2005 Honda Element with 331,000 miles and Automatic Transmission. Transmission slips when the vehicle hits 70 MPH and will not go faster, it will go back to normal after the vehicle drops to about 65. It also slips randomly while shifting (4 to 3, 2 to 3, 3 to 4) but not all the time. What would cause this? Would Lucas Stop Slip help?
Does it have sufficient transmission fluid? What is the fluid condition? Any transmission codes (pro scan tool needed)? With that mileage the trans is probably shot but it it should be diagnosed to see what's actually going on. Lucas Stop Slip is a temporary band-aid at best but if the trans really is worn out you have nothing to lose by trying it.
Yes. As far as I know, it has never been changed. No transmission codes.
If the fluid has really not been changed in over 300,000 miles it's amazing the thing is working at all. If that's the case I would expect the fluid to be in very bad shape (burnt and gritty) and if there's a filter it would be all plugged up - is that what the fluid looks like?
It's a good bet that transmission is worn out. You can take it to a transmission shop for further diagnosis, or take a chance and try some Lucas additive. If you do add Lucas you need to drain enough fluid out so as not to overfill. It's not going to be worth spending thousands of dollars on a rebuilt transmission for a high mileage 17-year-old vehicle so it's probably time to start shopping.
The fluid may have been changed before my parents owned it but I'm not sure. I think it is just old but not burnt or gritty.
If that's the case you could try doing a partial fluid change (drain and fill) since you need to drain some for the Lucas additive anyway, if you decide to give that a shot. Change filter as well if it has one. Bear in mind that depending on exactly what's going on inside that transmission this could make things worse.
Considering the mileage and the symptoms you have, I would say the transmission is worn .For Honda automatic transmission, do not use Lucas additive. Those transmissions are very sensitive and you should only use Honda OEM fluid, not anything else.
I've heard that about Honda transmissions but this one may be at the "nothing left to lose" stage. Here's a couple of reviews using Lucas in Honda products showing some benefit but of course those are short term results. The Project Farm test video indicates that the Lucas additive does little more than increase viscosity.
https://www.youtube.com/watch?v=LMyfq8ZxZc8
https://www.youtube.com/watch?v=aI0GXcPokT0
I think you are right about the "nothing left to lose" stage. Hopefully it helps this situation and he can get more miles out of the Honda.