I have an 09 4.6 3V SCREW and tow a 21' boat on a tandem axel trailer. Approximate all in weight of boat, motor and trailer is 4,200 lbs. I added Air Lift air bags to the rear suspension to stabilize the rear end when towing (great addition BTW).

I recently towed the boat from Ontario to Florida and found one very annoying problem. At highway speeds (65 to 70 mph) on the freeway and when in "Tow/Haul" mode, the transmission hunts constantly between 4th and 5th gears on flat runs. Even with the cruise control on, the tranny will slip into 5th nicely and then the rig starts to slow down and the drops to 4th to get the speed back up.

Is there a way around this through driving technique or some mod such as a tuning device? Your advice is very welcome. I want this to be corrected before I return home to Canada in March.

 

Not really an easy fix to this. If you had the 5.4 it maybe wouldn't hunt for gears so much. I would advise not using the cruise.

 

Thanks. I did find that it was better not to use cruise. I could hold it in 5th for a longer period, but eventually the speed drops off and the tranny goes into 4th to regain the speed limit. I know the 5.4 would have been a better choice for this application, but I had the truck before the boat. The engine seems to have lots of power to tow it, and at speeds above or below freeway speed, it stays in the proper gear. However, I don't like the idea of driving at 60 mph or 80 mph on Hwy 95.

 

The closer you get to 2k rpm and above the better it will pull. When I tow I have found that 75-80 works great and on only inclines like bridges and hills does it downshift. Granted, I have the 5.4 but yours still has the same power range/sweet spot. 2k rpm is the key.

 

What rear-end gears are in your truck now? You may try swaping to 3:73's if you don't have them already. It may help some.

 

what if you take it out of tow haul mode at cruising speeds? not sure what this would do

 

Do you think playing with the shift points with a chip would help?

 

That's what I would do. If the road ahead is relatively flat and perhaps cruise control is engaged, I would take off the tow/haul mode and let the truck gear up/RPM down. Once you see hills and such, re-engage the tow/haul and let the tranny do its thing.

Watch the tranny temp guage. Tranny heat comes from a lot of shifting.

Oh, perhaps 75 MPH might be a little too fasy with a 4,200 lb load..

 

That will help some also. Tow-haul mode just holds the lower gear for a longer time and may cause it to shift more. I would still NOT use the cruise too.

I pull a trailer 7by14 6,000lbs, 6 days a wk for my business. I have tryed many combo's to see which works best for me. So your not speaking to someone that just does this once in awhile.

 

you need to find your powerband and stay in it