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Old 01-01-2013, 10:22 AM   #1
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losing speed on hills - diesel & fifth wheel

I recently took my new-to-me fifth wheel and truck on an interstate trip to get some experience towing. The fifth wheel is a 2007 Mountaineer, 36' and weighs ~12k lbs. The truck is a 2003 GMC Sierra 3500 dually with the 6.6L Duramax diesel. I was disappointed to find out that it was not uncommon to lose 10 mph (or even 15 mph) while going up sustained hills (starting from 65 mph). These were not huge hills or mountains, either. This was in east Georgia on I-20, if anyone is familiar with the area.

So, does this sound normal? I was surprised as everything I'd read seemed to indicate the 3500 diesel ought to be able to pull this load fine. I had the pedal floored on a lot of these hills and kept losing speed anyway. I was using the tow/haul mode. I was expecting a downshift from the transmission to compensate for the hill and hold my speed, but this didn't happen.

If this isn't normal, does anybody have suggestions on what to check or have looked at?

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Old 01-01-2013, 10:41 AM   #2
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Have you had your codes pulled on the OBD II port to see if all is well. Also, it depends on what percent grade the hills are.

Also, have you checked the actual weight of the trailer?

My 2012 F350 Ford diesel has no troubles with "hills".


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Old 01-01-2013, 10:47 AM   #3
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I too have lot's of questions...
Assuming there is nothing wrong with your dmax, I too would think it would do as well as my 6.7L ford...

What's the gearing /hp/ torque/ towing capacity of your TV ?
have stock tires/wheels ?

What rpms do you normally run at ?

I like to keep mine in manual 6th gear and let the turbo pull the smaller hills
but while my max tq starts @ 1600 rpms, on bigger 'hills'(> 3% for any lenght of time) I have to let it downshift to keep the rpms up above there or I too loose speed with my load...

I think you'll be ok, just learning how your truck likes to work will improve it !
good luck...
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Old 01-01-2013, 10:57 AM   #4
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You need to find out the RPM where you get maximum torque and keep engine in this power band. Also an exhaust temp gauge to monitor how hard the engine is working is good information to know. Pay less attention to speed and more to RPM, EGT and coolant temp. and your truck will last longer and perform better.

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Old 01-01-2013, 12:30 PM   #5
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BF is correct. I want my duramax, pulling10k+, below 1800-2000 RPM and the EGR below 1300 degrees. Accordingly, I will manually shift to a lower gear if need be. Typically though, Im on a fairly steep/long grade and not waiting for the allison to downshift. I don't want to chance lugging the engine.
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Old 01-01-2013, 12:52 PM   #6
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Change your fuel filter, preferably with an ac delco original equipment filter. This maybe the reason for your transmission not down shifting also, although I am not positive on this one. But a partially plugged fuel filter will cause these types of problems.
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Old 01-01-2013, 01:03 PM   #7
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gtmatt is this your first rig ?? running you rig with your foot on the floor is a sure way to gut the motor ... if you don't have a prometer have one put on and drive keeping it within temp ranges. first off rving is all about the enjoyment of the trip, not how fast can i get there. i don't care how much torque you have there will always be a hill that you lose speed on. that being said you can always have your power turned up.
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Old 01-01-2013, 01:17 PM   #8
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On that chevy 6.6 L drive train, Have your mechanic check the turbo output. We have numerous troubles with these engines on our fleet bucket trucks!
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Old 01-01-2013, 01:40 PM   #9
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All trucks will lose speed on hills. I have trucks with 625 HP and 2050 FPT. And they lose speed on hills and in high winds. Not a lot and it depends on load, wind and the steepness and length of the grade. And these thing have 18 speed auto tranys, and may sometimes lose 2 or 3 gears on a interstate grade. That why they have the extra slow lane on hills, for trucks and pickups pulling loads. It's just a matter of physics. There is probably nothing wrong with your truck. It's just a baby throw away diesel. (No piston liner engines are called throw away motors in the trade) I think all Power stroke engines are also. Not sure about the little cummins in the dodge pickups. Just drive it and let it do its thing. I don't even want to talk about my brand new MH with the Ford V10 and what it loses on hills.
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Old 01-01-2013, 02:19 PM   #10
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My rig is around 13,000 lbs and I set my cruise at 65 and it holds very well on all but the steepest grades. I only lost speed in 2 spots - one north of Asheville on I26 and another in West Virginia on a 5% grade. I never dropped below 60 though.

I think something must be wrong with your truck, or your trailer is alot heavier than you think.
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Old 01-01-2013, 03:42 PM   #11
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I'm almost thinking his trailer is heavier than he thinks. Let us know what you find out. I'd definitely weigh the trailer first, since that's the cheapest fix.
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Old 01-01-2013, 09:34 PM   #12
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Your truck's Duramax engine is rated at 300HP in stock form. Your Allison transmission is properly matched to the engine through the ECM of the transmission coupled with the ECM for the engine. Read that Allison FAQ link to become more familiar with how it operates. It takes about 100 shifts for the Allison ECM to learn how to shift according to your driving habits. Tow/Haul is designed to be used when the truck is at or over 80% of maximum load according to that FAQ page.
I second replacing the fuel filter. You may order OEM fuel filters from the internet for much less than you pay at your local auto parts store or a dealer. Another item to check is the rubber connectors in the air intake pipe. If one of these develop even a tiny leak you will have little power on hills. The LB7 engine (up to mid-2004) has a well-known injector problem. Check your engine oil frequently for fuel in the oil. It is a sign at least one injector has failed.
I wouldn't worry about aftermarket gauges on stock diesel pickup. The onboard computer has safeguards in it to prevent most common diesel problems. If the exhaust gas temperature reaches 1,350*F the computer will reduce rpms and power until the temperture drops to a safe level. You will notice your "gas pedal" is not connected to the engine, it only has wires leading to the computers, which control everything for the engine and transmission. Some speed reduction on hills is normal.
For a test, next time you are towing your 5er do not engage tow/haul and remember how your truck shifts for several miles. Then engage tow/haul, and without changing your driving habits, note what changes.
The main function of GM's tow/haul feature is to reduce or eliminate "hunting" for the right gear by the transmission, which is the main source of overheating an automatic transmission.
I suggest joining dieselplace.com , a forum dedicated to the GM diesel engines. No need to post questions at first, use the search feature to find what was previously posted about your problem/questions. You have a very reliable truck. The number one rule for diesels is Keep the fuel clean!
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Old 01-02-2013, 05:33 PM   #13
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Get a Ford.
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Old 01-02-2013, 05:47 PM   #14
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That truck is 10 years old, do youstill have 300 horses?

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