck because the manufacturer uses a spray in foam between the outter bed side and inner wheel arch for "panel sound deadening"!  It's literally a big sponge that holds moisture and ticking time bomb for rust to begin.  The rust I see is coming from the inside out and different from past vehicles rust on the same area.  It starts as a bright orange color spot showing on the paint and turns into a bubble and pushes the paint off.

[kyleag89]

It looks like something that can be wiped or washed off when it first started but it cannot be.

Edited July 2, 2025 by kyleag89

[newdude]

1 hour ago, kyleag89 said:

It looks like something that can be wiped or washed off when it first started but it cannot be.

 

 

That is definitely from the inside out I bet.  You can see the bubbling in the paint.  The inner box wheel well is probably pretty rough too or getting there.  Happens in the area between the outer box skin and the top of the wheel well.  Everything gets trapped in there and should be flushed out a couple times a year.  

Edited July 2, 2025 by newdude

[kyleag89]

Surprisingly the inner wheel well is unaffected so far(from the outside anyway) but I'm sure the inside is all rusted and it will start showing soon.  If it wasn't for the oil leak and the transmission shifting weird I would fix it but I'm not too worried about that now.  To fix it and not have to worry about it again in 1 or 2 yrs requires cutting the metal out and welding in new patch panels which isn't something I'm set up to do anymore.(Nor want to I hate body work!)

 

The transmission is what concerns me and I don't know if it's ever been worked on or rebuilt before.  Of I had to guess I'd say no and that it's 100% original.  I've always had very slow shifts in this truck since I got it which may be normal for it unfortunately.  I have never driven another vehicle with the 6L80 in it to compare.  What I mean by slow shifts is that while accelerating at average speed sometimes it won't shift until I let off the gas or it does shift but very late and high in the RPMs.  Also sometimes it will flair, I don't know how else to describe it other than a flair.  It's almost like a slip but finally catches itself and continues driving.  It also will downshift strange sometimes as well.  I notice it mostly in traffic when just letting off the brake and coasting enough to hit 2nd gear then hitting the brake again.  I'll get a clunky downshift.  I've owned multiple GM vehicles and trucks but they all had either a 700r4 or 4l60 in them.  None of them shifted as poorly as this 2011 does.

Edited July 2, 2025 by kyleag89

[newdude]

If its physically "excellent" otherwise as you say (no frame rot or cracks as these are known to do), I'd be dropping in a rebuilt transmission with a warranty, especially if the one in there is all original.  

 

Or, trade it or sell it and get something clean and southern.  

[kyleag89]

Where are the frames known to crack on the NNBS trucks?  Other than the wheel arches starting to rust the rest of the truck is rust free.  The entire undercarriage was coated with a thick layer of some kind of greasy undercoating which protected it well.  I don't know what it is and can only describe it as spray undercoating mixed with grease of some kind.  Makes working underneath a very messy job.

[kyleag89]

From the symptoms I described about the transmission does that sound like it could be electrical or mechanical issues?  I don't know anything about these newer electronic controlled transmissions.  I miss the days of the old 700r4 and AW-4 transmissions in my jeeps.  I put a B&M shift kit in my 91 Camaro RS when I was 18 in a few hours.  Swapped a SBC 350 into the same car when I was 17 and it was the first time I've ever done anything like that.  Did it alone borrowing tools I didn't have from a neighbor.

Edited July 3, 2025 by kyleag89

[swathdiver]

If yours were mine I'd change the transmission fluid.  If it was shifting funny, I'd put my scan tool on it and see if the TECHM needs rebuilding.  This you can do on your kitchen table from under the pan for about $150 plus fluids.

 

As for the rust, you can replace the whole box, get new side panels welded in, etc.  GM still makes the pieces that makes up the bed.  Pull your taillight and shine a light in there to see what's up. 

 

The 6L80 is far more enduring than our old 700R4s and takes less power to turn too.  I know, in some ways they were simpler with the TV cable, but these are pretty awesome and last a long time when run on the severe service schedule.