6sigma

Just to offer a ray of hope, my 70-75MPH vibration has been fixed and after 5000 miles is still vibration free. My fix was two steps; torque factory U bolts to 80 ft-lbs, and then balance tires. The U bolts resolved about 80% of my vibration and the tire balance resolved the remainder. I know there seems to be different causes of the vibration, but I can report my truck is now very smooth, so at least some cases can be fixed. My truck is a 15 double cab, 4x4, z71 with 20 inch wheels.

My 70-80MPH vibration is eliminated now, after I tightened the factory U bolts on the leaf springs to 80 ft-lbs and the dealer did a road force balance. The U bolts made about 75% improvement and the rebalance took care of the remainder. Now it is smooth and vibration free. I've put about 500 miles on it and it seems to be holding up. Fingers crossed it doesn't come back.

I tightened my stock U bolts on the leaf springs to 80 ft-lbs. This significantly reduced the vibration I felt at 70-80 MPH. There is still some vibration near 80 MPH but it seems to have smoothed out the 70-75 MPH vibration. At 80MPH it is significantly better. The U bolts seemed to be around 40-50 ft-lbs factory. Also, now the vibration seems to come and go whereas before it was constant at those speeds. I still have the mild vibration at 45mph when decelerating. I suggest tightening the factory U bolts as a first step for anybody with vibration issues at 70-80mph.

Does changing to aftermarket wheels help? Does it happen with the smaller 18" wheels? Does it happen on both 4x4 and 2wd trucks? Does it happen on all wheelbases - reg cab, DC, crew, crew long bed? thanks

So, I'm trying to get up to date on this issue. I've scanned this thread but there are 547 pages! Can I ask some simple questions, which might be helpful for others trying to gather info on current status. 1. Is the vibration resolved by changing to some higher quality tires? 2. Do the new 2016 trucks have the same problem? 3. GM has not been successful in tracking down the source of the vibration, in most cases? thanks

It has FHO but does not have FHS. This is a 2015 Sierra DC, 4x4, SLE with the 5.3, if that matters.

I'm running E85 in my 2015 5.3 without the FFV option. It runs it EXACTLY as I have experienced in previous FFV trucks and as others have reported on the 14+ FFV 5.3s. It is more responsive, smoother, shifts better, and seems to have more power. It is getting approx 20% less MPG, which means it is adjusting fuel injection for the increased ethanol content.

Yeah, and I'm on the second tank. It is probably more like E60 here in the winter, but still makes me wonder how it is adjusting.

I just read your thread on your E85 conversion, wow that is impressive! I did not know there is a separate fuel sensor (didn't see that documented in the parts diagrams I found). I may rethink running a lot of E85, although I gotta say it seems to run it just fine and the truck feels so much better I may do it anyway! It even shifts better. I wonder if it could be using the O2 sensors or some other way of measuring ethanol? My truck seems to be adjusting to E85 just as I have experienced and would expect from a FlexFuel engine. The injectors and fuel pumps are rated for higher flow, and the computer is commanding more fuel flow as it should.

I have a 2015 Sierra 5.3 that does not have the $100 FlexFuel E85 option. I've had FlexFuel trucks before this, and like the option to run E85, but this truck does not have the option. All 2014 5.3s are E85 capable, and the 2015s have it as a standalone $100 option. I did some research on what exactly the $100 option gets you, and I'm pretty sure it is just the yellow colored gas cap. According to GMpowertrain.com, and all parts information I can find, there is NO difference between the E85 and non-E85 5.3. GM Powertrain says the 5.3 is E85 compatible, period. So, I decided to run a couple tanks of E85 to see what happens. Turns out, it runs E85 exactly as I would expect and have seen from my previous FlexFuel trucks. It cranks a little longer on cold start, as others have reported. It get less MPG, maybe 20% less. It idles smoother and feels much more responsive than on 87 octane gas. I had to pull out into 50mph traffic from a stop so I hit the gas pretty hard, the truck smoked both rears and stepped sideways a little bit. It is not that responsive on 87 gas. I also noticed that the truck seems to hold higher gears more, rather than downshift early. I think on 87 octane gas it will downshift much sooner as an anti-knock protection, to prevent a load on the engine at lower RPM. If I would breath on the throttle with 87 gas it would want to downshift. This makes sense, but it also tells me the engine computer is sensing and adapting to E85. It also runs in V4 mode more often and longer on E85, another sign the engine adapts to E85. It is also injecting more fuel with E85 (as is expected with a flexfuel engine), so it must be measuring ethanol content. I'm convinced the $100 E85 option is nothing more than the yellow gas cap and a way for GM to offer a choice. I wonder if there is some advantage to GM in CAFE or fuel economy calculations by having E85 as a customer option, versus a factory default. I see a very noticeable difference in smoothness and responsiveness on E85, and I like how it holds a gear longer. I think I'll try a couple tanks of 93 octane next then 89 octane, to see if there is any difference.