1st real tow, starts bad, ends good

[bluenote]

I towed our 5er (10K lbs.) up deer hunting a couple of times in the fall right after I bought my Silverado, but this memorial day weekend, Blumax had his chance to show me what he can do on a little bit longer trip, with a few bigger hills. On the trip up, I tried running in cruise, and was very disappointed with the constant downshifting. I can now see how some Duramax owners have been upset with this. At 2K rpm's, I don't see why the tranny needs to downshift and run the rpm's up to 3K, it killed my mileage. Once we were at the campground, I did remember that I hadn't done the 60mph to 1st gear thing that I wanted to do before we left. On the trip home, the cruise gave the same kind of performance, so I ran the throttle myself...BINGO! Only one downshift, and that was accelerating uphill to pass someone. The engine has all the power I need. Although the performance chips sound fun, I'd be even more interested in a product that would force the tranny to hold 5th gear until the rpm's drop below the powerband. Once I gave up on cruise, I found out I have a pulling beast, easily passing most other RV's on the hills, again, without downshifting. Give me a better TCM program...PLEASE! I'd love to have that same performance in cruise (yes, I have had a reflash).

[TBC]

There is a dealer fix for the busy shift problem in cruise and OD. The Service Bulletin number is 010730034. It takes about 30-45 minutes to do the job including check in if the dealer is accepting drive in appointments for the fix. I had mine done. It has been a big topic on this and other sites. I have towed for a short distance one time since and didn't experience any problem. I will know for sure starting Friday as I head for CO and AZ.

[LarryM]

bluenote:

Amen to that! This is the first vehicle I have owned that gets better fuel economy with the cruise control OFF!

Even using the foot feed, it's impossible to maintain 1800 RPM (torque peak) on the 6% grades, towing, because the thing downshifts well before you get to max throttle. I just had the March release software flashed in last week. Wednsday I'll pull the trailer north (1500 miles) again and see how this one does. I don't expect any improvement, I hope I'm wrong.

[RealBigTruck]

I don't think your downshift issue is a real problem. I have a ZF-6 tranny and I pull about 8500# enclosed trailer. Going up steep grades at 2,000rpm and full diesel pedal, I see only 12-13 psi of boost pre-intercooler while the truck slows down. If I downshift to 5th and go full pedal, the truck accelerates up the same hill and I see boost pressures of 20psi.

Several people said this is normal from this truck. Apparently the engine defuels at lower rpms, thus not generating enough heat to spin the turbo to full boost. If this truck had full boost at 2,000rmp, there wouldn't be any downshifting issues.

[pullinpower]

GM are you listening......we need more fuel and full boost at 1800 rpm to hold 5th gear before downshift occurs. Wish we had that button jk was talking about to "hold" any gear we choose.The downshifting will drive ya crazy,I used the coaxing method(downshift manualy to D back to OD quickly)and it helped kick it into 5th sooner.We shouldnt be going threw this with the almighty Allison tranny ;) :D

[01_Duramax_Dually]

Just got back from another tow to Las Vegas, I never use cruise going up any hills towing but do use on flat land. Noticed what you guys are saying about the lack of gear holding. I have to play tricks on mine and then can get it to hold. Hate the deep drop and the 3K RPM deal but that does not happen frequently.
I like 4th gear going up steep grades. Pulls excellent and actually builds speed. 3rd just shows alot of RPM and power but never goes fast.

All in All. I am still 100% satisfied with my truck. I can tow up 6% grades at 60MPH with a 10K trailer. A friend towed behind me this weekend to the race. I had to leave him for awhile and then pull over and wait. His truck struggled to hold 20mph and he had his car on a open trailer weighing all of about 5K.

Carry On.... :D

On Edit, My friends truck was not a Duramax. Did not want to bash other manufacturers, was only using as a reference to support how really powerful the DMAX is...

[ 05-28-2002: Message edited by: 01_Duramax_Dually ]</p>

[Kennedy]

Mine will boost quite solidly at 1800 RPM and above. With a power box, it will boost MUCH better. Key is AIRFLOW. You only get so much fuel during power enrichment, and if it is not sufficient to gain airflow, you won't get any more fuel.

As for GM making changes, I think the EPA has them by the short hairs on this one.

A bit more aggressive turbo :D could also help this...

[bluenote]

I did have the computer flashed back in October, shortly after buying the truck. Would the March release update make any difference? Have they truly improved on the previous programming? Question for Kennedy...I have the stock air filter and box, so give me your recommendations on what would be the most help with airflow?? I pull anywhere from 2K to 4K miles in the summer, and the rest is as a daily driver, unloaded (if that info would be helpful). Thanks!

[Jackmchome]

I suspect you did not get the TCM flash you need. After having the re-program done you will find the truck will loose approx 5mph before it downshifts. This to allow the turbo to build a littel boost. As to up-shifting in cruise, it will not do it itself until 60 to 62mph. Any sooner and is is not in it's high touque range.
My $0.02

[zrone]

While towing my 5th wheel 14,000lbs I set the cruise at about 35mph and then drove "manually". This allowed the speed to drop almosst 10mph before downshifting. It worked better in the hills then any other way I drove it. I also had all the updates.

[dmaxstu]

I still drive PT after retiring from a 40yr driving career. Some of the buses I drive are 45'MCI with series 60 Detroits and Alison 6 speed transmisions ALL of these buses (fleet of 30) shift down from overdrive 6th gear as soon as they go up any hill.
When we drove our Dmax I put it in cruise and TowHaul and drove to the Mexican border from the SF bay area. It shifted out of overdrive quite a bit further up the hils than the buses I mentioned.At all times did the RPM stay where it should be.If you guys want to pull long grades in overdrive with a diesel engine I think you will be dissapointed and soon ruin your engines.
The very reason 18 wheelers have 16 gears is to keep those RPMs up. To a professional driver it feels real good when a rig shifts down and holds its speed or nearly so, it feels even better when you don't have to double clutch it to have it happen. I think your fuel consumption will suffer more when you try to lug it up the hill in overdrive. Now if you don't want any shifting to occur change you diff. to a 4.10. Just my opinion. Stu

[Kennedy]

And the winner is... dmaxstu!

SoMn and I were sharing the exact same notion, that a 4.10 ratio would be very nice when pushing it in the hills. Then a 6th gear for the non towing miles...

[dmaxstu]

John
A friend of mine who drives a PSD bought such a large 5th wheel that he was having to weigh everything he loaded into it. He ended up installing a gear vendors under/ overdrive and a 4.30 differential I think and ended up with something like what you mentioned. Talking about having to change gears often . He sure ended up with a lot of gears. I think I would prefer a factory option of an Alison 6 speed. But it can be achieved with after market mods. Stu