Picking Up 2008 2500HD Sierra w. 6.0L - Need Advice On Maintenance & Performance

Drew Weidner · November 21, 2019

Hello,

I will soon be picking up a very-nice 2008 GMC Sierra 2500HD with a 6.0L V8 and 6L90E transmission. The truck seems to be very-well maintained and it only has 72,000 miles on it. I keep my cars/trucks for a very long time (my current ride is a 96 Suburban with 206,000 miles), so I want to sure I get any deferred maintenance done asap. I also would like to make some sensible performance upgrades (I'm 52yo and looking for something civilized, but maybe with upgraded power, stopping or ride quality).

The transmission fluid has never been changed. It has 72k on it now, so I would like to do at least do a fluid & filter change. I bought my Suburban with 120K on it and the fluid had never been changed. The advice for my 4L60E was to drop the pan and install new filter and refill fluid that was drained -- then do that again in about 10,000 miles. The opinion was that changing the old fluid gradually, rather than doing a 100% fluid exchange all at once was better for the transmission. That seems to have worked out well for me, but I know this tranny is different. If it were yours, would you just drop pan, install new filter and refill, or would you change filter and do a 100% fluid exchange? Also, I used Valvoline MaxLife trans. fluid in the Suburban (that's what people recommended for the 4L60E), do these 6L90Es like MaxLife or would I be better going with a full-synthetic Dexron VI?

Transfer case fluid has been changed regularly - once a year. I think I'l leave that 'till next year (it was done about 7,000 miles ago), but just wondering if I should replace OEM fluid with something better to increase the life of my transfer case?

Differential fluid has never been changed, so I think it's time to do it now. Any recommendations?

Everything on the truck is bone-stock. It is a one-owner truck - owned since new by a man that is now in his 70s. Even though it is a HD truck, it won't really haul or tow heavy loads. It will primarily be my daily driver - occasionally hauling some furniture, a load of mulch, some firewood. It will get relatively light use and will travel mostly unloaded. Do you guys have recommendations for mods that will make the truck ride smoother, get better fuel economy or maybe a little bit better performance without being anything crazy?

Like I said, I keep my vehicles a long time. I'll probably keep this for 7-10 years. Any advice for maximizing longevity? Also - any common problems with these trucks I should keep on the lookout for?

Thanks!

casey.mikl · November 21, 2019

Change the trans fluid and filter. The transfer case takes the same fluid as the trans. Dextron 6. The 6L90 is a great transmission. I got 100k out of mine before it needed to be rebuilt. The torque converter went bad and sent metal throughout the transmission. Change the diff fluids with the correct weight gear oil and regular oil changes. I used 5-30 Mobil 1 full synthetic. I go off of the trucks oil life in the cluster. I drive mine maybe 5k a year and almost every time it’s hauling a load. The 6.0 is a great motor. Makes great power. My truck has a K&N Intake (was on it when I bought it) plus side is the cleanable filter. A tuner would help out a little with power and fuel economy. If it is an extended cab 2500HD (not sure if they had it in 2008 but they did in 2011) the center link for the steering will have a rubber bushing in the pitman arm. You can get a non isolated center link and pitman arm off a 3500hd and it will bolt right in. Mine had it and the bushing was shot causing a lot of play. The non isolated model has more feel in the steering wheel. But no bushing to go bad.

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casey.mikl · November 21, 2019

I would probably not do a flush, but I would drain it all out and add new fluid. When I did mine before I had the trans rebuilt. I disconnected 1 side of the cooler (passenger side) and put a pipe over it. Started the truck and pumped the old out and added new in. Probably not the best to do. But it worked for me.

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Formulabruce · November 21, 2019

KEEP the original Trans pan Gasket. Its rubber, with metal inside. The one that comes with a filter will be a folded

piece of thin gasket material. Re-Use the original gasket!

Colossus · November 21, 2019

You are safe to flush it. And I recommend it. As long as someone is doing it the right way. if it had 200,000 miles and never serviced then I'd probably say avoid flushing it and do a drain and fill but since it has been maintained and has low miles, flushing IS SAFE.

Drew Weidner · November 21, 2019

2 hours ago, casey.mikl said:

Change the trans fluid and filter. The transfer case takes the same fluid as the trans. Dextron 6. The 6L90 is a great transmission. I got 100k out of mine before it needed to be rebuilt. The torque converter went bad and sent metal throughout the transmission. Change the diff fluids with the correct weight gear oil and regular oil changes. I used 5-30 Mobil 1 full synthetic. I go off of the trucks oil life in the cluster. I drive mine maybe 5k a year and almost every time it’s hauling a load. The 6.0 is a great motor. Makes great power. My truck has a K&N Intake (was on it when I bought it) plus side is the cleanable filter. A tuner would help out a little with power and fuel economy. If it is an extended cab 2500HD (not sure if they had it in 2008 but they did in 2011) the center link for the steering will have a rubber bushing in the pitman arm. You can get a non isolated center link and pitman arm off a 3500hd and it will bolt right in. Mine had it and the bushing was shot causing a lot of play. The non isolated model has more feel in the steering wheel. But no bushing to go bad.

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It's a Crew Cab with a 8-foot bed.

100k miles doesn't seem like a lot of miles on a transmission before needing to be rebuilt. That's what I'm trying to avoid.

My 96 Suburban had 125K on it when I bought it. It has 206k on it now. I change the fluid and filter every year, or so. I think I've done it three or four times. Tranny shifts a little rought sometimes, but is still relatively smooth.

I've owned a bunch of Toyotas in the past that have gone over 300K on the original trans.

What is the typical life-expectancy of the 6L90E trans?

CamGTP · November 24, 2019

Hard to give you a decent answer. Anywhere from 125-200k is pretty normal. Seen several with far more miles than that on a original transmission. It's the luck of the draw sometimes with as many of these trucks on the road.

We just lost a transmission on one our work box trucks. 2015-2016ish GMC van chassis with a cube box on it, made it all the way to 150k miles before it let go. It was loaded every single day of it's life when it left every morning. Probably 9,000-11,000+ lbs, got 10mpg no matter what too.

As far as gaining performance and mpg, it's not going to change a lot without spending a decent chunk of money for the power part. I've been tuning trucks/cars for a few years now and gains are hard to come by without adding/changing parts.

A tune on the stock engine can help make it respond better to your foot, give it a better get up and go but the power gain won't be a lot. It may feel/respond faster but we aren't adding 30hp type of thing. A lot of the tuning can help the transmission if the person knows their way around the 6 speed calibration.

MPG gain is going to come down to far more factors than a good tune. Those really can't gain you any magic fuel mileage. Driving style, vehicle maintenance and tires play a role in mpg too. In the end it's a 7,000+ lbs truck with a 6.0 V8 engine, getting mid teens is about what you can expect.