98 Transmission Thump / Hesitation

[handygal]

I have read through pages of posts regarding transmission issues with mostly older LS's. I need some help as to what I should try having inspected / replaced before I have the transmission changed in my beloved 98.

Currently, the car seems to have a problem downshifting. Whenever I'm coasting there is a good chance that when I step on the gas again, the car won't find the gear, hesistates, catches with a thump and a jerk. At one point I was driving with the window open and it almost sounded like a slight backfire pop, imagine a Lexus with a backfire! Oh my. I was in a neighborhood with a series of stop signs and I did stop, barely, and when I stepped on the gas again, I heard the thump/pop each time. It is getting worse. I think I completely avoid it if I downshift manually. So now when I have to slow down but not stop (a changing traffic light, speed bumps, turning,) I shift down to at least 3rd gear, even if the car really needs 2nd. It seems to find second if I tell it to downshift. It's a bit of pain, would be smoother if I had a clutch!

The only transmission related maintenance I've done is tranny fluid changes. Not the full flush, just a few 2 quart changes (Toyota fluid, was it type IV?). I'm not sure if it helped or not. It seemed to at first but maybe not. The car has 72K miles on it. My mom was the original owner, I've had it for 4 years. I'm a sports car kind of girl, so I'm not exactly a gentle driver. I live in south florida, so heat and rubber degredation is always a concern of mine.

I was going to have the mechanic start with a new transmission mount. The miles are fairly low on the car but considering the age, I thought it might be a start. Can anyone offer more suggestions?

[lexus082]

If you replace the transmission mount it should take care of that thump. Even though you have low mileage that mount is constantly under stress and is probably just worn out. I had the same problem and it disappeared after I changed the transmission mount.

[SKperformance]

Tranny mounts should help also keeping it out of O/D and in pwr mode will help as mine is similar due to the drive by wire system.

[handygal]

My mechanic insists that the transmission mount is fine. Then he took it for a drive and heard the thump/pop. OMG, I think I want to faint, he promises that it really is a backfire. I thought it sounded like a slight backfire when I had the window down. I never ever want to hear a Lexus backfire.

The mechanic started it up on Monday morning after it sat in his yard all weekend and said it smoked up the whole yard. Now they are chasing a leak and possibly fouled plugs (new a few months ago) and then on to coils and so on. Anyone have any words of wisdom to share with me? Any similar experience?

[Cornwall]

I am not sure if the fault on my GS300 auto box has the same cure as the LS, but this happened to me in England

I cleaned out the throttle body etc a while back, and have done a fair amount of work due to a blown head gasket, which is now done (very easy once you remove the manifolds

I then was getting a high tickover and when engaging gear it was like i had been rear ended the take up was so sudden and harsh, and changing gear on the move was so severe I thought my driveshaft was going to break

Nice man at Lexus told me to adjust my throttle body sensor switch as it was over fuelling on tickover and to much power when engaging gear, apparently it can also sense the gearbox somewhere in the electronics

So I adjusted it and the problem dissapeared, the car ran smooth again and tickover was good

The Lexus technician said i was the second with the same fault and the first one had been told his gearbox was knackered and $4,000 for a new one by his local garage

Takes a few seconds to set up the sensors but what a difference it makes