Hey guys, I have a TL 2008 with 150,000 miles, and the vehicle is consistently slowing at highway speeds over 75mph + (about 5 mph every 2-3 seconds) when I知 not giving the car any gas. I do regular oil changes, which always helps alot, but did a full synthetic/ high mileage oil & filter change less than 2,000 miles ago and feel like the problem is not an oil issue. The fuel economy has been sucking, and I知 hoping this is just a sensor issue or normal. Not experienced enough to tell.

*used vehicle, bought at 140,000 miles, well maintained.
*No check engine lights on.
*Emissions System Code: P0443
*Tire Pressure Monitor Code: 81 (front @ 41psi, rear 40psi/// Sensors read 37psi & 36psi)
*automatic transmission
*shifting into reverse was getting a bit jerky until I added more fluid, which resolved that issue (fluid low & a bit darkened, but not burnt).
*added 3/4th quart of ATF Valvoline/ Maxlife to transmission dipstick yesterday which seemed to help shifting, but not highway speed maintained.
*rpm痴 go into 3,000 sometimes switching from 3rd to 4th gear (low speed). Vehicle maintains speed of 80mph at 2,250 rpms.

Changing oil seems to resolve the issue temporarily, but again, only for about 1,000 miles or less. I知 not a certified mechanic but am intuitive enough to figure out how to find and fix most things with vehicles. I知 still learning, so any guidance would be helpful. Not sure if I知 just overthinking it, or if something is wrong.
Thanks!

 

I'd recommend doing a 3x3 transmission fluid change using Honda DW-1 and go from there.

Regarding your engine oil comments, unless the engine oil is dangerously low, changing the oil (with the correct grade) will have exactly zero impact on how the car drives, regardless of the type of oil.

 

Thanks for your suggestion! I've driven the vehicle around 10,000 miles now without doing any work on the transmission.
And that's really good to know about the oil. I haven't performed a transmission fluid change/flush before, so will need to start learning.
I'm hoping that adding the Valvoine Maxlife will not do any harm to the transmission in the time being. I didnt have DW1 readily available at the time.

 

A few ounces shouldn't negatively impact the transmission.

Another thing you might want to consider is changing the pressure switches for 3rd and 4th gear; you can search the site here for instructions on both the 3x3 ATF change and Pressure Switch change.

 

So you let off the gas at highway speeds and your car slows down? And you are concerned why? No gas, no go.