GMC AT4 6.2L vs 3.0 diesel

Excellent info tiller! I may consider using it from now on.

Tiller my truck was almost empty so I filled up with 91. Holy smokes what a difference. The truck was unreal before but now I’ve awoken an absolute beast! I did notice a tiny bit of pinging before but only when I really stomped on it. All that is gone! Whoa I’m going to get into trouble!!!!

Did you have any of the issues that plagued the 8spd? Like not going into 8th gear? Just curious. I never had one myself.

Not that I was ever aware of. I still have the truck but my son drives it and if the boat is pulled, the 3.0 does the pulling now. Just got back from a 1000 mile each way trip to lake Erie and it averaged 13.9 for the trip pulling a 621. I have 3 friends that made the same trip with same boats but they have the 2500 diesel’s and they all averaged around 11-12. I will take it!

Thanks Red! You might have had one after they made the changes to get rid of that problem. It was actually an easy fix, just changing the dang fluid to a Honda fluid believe it or not.

kidfish wrote:

Do you guys with the 6.2L use the 91 octane gas? I thought that’s what you needed to use with that motor?

Thats all I have ever used. 109k on it now and the motor seems strong as ever. The Trans however does not operate like new.

My understanding is a trans flush and new super duper fluid makes a HUGE difference with the 8 speed tranny. Just what I’ve heard.

MX1825 wrote:

My understanding is a trans flush and new super duper fluid makes a HUGE difference with the 8 speed tranny. Just what I’ve heard.

A flush or just a gradual drain and fill?

LabDaddy1 wrote:

<div class=”d4p-bbt-quote-title”>MX1825 wrote:</div>
My understanding is a trans flush and new super duper fluid makes a HUGE difference with the 8 speed tranny. Just what I’ve heard.

A flush or just a gradual drain and fill?

On the 6.2 and 5.3, I would pull the pan to drain that fluid, add new fluid(slightly overfill) then pull the hard line that runs into the transcooler(it runs through the radiator), start vehicle up, put in gear to the pump with run and push most of the old fluid out. Once the fluid starts to “clear up”, shut down, refill and repeat until you’re happy with the results.

One thing, to pull the pan you need to either remove the y-pipe or you can also get creative to “move” it out of the way. There is a flex pipe that give you just enough room to pull the pan when you give the exhaust a little persuasion.

You can just stick with Dextron HP(8 Speed) or VI(6 Speed. Also, Amsoil makes a ATF that is HP equivalent. You’ll likely need about 10-14 Quarts to do the method I described above.

I would never flush a transmission. Drain it, replace filter and refill. A pain, yes, but you are not pushing metal and lord knows what else throughout the system

Yeah, flushing any sort of fluid in a motor is kind of foolish. I wouldn’t be worried so much about metal, if you have pieces large enough to clog up a passage in a auto tranny, then you have bigger problems. But you can dislodge debris/dirt/sludge. Even more so, the benefit is what? You don’t flush motor oil out of an engine to get every last drop, although I have seen some advertisements for this process. Every fluid can be just drained and call it good.

This thread reminded me I need to do an fluid change on my transmission before I head to Canada next month.