dakrause

1999 RX300 with 360K miles, original engine & transmission
Sunday on the highway I suddenly encountered 20 minutes of stop & go traffic, after which time the engine started shifting oddly. Specifically:
70 mph at 2800rpm harsh shifting, engine temp on the low side but within normal range
I stopped at the next rest area, popped the hood and allowed everything to cool down for 20 minutes
A mechanic happened to have just parked next to me at the rest stop, asked me to start the engine and pulled the transmission dipstick, level looked normal and then smelled the dipstick and proclaimed "Yup, smells burnt, the transmission was overheated and you should ask your mechanic to top it off BUT DON'T HAVE HIM FLUSH THE TRANSMISSION".
I thanked the mechanic(no payment other than my thanks) and continued on my way. Engine and transmission seem to be working normal from that point until now.
Does the mechanic's diagnosis make sense? And does it make sense to top off the transmission as suggested or do anything else for the transmission?

maxSteel

After a scare like that I'd probably drop the pan and change the fluid and filter (I do not have the screen any more)

MattRX

1+ Get a look at the factory screen for metal shavings then replace it for a media filter or clean and reuse screen. I opted to replace my screen w/ a media filter but both are available.

Part numbers:
https://www.toyotanation.com/threads...#post-14842478

salimshah

My 2c on "topping of" .. Your problem can be too much fluid. With heat the volume expands.

Proper procedure is summarized below

1. With engine + transmission cold .. check level on the transmission dip stick and ensure it is above min-cold.
2. Warmup engine + transmission [drive for about 1/2 hr] in mix-driving condition.
3. Park at a level surface and let the engine idle. Shifter in Park.
4. Check the fluid level (hot mark). [Good luck with that as mostly I had difficulty in making a good read]

Salim

salimshah

Please confirm, but I think the screen was superseded with the media-filter. The screen was more free flowing and the fiber captures the floating grey stuff better. If you do decide to go with the screen, please replace it regularly. With the screen, you can opt to do drain and fills without dropping the pan.

Salim

dakrause

I live in the city so cannot do much work myself, so I asked my mechanic for a suggestion and he said to change transmission filter. Which he did and said the filter was dirty but the fluid otherwise looked fine. Mechanic did mention that I might need to "add" a transmission cooler within two months.

A few days later on an hourlong trip with temperatures in the 90s and A/C at full blast, the transmission again overheated(same symptoms). So I stopped, let the car cool down for half an hour and continued at a lower speed without A/C and made it home without any more overheating. So the problem has not been solved.

According to the VIN, my model is: 1999 Lexus RX300 AWD 6 Cylinders F 3.0L FI DOHC 183 CID
Which I understand does already have a transmission cooler. Looking in the passenger front wheel well I can see what should be the transmission cooler:

Digram of transmission cooler

Photo of transmission cooler

This suggests to me that the likely diagnosis is a blocked transmission cooler, which my mechanic does not know even exists.
Does my diagnosis make sense? Am I missing something?

maxSteel

Maybe the cooler radiator is covered with crap. See if you can clean it.
I don't know if all 1999 RX300 have a cooler. I think it is part of the "Towing Preparation Package", don't know if that was standard or optional

dakrause

The transmission cooler radiator looks fine and feels warm/hot to the touch after driving ten miles, so appears to be working as intended.
I'm stumped- what could have changed at 360K miles for transmission overheating to suddenly start to become a problem?

salimshah

The mesh may still be clogged.

Salim