Very repeatable issue. Car is a 99 LeSabre with the 3.8, 72k miles. Great car, might need shocks but otherwise great car. But have a problem that has risen within the past 1k miles or so.


I can be going in 4th and TC locked up; 45-50MPH/less than 1500RPMs, come to a hill/incline to put the car under load and it "shudders", if feels like a VERY Hard TC lockup or lockup and unlock real fast but the RPMS do not change, it will do it all the way up the 'hill' or until I give it more throttle and it comes out of lockup. I can repeat this scenario easily and just about tell when it is going to start shuddering.


The SES light has not come on.


What to do first?

 

I think Ive heard of this.

Two possibilities I can think of.

Does sound like the TCC valve is leaking ... In that case its called a CHUGGLE.

Check out the symptoms on research of CHUGGLE.



I think there are prom changes that help during shifts with but its not a fix just a work around.



If it is it might help to change the transmission fluid.. and maybe a
additive. The valve fattens up a bit with the additive.
Elsewise a trannie workover..



The other possibility (but your throttle mention disuades this) is just
a plug giving it up and misfiring somehow. Under high cylinder
pressure the resistance increases across the spark plug and the spark
finds another route like a wire to ground or inside a coil pack.





Good Luck.. hope this gives you some ideas.