03 Buick trans sometimes slams 2 to 5 mph

[geo buick]

Hi friends. I've looked at prior threads but could not find a problem similar to mine. My 03 Buick with 3.1 will sometimes slam like someone hit me in the rear when starting off from a stop. It does not slam when shifting into drive or reverse or upshift or have any other problems what so ever. If I am really light on the gas or really heavy I will not notice it. But on normal starts 50% of the time it will slam. It will happen after it starts rolling but before 5 mph. Could this be something simple like the modulator or big like the torque converter?

[gpse3400]

Seems like the unit is going neutral, then reengaging uncontrolled. The forward servo could be at fault. once you raise the pressure in the system, the forward servo reapplies.... That is on the back side of the transmission, not easy to get to... That is my guess if everything else is alright.

[geo buick]

Thank your gpse3400. The raising pressure thing makes a lot of sense. Is the servo on the back something accessible externally on the trans? A trans guy suggested to me that it might be an axle shaft, but it seems to me that it would have to be very worn to have that much play. Since the car starts rolling normally immediately upon taking my foot off the brake the axle has pressure on in in the forward direction and it should not be buckling any more. If there was that much wear I should probably notice other symptoms such as a clunk going from N to D or R but everything else is normal. I agree with you that it seems to be pressure related and very light or very heavy acceleration hides the pressure issue.

[gpse3400]

Yeah the servo is acessable, though I'm not sure I wil say tis easy, you might have to drop the back of the frame and keep the engine/ transmission suspeneded to get at it. It has 3 screws holding it in. One way to further verify servo fault is to do a manual select to 1st and see how it moves from a stop. If it does not slip/bang then I would definately start thining of checking the servo. When you select manual 1st the servo gets line pressure which is more than enough to hold the forward roller clutch.

[Azrael]

Check to make sure all the sensor connections are good on the motor. I've seen a few people forget to reconnect their MAF sensor after an aftermarket intake install. This caused the computer to command maximum line pressure in the transmission which resulted in severe-feeling shifts. I think this usually triggers the 'check engine' light though.

[geo buick]

Thank you for all the replies. I may have found the problem. Please let me know if this makes any sense. Two weeks ago, my Service Engine Soon light came on. The code was P0420. I had a suspicion that my intake gaskets were bad because I was getting too much condensation in the valve covers although really not too much antifreeze was lost. So I had the gaskets replaced along with the Cat and O2 sensor on the cat pipe. Since that was done, a week ago, I have not experienced the slamming that I described. Amazing! Sometimes the slam was so hard that I looked in the rear view mirror to see if someone hit me. Now, I have not had it in a week.

[DefEddie]

P0420 is simply a cat efficiency code,you need a convertor.

The slamming as you take off is probably the torque convertor solenoid sticking,and not engaging the torque convertor. The slamming into gear is the torque convertor engaging.
Get a trans flush,change the filter and solenoid.

That's what I'm thinking.

[MidnightriderZ24]

which trans is that? the 4t65e? if so those pressure controll soleniods take a shit all the time. but it requires a new valve body as well.

[robertisaar]

The slamming as you take off is probably the torque convertor solenoid sticking,and not engaging the torque convertor. The slamming into gear is the torque convertor engaging.
Get a trans flush,change the filter and solenoid.

That's what I'm thinking.

not likely. the torque convertor won't be commanded on until at a minimum of 2nd gear. 3-5MPH is without a doubt 1st gear...

which trans is that? the 4t65e? if so those pressure controll soleniods take a shit all the time. but it requires a new valve body as well.

an 03 having a 4t65e would make sense...

[torq455]

yeah, TCC can't physically lock in 1st gear, and not commanded to lock until a certain mph/tps while in 3rd gear.

[geo buick]

Thank you all for responding. After the P0420 code and getting the intake manifold gaskets, thermostat, catalytic converter pipe and O2 sensor replaced three weeks ago the car has been good. What seemed for sure to be a trans problem was not? The car has the 4T65E so there is no vacuum modulator. For the months of slamming, did the computer see a problem with intake vacuum because of bad intake gaskets and cause a glitch with the trans but never set a code?

[MidnightriderZ24]

very common problem on these trans are the pressure control soleniods. have someone with a gm tech-2 drive the vehicle looking at the pcm and watch the pressures etc. when i have this problem, i ALWAYS put a new valve body in.