HELP! 99 GMC 3500 Dually 7.4 struggling to tow on mountain passes

[chevytruc1658]

Hi Everyone, first post on here but have been a long time owner of numerous c/k chevy/gmc trucks. I currently own a '99 GMC K3500 crew cab dually pick-up truck with a 7.4 454 vortec engine with 160k miles. I recently purchased this truck just to tow my 7,100 lb travel trailer, not a daily driver. When I purchased it, I put a good amount of money into it, most of the things i did was not necessarily because it needed it but more because I wanted peace of mind knowing I had a truck that I could rely on. The truck drove and ran fine but did have a hard start when cold. I replaced the fuel pressure regulator but figured while I was under the plenum I might as well change out the notoriously troublesome injectors with new Bosh injectors. After that was done the hard start was fixed and the truck ran excellent. I then tested the truck by towing it up a nearby pass to see how it would perform and it severely struggled to go over 30 mph and shifted very hard into gear. I took the truck to the local Chevy dealership and they came back and said that the acceleration issue was a result of a bad fuel pump. I had that replaced and the hard shifting was gone!, and the truck ran even better. I then took it on another test tow and the acceleration was a lot better towing uphill in 3rd however it still struggles to stay above 45 and really has a hard time. So heres the deal, I totally get this is a 20+ year old truck and I am not expecting to do 70 up a mountain pass but do hope I can maintain 55-60 and not feel like we are going to slow down to a near stop when towing up a steep incline. I have been on a camping trip the last two weeks and the truck has been performing great aside from climbing the passe. What I am looking to get out of this post is if there is anyone who has experienced the same issue and what did you discover or upgrade to get the truck performing at a higher level. I really don't want to get rid of it and don't mind doing some minor upgrades like exhaust, headers, ECM upgrade but just don't want to throw money at it for very little to no gain. I have a attached a pic on the rig. HELP!

[Brian Lindsey]

you might have the wrong gearing in differential for towing in mountains

Edited July 20, 2020 by Brian Lindsey

[newdude]

I'd crack open the glovebox and look at the RPO label.  Head in to the G and or H section and you should see one of the following codes there:

 

GT4 - 3.73 gears

GT5 - 4.10 gears 

HC4 - 4.56 gears

 

Also, how heavy is that trailer?  Dry or the GVWR of it is fine, whichever is easier to find.  

 

 

Edited July 20, 2020 by newdude

[chevytruc1658]

Thanks for the reply so far! 

 

I looked at the codes and I have a GT5(4:10) 

 

GVWR 7600K and Unloaded weight is 6K

[CamGTP]

Well, it was 4.10's. I say that because those look to be oversized tires compared to stock. So it did reduce your gear ratio by some with those larger tires. Towing performance will go down because of that.

 

Without a scanner hooked up and possible a back pressure sensor on there, I can't really see what it's doing and if there is a possible exhaust restriction before the cat converter.

[chevytruc1658]

Thanks. Yea I purchased with those tires and I had a feeling that would be an issue. Do you recommend going to a 4:56? Or re programing the shift points? 

[CamGTP]

Going to 4.56's would help because of the tires and the added weight they bring but that is a ton of money unless you are doing the work yourself.

 

It would be half the cost or more to go with smaller/lighter tires and sell the oversized tires to make some money back and make it even cheaper.

 

Tuning it can help but hard to say how much of an improvement it will make.

 

 

 

 

[Super 8]

265 vs 245 tires, brings your gear ratio 410 down to 373.

[diyer2]

Colorado?

[aseibel]

Jeeze, you got a 1 ton with a 7.4 liter and struggle to pull a 7,000 lb TT.

Don't show this to the guys with new half ton trucks pulling the same size trailer.

 

I'd second the idea to go back to OEM size highway tires. What you have on there now isn't helping you on the pavement.

[chevytruc1658]

Yea tell me about it. Stock tires say 225 75 16 and I have 235 80 16 on it. 

Edited July 22, 2020 by chevytruc1658

[Super 8]

This will show you the difference between the tire sizes;

https://www.tacomaworld.com/tirecalc?tires=225-75r16-235-80r16

[CamGTP]

The picture make them look bigger than 235/80's that's for sure but if they are mud tires that are really heavy that is still hurting things some.

[chevytruc1658]

Yea that’s all they are but they are very big treaded mud tires. 

[chevytruc1658]

Hey everyone, one other question. I got a line on a set of 6 very nice Alcoa 225 75 16 tires (stock size) but they came off of a 2007 Ford E450. They are 16” and 8x6.5 bolt pattern which is the same as the Chevy. I know the hubs on Fords are larger then Dodge and Chevy, and these hub openings measure 4 7/8” Which are in fact larger then Chevy so they should fit right?