My 04 GT with auto runs great, but for the past week I've noticed when accelerating moderately around town in (I think) third gear, around 25-40 mph, the car starts to rumble and feels almost like it's driving over rumble strips on the side of a freeway until I let off the gas or it shifts, any ideas?

77k miles, no codes, all fluid levels fine, stock tune, newish spark plugs

 

For the forum,do these automatics have a lock up torque converter? If so,sounds like it's not disengaging properly.
For the poster,have you ever serviced the transmission?- New fluid and filter?
Good motor and transmission mounts?

 

I would expect a P0743 code to be present if any problems with TCC. But you are likely hitting on the problem. It definitely sounds like a Torque converter problem.

OP I suggest you change fluid and filter,,, dont forget to remove the plug from the torque convert to get all that old fluid out as well. Ad a bottle of shudder stop,, its a friction modifier and it has been known to cure the shudder.

 

Yep. When I lunched the transmission in my 02 it sounded a lot like what you have going on. By all means, pick the low hanging fruit first. I did a flush and tried all of the 'miracles in a bottle' I could find but the upshot was, I need to rebuild it.

 

lol I agree, but anything is worth trying just so you dont have to pull the trans and rebuild.

This is why I try to avoid Automatics if at all possible. If I had the cash the Mark viii would get a 6 speed.

 

I'll try doing the filter and drain and fill, the fluid still looks nice and red but I guess that's not a good way to judge how good it is.

I'll have to just push it more around town, it only seems to do it when it lugs in third gear right at 25 mph, if it downshifts it doesn't do it, it's also fine on the highway even if o/d is turned off although maybe the converter is locked up at those speeds.

The only thing else it's done for the past two years is the whole car shudders bad when I completely let off the gas pedal when it's cold out, until the car really warms up, then it doesn't do it.

 

Double-check the spark plugs and make sure that none of them are loose. It is possible to get a slight misfire that won't trigger a code, and it can result in a shuddering just like what you're experiencing.

 

Camster, they do have a lock up 1:1, OP the overdrive light will flash if its the TCC solenoid or for that matter any of the solenoids/electrical components, that at least has been my experience, check ECU for codes.

77k on a 4R75W is nothing at all, with that being said IDK your driving habits