Ok. . .my $0.02, I think it's either continued funkiness with tires, or potentially out of true rims (40% chance), a transmission gremlin (40-45%), or something that is beyond me (15-20%).
- Have you rotated the tires/wheels that are on there now (i.e. with the new tires?)
I'm leaning towards a transmission gremlin because this statement:
It always only occurs at about 22 mph and stops at 25 mph. It does not occur during moderate to heavy acceleration. It is more noticeable after the engine is warm. It does not occur during coasting. It can be prolonged and made more noticeable by driving up a steady smooth hill at 23- 24 mph.
Sounds like very routine transmission functions gone slightly awry. The "moderate to heavy acceleration. . .not during coasting" sounds like either it's screwing up a kickdown (aka downshift) into a lower gear to accelerate, or depending on where you are in the rev range, it's having a hard time upshifting into that next gear as it accelerates (hard to say without being in your car with you and looking at your tach as this happens). It's funny because the transmission behavior in "moderate to heavy acceleration" would be exactly identical only to going up an incline, which you said is the other situation that this happens.
So. . .obviously the tranny is extraordinarily complex and is hard to troubleshoot without having a lot more technical knowledge than I have and also the dealer's HDS technical computer. . .but there are a bunch of things you can do to maybe hone in on it and at least guide the dealer into things it could and couldn't be, so hopefully they can get it resolved in as few visits as possible. If one of the 100 or so sensors in the transmission is smart enough to realize that a part is failing or not sending a signal it will wither illuminate your check engine light and/or blink the "D" indicator in your gauge cluster. However, there's lots of stuff that can happen that won't trigger a DTC (the code that trips the light).
- First thing: check your ATF level under the hood, and also look at the fluid, it should still be bright and translucent. . .if it is dark or gunky in any way that is a major red herring.
- While parked in your driveway try some of the following and note if any of these moves seem to: take a long time to engage; engage too forcefully/with shock; take a long time to disengage.
P to D
D to P
N to D
D to N
P to R
R to P
N to R
P to D3
D3 to P
P to 2
2 to P
P to 1
1 to P
1 to D3
D3 to 1
1 to D
D to 1
N to 1
1 to N
2 to N
N to 2
(if funkiness is detected, note which shifts cause it)
- Can you replicate the same shudder when the tranny is in D3 for the entire episode? If you cannot replicate it in D3 that means the problem might lie with 4th or 5th gear.
- If you manually shift from 1-2-D3 as you accelerate from a stop can you cause the shudder? If so where.
Here are some diags from the dealer service manual. . .before I do that, FYI the torque converter will kick in when you're at a constant speed once you're above 31mph. Torque converter lock/unlock problems are a malady pretty easy to diagnose because they are semi-violent feeling and very throttle sensitive. However it doesn't sound like your issue happens at that speed or above.
So, if we were to think that this is a tranny problem, specifically a problem with a clutch/valve/or solenoid on a gear 1-5, here is where the shifts normally occur under light, medium, and full throttle acceleration. This can help you hone in on what gear might be acting up.
And here are the downshift points again under light, medium, and full throttle.
(note - you're going to want to do that 5th to 4th downshift at full throttle test between 111-121mph at least 5 times to give us any useful data
Ok - if it's a tranny problem, you'll probably be able to verify that fact and isolate where it's happening (i.e. which particular gear under what kind of throttle, or is it torque converter related [probably not], etc). Obviously this isn't 100% of the tranny diagnostics, but I think this is definitely enough for you to poke it hard enough to get angry. . .if all of this doesn't help isolate it then at least you have some good trial and error to share with your dealer tech.
hope this helps and look forward to next update