Jump to content
  • Sign up for FREE! Become a GM-Trucks.com Member Today!

    In 20 seconds you can become part of the worlds largest and oldest community discussing General Motors, Chevrolet and GMC branded pickups, crossovers, and SUVs. From buying research to owner support, join 1.5 MILLION GM Truck Enthusiasts every month who use GM-Trucks.com as a daily part of their ownership experience. 

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

Recommended Posts

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!


Link to comment
Share on other sites

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 by newdude
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.





Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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? 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.