Gs300 Tranny Sluggish (oil Cooler Bypassed Mystery)

[shadetree99]

Hi All,

I'm new to this forum. I aquired a Lexus GS300 1999 in ~2008. Has had what I would describe as slow to shift from Neutral to Fwd or Rev. If I apply the gas too quick, engine revs and then the tranny engages with tires spinning. This also happens if I am slowing as if to stop, then accelerate - seems like it likes to downshift to 1st gear prematurely. Recently I was replacing my radiator hoses and after removing the air dam under the engine to gain access to the bottom hose, I realized that the transmission had an oil cooler, and it wasn't hooked up! The original hoses on the rad were just lying on the air dam for who knows how long. Looks like one had found its way to the drive belt pulley and had a section worn through. The tranny fluid lines had a short hose completing the loop - just stuck on the pipes with no clamps. I have been driving this car for ~ 1 year like this... Here is the history as best I know...

Car purchased new by my Mom in 1999. She hardly drove it.

Serviced at regular intervals only at Lexus dealership

Car involved in accident mid 2002. Front end damage. (Other car hit this car in the right (drivers) front side ( I believe) I assume radiator was removed and reinstalled as part of repair. I also assume that at that time the tranny was jury-rigged with a "U turn" hose to keep from draining all the tranny fluid...

Oct 2003 she had 14,735 miles and Lexus did 15000 mile maintainance where 3 qts ATF T-IV were replaced and trans gasket replaced. (If "bypass" hose was left inplace from 2002 repair, I'd think it would have been restored then...

Fast forward to April 2005. Another 15,000 mile maintaince - Car had 16,228 miles (~1500 miles since first "15,000 mile scheduled maintainance). In this service 4 qts ATF T-IV was replaced along with a new trans gasket.... Again - wouldn't the oil cooler bypass have been noticed? (Is it normal to swap out 4 qts of fluid after 1500 miles? or was it because it was hardly driven? or did they see $$$ when Mom came in the door?)

July 2006 my sister borrows the car and does ~3000 miles of highway driving. On the trip, the tranny overheats and she sees white smoke trailing the car. She drives it till the check engine light comes on. Tranny begins to slip and shift poorly. Finds a Lexus dealership where they discover the tranny fluild is low. They add 3.5 qts and she's on her way.

A few days later she takes it to Mom's "regular" lexus dealer that has done all the maintainance/repair work. They inspect it and say the tranny was overfilled and remove a qt (don't know for sure how much). They see nothing wrong or damaged with the tranny from its little overheating episode.

I know the car has not seen any other service center prior to me discovering the oil cooler was bypassed. I find it hard to believe that a mechanic could overlook a jury-rigged bypass hose with no hose clamps and two loose radiator oil cooler hoses flopping in the breeze - especially after being asked specifically to determine the health of the tranny.... but there it is.

So my theory is that the tranny oil cooler has been bypassed since the 2002 accident. The small amount of driving Mom did wasn't enough to cause any issues. After hardly being used - it sees alot of high speed driving in the summer and the lack of the oil cooler takes its toll. I've had the car for ~ 1 year. It had ~19000 miles a year ago. It has ~ 23000 miles now. Its only seen short commutes since I was suspecious of the tranny smoke incident - with no understanding of why it happened. Over this period of time, I have routinely checked the ATF level and its always in the normal range. The fluid has always been nice and red. All the while, I'm wondering if its normal for the tranny to be sluggish in shifting fwd-rev. It doesn't seem to slip under normal driving conditions. It will however, seem to downshift to 1st gear and rev up prematurely. We have a long down hill driveway. I stop at the top to get the mail, and everytime I start coasting down, it will downshift with a jerk to a lower gear, then shift back up. I now move the shifter to 3rd gear before coasting down and avoid the problem. Somehow, I don't think this is normal behaviour.

My course of action is to do a low pressure leak check of the oil cooler, and barring any leaks, re-connect the tranny pipes with new hoses to the cooler and run the car - recheck the fluid and add as necessary.

Should I pull the pan/filter and inspect/clean it? (never done that before...)

I've read in other posts about doing a flush, also about resetting the "learning curve" of the tranny. I'm hoping that putting the cooler back in the loop will somehow help.

Any theorys as to why the tranny oil cooler was bypassed? Could it be causing my symptoms? Thanks in advance for responding to my post. I'd like to understand the mystery.

[shadetree99]

Follow up to original post.

I've re-plumbed the transmission to run its oil back through the cooler as originally built. Found no leaks/problems with oil cooler. (so why was it bypassed in the first place????) In the process of breaking the "u-turn" hose that bypassed the cooler, I noticed the fluid WAS burnt red. I then dumped the oil in the trans pan (about 3 qts) and refilled with 3 ATF T IV qts. I noticed the transmission did shift better all around. I followed up a week later and changed out ~ 3 qts again. From what I've read here, the trans holds ~ 6-7 qts, so I'm only changing half each time. I guess I have 3/4 clean fluid now...

Much happier with the overall transmission performance. I still have one annoyance... If I stop my car at the top of my driveway hill, accelerate slowly then coast down the hill... as the car gains speed (~15-20mph) the trans suddenly downshifts momentarily, then shifts back up, causing the passengers to lurch forward. Is this normal behaviour? Has anyone else experienced this?

Anyrate, things are shifting smoother.