# Gmc Denali Struggling



## RVdogs (May 5, 2010)

Looking for some advice/insight here. We have a 2010 250RS (fully loaded weighs 6980#) pulled by a 2005 GMC Denali 6.0L with towing package: [email protected], 375 [email protected], tow rating 8200#. We load the TV with approximately 500# including passengers. For the most part, the Denali tows great without a hiccup. However there are a few destinations on our annual itinerary which have sustained 7%-12%+ grades that really tax the TV. My trans temp is right on the borderline of the red zone and I have to put it into either L2 or even L1 at times just to keep the vehicle moving.

Am I expecting to much from my TV or is this just a matter of driving smart and stopping occasionally to allow the TV to cool down?


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## duggy (Mar 17, 2010)

I also have the 6.0, so I know it's plenty strong for the 250RS. My first thought, before I got to it in your post, was to gear down and give the truck a chance. On seeing that you do that, I'm guessing you just need to give it a rest if things are too steep. I'm assuming you already have a tranny cooler with the towing package.


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## duggy (Mar 17, 2010)

Another thought; I assume you use the tow/haul mode. A friend of mine didn't like to use his, because it killed gas mileage. I reminded him gas was cheaper than a new transmission.


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## Nathan (Jan 2, 2007)

IMO, a sustained 12% grade is going to tax any vehicle loaded near it's limit. I am admittedly at my limits, and when I hit a sustained 8% grade, I was content that I maintained 25mph.....








Of course if you are at altitude, that will also reduce your towing performance. Now, if your trans is overtemping, you might want to think of a larger or secondary cooler.

If you want to pull it up the mountain with minimal drama, the solution is a bigger truck, preferrably with a Diesel. (Doubling your torque _WILL_ help!)








However, if you'd rather avoid that big payment, you might be content with your performance. Please remember however, that all component lives will be reduced when you push them to their max. Make the decision that makes you happy.


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## outback loft (Sep 22, 2008)

The 6.0 has the power for that trailer, but when you hit steep grades any vehicle is going to slow down. I have the 6.0 Vortec max and I am pushing about 420 hp and 450 ft.lb of torque. By the specs you are giving I think that you actually have the Vortec Max motor and it shouldn't be too bad. The only thing is I have the Allison 5 speed tranny and 4.10 gears.

The diesel will slow down less on a grade but it will still happen. The only difference I have seen when pulling my trailer with my gasser over my 7.3 f250 is when I hit a grade.


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## Duanesz (Jul 10, 2009)

Maybe just swap gears if you like the Denali. Get a bigger trans cooler and you will be fine. What percentage of towing do you do on steep grades? Any truck will be running pretty hard on a 12% grade.


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## rdvholtwood (Sep 18, 2008)

We have a 2006 Dodge RAM 5.7l Hemi with 3.73 gears pulling a 250RS and it struggles up the hills - especially from a complete stop. For the most part; however, on the highway the TV does great. When I hit the steep grades, I just let the truck take its time going up the hill.


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## RVdogs (May 5, 2010)

Thanks for the feedback, everyone. I'm quick to put it into a lower gear to reduce strain on the motor. I also use Tow/Haul mode on everything except open flat highway. However I appreciate you mentioning it as you never know who your responding to.







I really like the Denali so I'd prefer to keep it. It sounds like it's performing as one should expect. The 250RS is our first travel trailer and my first experience towing this much weight. I've been towing for years but only half the weight so this was new to me. Fortunately only three of our 8-9 annual camping trips have anything more than a 4-6% grade so it's not a common occurrence.

If it really starts to be a problem I'll consider the bigger/secondary trans cooler. I'll also quit stressing about it and just take my time up the mountain.


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## rsm7 (Aug 8, 2009)

RVdogs said:


> I also use Tow/Haul mode on everything except open flat highway.


You may want to review your owners manual on towing. If I remember correctly you should never tow with a GM trasnsmission in OD. Maybe a 3/4 ton but never a 1/2 ton. The temps get too high and it will actually boil your fluid right out the dipstick and fry your tranny. This was the case with the older 4 spd. Perhaps its different with the new 6 spd. I dont know. In any event I might just leave it in tow/haul all the time, depending on what the manual says.


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## Justman (Jul 22, 2006)

Like Nathan said, go with a transmission cooler and consider a lower back end gear ratio. Most Denalis, Yukons, Suburbans prior to 2008 (or so) were manufactured with 3.73 rear gears. You'll want to go with 4.10. The 4.10 will cost more in gas over the long run due to reduced gas mileage, but they should get you up the hill a little better and help save your transmission.

