RX300 MCU35 Gearbox issues

nowakkam · February 1, 2023

Hello people 🙂

I have a question to people that have had issues with gearboxes on the RX300s.

Long story short - ATF change looooong overdue, car started making whining and then grinding noises (gradually louder within a space of a few days). Then one day desperately trying to get to work the transmission went (everything apart from P was like neutral, after engine restart things went back to normal for maybe 60 seconds after which it felt like clutch slip in a manual, then again gone back to normal after restart in a loop until I get it back home 2 miles down the road). Throughout those days the gearbox oil temp was going all the up to the high 80s celsius.

Since then I have dropped the pan, cleaned the magnets (no porcupines but the ATF looked like hot chocolate), changed the filter (mine was already a non-metal filter so didnt attempt cleaning it, just swapped it for a new one), and filled her up with fresh cherry red T-IV.

Now here is where I am confused - car started beautifully, no noises whatsoever, drove absolutely perfect (and smoother than before) for exactly 5 days until today when I can hear the exact same loud whine from the gearbox, mixed in with a slight grind. Also suddenly there is a metallic thud when switching between D and R, and when i pulled the ATF dipstick out after an hour drive, the fluid was brown actually with some bubbles on the stick (level within the hot range though). Transmission running at 65-75 degrees celsius.

Here's the question - is it worth draining and filling her again or send her to a garage straight away (which I'm less willing to do as the bill starts going as soon as they touch the gearbox)?

Sorry for the long post,

Kindest of regards

Kamil

katabrontes · February 1, 2023

Interesting. I had my ATF changed( well most of it as the gearbox won't drain completely) after watching a UTube video from the Car Nut. He advised changing the ATF at 60K even though Lexus state it never needs changing and my garage said the same. Car Nut's point was that ATF is cheap but the gearbox is probably the most expensive repair on most auto cars. Having read your post I am very glad I did.

nowakkam · February 2, 2023

1 hour ago, katabrontes said:

Interesting. I had my ATF changed( well most of it as the gearbox won't drain completely) after watching a UTube video from the Car Nut. He advised changing the ATF at 60K even though Lexus state it never needs changing and my garage said the same. Car Nut's point was that ATF is cheap but the gearbox is probably the most expensive repair on most auto cars. Having read your post I am very glad I did.

Yeah I feel stupid for listening to this kind of advice. Also I had a slight confirmation bias thinking that the ATF doesn't get anywhere near the engine oil temperatures so it won't get burned. And here I am asking fellow owners for their experiences 🙂

m4rkw · February 3, 2023

Changing it again isn't going to hurt anything and it's certainly true that draining it never gets all the fluid out so if it was sludged up there may well be sludge left inside it.

Whether that will fix the problem or not isn't certain however I like your odds because you had symptoms before the first change which resolved for 5 days, suggesting that it may have been ok until the sludge got back around to where it was causing a problem.

If it were mine I'd try another change and then stop driving it at the first sign of trouble if the problem persisted.

m4rkw · February 3, 2023

If that doesn't work I would definitely talk to a gearbox specialist. Under normal circumstances I would never recommend anything that contains the word "flush", however a friend of mine did once have a gearbox issue that was resolved with a flush and if your symptoms are being caused by sludge stuck in the system then it's not inconceivable that that could possibly help. However I would hasten to mention:

1. Many manufacturers explicitly state "do not flush our transmissions/engines", flushing can damage transmissions

2. Places that flush gearboxes are often running a numbers racket, flushing the units with the same generic fluid and making a lot of money in the process

3. If I were ever contemplating this I'd be very very careful who I let carry it out and I'd want to make sure they were using the correct fluid for the unit and I'd want a detailed explanation of how it could feasibly rectify the issue

More info on transmission flushes here: https://www.agcoauto.com/content/news/p2_articleid/238