Transmission issues

[Parmy]

Hi guys, I'm new to the forum and should have come on here years ago. Here is my story, hopefully someone can help with my issue.

I have a 2007 GMC MAX, 1500 4x4 with a 6l and 4:10 gears.

When I bought the truck 4 years ago I was getting about 600-640kms to a tank of gas...winter came and mileage dropped to about 530kms....the service guy said this was normal. I also complained the when I pulled up to a stop, I would feel a clunk...almost like a delayed downshift into first. Summer came and mileage went back to the 600 range. The next winter came and mile ages went to 400-450 km range....I'm now at the 540 km range to a tank. I took it to a tranny shop and the mechanic seemed confused. He said it was shifting fine however he said it wasn't going into lockup...which explained my higher than normal cruise rpm...90km/h is about 1800 rpm....used to be 1600 ish. He watched the rpms jump up and down as we drove.....said it was going in and out of lockup. He was confused as to why it was not throwing a code for the converter not locking up. Any thoughts as to why it's not locking up.......the lack of gas mileage is killing me.

[gmtech4]

Slip yoke into rear of t case , remove and lube splines , common problem ,

 

I can't remember what grease use other than GMs

 

Someone here will chime in

[Parmy]

Just curious, why would that cause my fuel mileage to be so bad? I did read about that problem, but thought it was only for the clunking sound when putting it into gear.

[gmtech4]

Nope, just coming to a stop and sometime when taking off a weird jolt is felt

 

Nothing to do with milage , winter will change milage , not sure where u live using my cell phone writing this

[Chris]

Winter fuel destroys fuel mileage. Your range will be back normal with summer fuel.

[Parmy]

I know winter kills mileage, however mine has never come back after the second winter....I'm getting roughly 540 kms to a tank at best...sometimes less than 500. I live in Ontario and nothing has changed as far as the type of driving I do....mix of city and country driving. I should be getting 600 kms per tank...I'm getting winter mileage in the summer and it been like that for 2 years now. Could my torque converter be pooched?

[gmtech4]

if there is a slip ,,,there will be a code ,,,,,go see a tranny guy and have him take a peek

[txab]

if there is a slip ,,,there will be a code ,,,,,go see a tranny guy and have him take a peek

 

 

 

 

I took it to a tranny shop and the mechanic seemed confused. He said it was shifting fine however he said it wasn't going into lockup...which explained my higher than normal cruise rpm...90km/h is about 1800 rpm....used to be 1600 ish. He watched the rpms jump up and down as we drove.....said it was going in and out of lockup. He was confused as to why it was not throwing a code for the converter not locking up. Any thoughts as to why it's not locking up...

[efritz]

As for the slip yoke, I just use high temp lithium grease. As for the TC not locking, it may not throw an 1870 code as GM added an input shaft speed sensor to their trannies in 05. It would be a more specific TCC related code. You really just need a good tranny guy, not a chain like AAMCO look at it. What tranny do you have, either the 4l65e or the 6l80?

[gmtech4]

 

 

 

i meant slip of the TC

[Parmy]

Thanks for the help guys, I guess it's off to the tranny shop....and I hope it's a simple fix....I'll keep ya posted as to what it was.