So I noticed today while driving, when stopped, when i press on the gas, not all the time but sometimes the car will hesitate and then jolt forward. It feels like being rear-ended by someone and there's a bang too. It only does this sometimes. Doesn't do this while driving, not that ive noticed anyways, and I do pay attention to that while driving, only when at a complete stop and then accelerating again. The engine and transmission are at full temp when this happens and its not very cold out. It did this 3 or 4 times today but has done this in the past as well. Could the transmission be slipping? or something wrong with the torque converter? Could it be something else entirely? I know these transmissions weren't manufactured with the best quality parts so im leaning towards it being the trans, but I dont know for sure. Maybe it could be something else.

 

That's not good. Assuming the engine revs up when that delay happens, then its 100% the trans. I had that in my 08 Impala and it always worried me trying to pull out on a busy road because you never knew when it was coming after stopping.

Since it hits that hard when it engages, It's most likely the contact area for the input clutch piston. The wear causes the seal to occasionally start to leak significantly. When the engine revs up (almost like its in neutral at low rpm), the fluid volume builds and once it has enough volume to overcome the leak, it suddenly engages and it lurches forward like you just neutral dropped it. At first it'll be very sporadic, but the frequency will increase the more it wears. As you can imagine its also very hard on the hard parts inside the trans as it is really like a neutral drop in severity.

There is an aftermarket seal (called the long seal vs the short style OEM seal) that helps prevent this, but obviously its a trans tear down to fix the issue.

Two other potential causes:
-Boost valve (unlikely in your case with how hard you say it hits)
-Extremely clogged filter. Although TBH if the filter is this clogged with material the trans is toast anyways

 

Well to be clear its not super hard, its just a thud noise. But that still cant be good. The engine doesn't seem to rev but i cant remember to well. I think it just hesitates and then thuds. but that cant be normal either. Something is wrong for sure. I guess i'll be spending $2800 for a rebuild then. yikes. When I have it rebuilt can i ask to have them use higher quality parts? I dont want them using the crappy OEM ones and have this happen again at 200,000 miles.

 

My 2004 Monte did something like this for years. The jolt was not too bad. I was able to hide it a bit with with the poly radiator mount bushings AND increasing the line pressure settings a little in the PCM. I finally decided to have it completely rebuilt. I did learn with the right scan tool, you can find out how much the adaptive shifting is being pushed to determine how bad off the trans might be.
My dad has a 2008 Impala that has been doing the same thing for years.

Very first thing, just ensure the trans has the right amount of fluid in it (that this is not a slow leak that is finally got the fluid low).

As for having it rebuilt, I know the really good performance parts have been gone and off the market for a long while. BUT given common fail points, I am not sure what is out there for a routine daily driver. TripleEdgePerformance for years have been the go-to shop for issues with the 4t65e in these cars.

 

My Monte did that as well. When I upgraded to the ZZP 1.0 computer box or whatever it was, completely fixed the issue.

 

You can upgrade the computer on the car? was it difficult to do? or just plug and play? I would liek to make those kinds of upgrades to my monte as well and im interested in what the computer was, and what other little modifications i can make to my monte like that.

 

Look up "ZZP PCM" for the 3800.

 

There's 2 options- take it to someone local to plug into the OBD port and tune it or find an online store that does tuning (as you tell them what car you have and what changes you want, and they send you a tuned pcm that you swap with yours, then you send your old one back as a core).

If I remember right, you have a 3400. I'm not familiar enough with them to know the common tuning shops for them- zzp tends to focus on 3800 stuff for the W platform.

The one thing I'd caution is that IMO stuff like that is a bandaid if the clutch piston surface is worn. Some people do mechanical modifications or additives to the fluid for the same effect, but TBH it often comes back with time.

 

Yeah I have the 3400. I don't want the new computer for fixing any trans issues, I'm just wondering if a new upgraded computer system will offer any kind of benefits to this old car. Im good with computers but I know very little about a cars computer. If there are any benefits then it might be worth the upgrade, but if not then I might just skip it.

 

Oh yea, 3400.... Duh forgot. Theyre still available.