I've been having issues with my 01 Millenia 2.5. Occasionally on the highway (60+ mph) the rpms will drop by about 250-500rpm and will lose speed as if I were coasting or the transmission was in neutral. If I press on the gas it will jerk and begin accelerating. Letting off the gas produces the above mentioned coasting/rpm drop. Getting off the highway and onto city streets (35mph) the rpms will bounce up and down and the car will jerk back and forth. I've got no CELs, and have been through all 78 pages pertaining to the Millenia and have found nothing that has helped.

 

Hi, did u ever figure out what was causing the issue?
I'm having the exact same issue.

My car drives fine but once at a constant speed and I try to maintain that speed(ex 70mph) or I turn cruise control on, it start bucking and surging. RPM's drop or surge about 200-300 when it happens. Constantly I'm jolting forward every few seconds bc I'll hit the car in front of me if I don't try to cruise at a constant speed.
In a way it seems that at highway speeds when it bucks the first 1/3 of the gas pedal doesn't do anything at all. Only once you press down past 1/3 does the car surges forward again.

​​​​​At low speeds the same thing. (Updated)

Just replaced transmission speed sensor after I got the code for it. While there I cleaned up all the ground connections I could fine. No cel lights other than that.

Does anyone have any ideas what this could be? Why isn't there a cel light for this issue?

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In order for help you need to post your miles engine type and year

 

2.5l
205k miles
2001

​​​​My bad, I forgot to add them to the post.
I made a video. I'm trying to keep a constant speed when it starts jumping. When the rpm jump a lot and stays that's me stepping on it, which makes the issue go away(at least it feels like that)
https://youtu.be/BJ5JaxyJc8w?si=XScg4KXfN5_rKVOY

 

The age and mileage does make it somewhat of a challenge for any DIY!
The ECU for that year was barley into OBDII and really for your model the ECVU programing was more or less in the beginning. OBDII came around in 1995. In 2000 some better-quality ECU went in more expensive vehicles, and it was not until about 2005 or later until ECU programing was more or less very improved. So, for not getting a DTC (dinosaur's call them CEL)engine check light would not be totally out of the ordinary? However, a professional grade Diagnostics scanner can look into more system independently and run live data feed which can determine at least consideration for a starting point to solve the problem. They can also look into the memory and see if any DTC was generated but not permanent which is another reason not to get a engine check light. Otherwise, you are going to need to go to a service shop that has a skilled Automotive technician or spend a few hours watching YouTube and on forums with members guessing your problem.

BTW I can tell you have already been doing that because Automotive Diagnostic Technician stopped generally calling Diagnostic trouble codes (DTC) CEL years ago.

Your video BTW can mean about at least a bakers dozen reasons for the problem?