Thump noise when upshifting after set amount of time??

[Iggydrake]

Hello everyone,

I'm new here....I've owned a 2004 Pontiac Montana and it's been my best most reliable vehicle.
I take good care of it, and it has taken us across Canada a few times. Has 250k kms (155k miles) or so on it. My good friend who sold it to me also loves Montanas, running them up to 400k+ before buying yet another and doing the same.

I just took a trip from out west to Toronto area. 3400kms (2115 miles) bout 5 days of 7h highway driving....but since I've been here my van has had its first complaint I've ever had in the 5 times I've gone from one end to the other...

It thumps when (automatic transmission) upshifting gears as I accelerate. Once it seemed to be clunking pretty hard.
What is odd, is that it seems to go back to perfectly smooth shifting once it's rested for some time, then of late it starts after driving for about 45mins to 1hr. It only does this when gears change during acceleration, never when downshifting as I slow down/stop

I have to get 3400kms across Trans Canada highway to get back home in 2 months and I've been up nights worrying about this....there's no other way to get home, and I need to bring stuff home with me.

Might anyone know what the issue may be? Could it be engine mounts even?
If never thumps when shifting from park to drive to reverse or any of that, only in the one case. Diagnostic Code scanner reports no problems at all.
Should I try new trans fluid and filter?
There are no other symptoms or any noises or nothing....I was told by one person fluid/filter change could cause problems?

Would this possibly be fixable? Can I make it home?

Thanks for reading, and any help would be much appreciated!

 

[melsg5]

You cannt check a transmission for codes using a normal OBD scanner.
Is there a reason you havent taken it to a shop for the issue as opposed to having people on the internet guess?
Have you had the fluid level checked?
As long as you dont Flush the transmission there is no issue with changing the fluid and filter and adding a bottle of Lucas transmission treatment.
The transmission solenoids may be sticking causing the hard shifts.

 

[Iggydrake]

You cannt check a transmission for codes using a normal OBD scanner.
Is there a reason you havent taken it to a shop for the issue as opposed to having people on the internet guess?
Have you had the fluid level checked?
As long as you dont Flush the transmission there is no issue with changing the fluid and filter and adding a bottle of Lucas transmission treatment.
The transmission solenoids may be sticking causing the hard shifts.

I haven't had it checked because right now I can't afford to. Not for another week so I thought I could ask in meantime and see what anyone who may be skilled with vehicles would have to say. Like see, already it doesn't sound like total doom, so I can least sleep at night. But I do know of someone in this small town who works on transmissions, will see if they will agree to look at it and I will try replacing fluid and filter along with that Lucas stuff (didn't know they had that)
and see if that helps.

I heard people say when transmission is going out it makes who it makes whining and such noises but this van acts completely normal, makes the clunking then goes totally quiet again.

I'll ask them to check solenoids specifically as well