My Yukon XL came with a very small transmission cooler. One of the first mods I did when I got the truck was to install a larger transmission cooler---one more than twice the size of the stock cooler. I don't have temp numbers to show how much it helped, but with a 5.3 engine and 3.73 gears I didn't have any temp issues going through the hills of S. Illinois a few years ago. Transmission coolers are passive, so the cost is pretty low given the benefits they provide. Believe the one I got was called a "Tru Cool Max" or something of that nature. You'll want to be sure and place the new cooler in about the same position as the old one---in the primary air flow in front of the radiator. Don't block it by putting it behind something else.


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## duggy (Mar 17, 2010)

RVdogs said:


> I also use Tow/Haul mode on everything except open flat highway.


I would recommend using the tow/haul mode ALL the time. Even on flat highway you would be saving the transmission due to higher line pressure, unlocked torque converter, and proper shift points for the load. You may think none of that matters on flat highway, but you will still have changes in speed, and other changes on the towing strain, which will tax the transmission in normal mode. You also may hit some hills, and forget to push the button for a while. Simpler to make it a habit to use tow/haul whenever the trailer is connected.


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## wolverine (Mar 5, 2009)

I always use my tow/haul mode when towing my trailer and I leave it overdrive like the manual says. If I was towing in the mountains I would down shift as needed.

My cousin who lives in Colorado added another trany cooler that runs along the frame rail to keep his 2500 HD trany temp down.


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## Scoutr2 (Aug 21, 2006)

Justman said:


> Like Nathan said, go with a transmission cooler and consider a lower back end gear ratio. Most Denalis, Yukons, Suburbans prior to 2008 (or so) were manufactured with 3.73 rear gears. You'll want to go with 4.10. The 4.10 will cost more in gas over the long run due to reduced gas mileage, but they should get you up the hill a little better and help save your transmission.
> 
> My Yukon XL came with a very small transmission cooler. One of the first mods I did when I got the truck was to install a larger transmission cooler---one more than twice the size of the stock cooler. I don't have temp numbers to show how much it helped, but with a 5.3 engine and 3.73 gears I didn't have any temp issues going through the hills of S. Illinois a few years ago. Transmission coolers are passive, so the cost is pretty low given the benefits they provide. Believe the one I got was called a "Tru Cool Max" or something of that nature. You'll want to be sure and place the new cooler in about the same position as the old one---in the primary air flow in front of the radiator. Don't block it by putting it behind something else.


Justman is correct abot the rear axle ratio. I would bet, from your description, that you have the standard 3:73 rear end. While it helps with gas mileage, it causes your engine to struggle under an unusually heavy load. The 4:10 axle ratio in your rear end would help you out a lot in towing, but cause your engine to be thirstier whether towing or not. You must weigh your options and decide if you want to spend the money to replace the rear end (which could cost $1000+), or stick with your 3:73 gearing, which keeps you driving slow up steep grades, but saves you gas money year round. And remember, this is more than just a simple mechanical swap-out. Your engine ECM must be reprogrammed for the new axle ratio, because shifting points will change - the same engine rpm will get you slower speeds. The speedo will not read correctly either, but I'm not sure if that is still a simple speedo cable gear replacement or whether it is an electronic fix, like the engine/transmission shifting points. In any case, I'd have a dealership do the work, just to be sure there are no problems with powertrain warranty down the road.

Also - does your vehicle have a torque converter lockup clutch? My 3/4-ton with the 6.0L does, and when I hit a long grade up, the clutch is the first thing to unlock, putting the vehicle in torque-converter drive (as opposed to straight mechanical drive). When the clutch unlocks, I see the engine rpm jump up a couple hundred. If that is not enough torque to maintain speed, the transmission then downshifts.

I have been in the Black Hills and other hilly or mountainous areas, and occasionally, I have to accept the fact that with a gasoline engine, I'm just going to have to cruise uphill at about 45-50 mph. It is the nature of the beast.

Mike


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## RVdogs (May 5, 2010)

Thanks for the details, Scoutr2. Nice to know my options with my tranny if I decide to go that direction. AFA the torque converter lockup clutch, I have no idea.


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## OutbackPM (Sep 14, 2005)

ditto on what the other have said and I would add one thing for extra margin. Synthetic oil can withstand higher temperatures 1) without loosing its properties and 2) Its life will be less effected as well so will last at these conditions.

If you baddly over heat the oil once it can be severely degraded without you knowing it but systheitc will not be effeted.

Good luck and take your time when needed.


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## duggy (Mar 17, 2010)

I believe GM automatics leave the torque converter unlocked all the time when in Tow/Haul mode.

If you're thinking about changing the rear end ratio, I'd recommend just driving in a lower gear (drive instead of overdrive if it's a 4-speed). That way you have good gearing for hauling, but don't pay the penalty in gas mileage the rest of the time, plus it doesn't require making big changes to the truck.

I'm not sure if GM had changed to Dexron VI in 2005, or if that happened in 2006. I understand that it is fully compatible with older GM transmissions, plus gives firmer shifts and has less breakdown due to high temperatures.


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