Mechanics

BRAKING, ACCELERATING & SHIFTING BY ITSELF

1999 Lexus RX 300 • 215,000 miles

I recently bought a 1999 Lexus RX300 that seems to not have been maintained well for the past few years. I am having issues with it not braking, automatically accelerating and suddenly shifting to a lower gear while accelerating or driving at a higher speed. To explain further, while driving it sometimes accelerates without my pressing on the gas. Then, if I try to slow down. It will not brake for a moment until I press harder on the brake then it will suddenly brake by jerking. I worry it will not brake at all sometime and I will run right into someone or some other vehicle. Furthermore, if I need to brake after I am stopped and accelerate again, it will brake with great difficulty. Additionally, if I am parking and need to back up a little and put it back in drive to park, it will brake with great difficulty. I get very close to hitting other objects or cars. It seems to be pure luck that this has not happened yet. I read in the manual something about this vehicle having two breaking systems and if one fails, I need to contact the dealer and never pump the breaks. I cannot spend a lot of money on this vehicle so I was wondering if there was ever a recall for these brake or transmission issues? I was also needing to know what basic things I should do first such as flushing transmission or using some fuel cleaners or oil additives. Do I simple need brake fluid or do I have a failed/failing break system? I appreciate your help and expertise.
Avatar
Pathaani
November 2, 2013.




A mechanic should look at the hard stopping issue with you there to show him what's happening if necessary, but I can explain some of your other comments. First of all, on much older cars if a rubber flexible brake hose popped a leak or a metal brake line rusted through, you lost all hydraulic pressure to all four wheels so you had no brakes at all. As a safety measure all cars sold in the U.S. After around the late '60s were required to have a dual hydraulic system. One worked the front brakes and one was for the rear brakes. If a leak developed in one system, you had the other half of the brakes to stop the car, but obviously it would take a lot longer to stop. A red warning light would turn on to tell you there was a problem. That light was usually the same one as the parking brake light. On old rear-wheel-drive cars, about 70 percent of the weight was on the front tires, so if the front brakes had the leak you still had about 30 percent of your braking power with the rear brakes. On today's front-wheel-drive cars about 80 percent of the weight is on the front. That leaves you with very little braking ability if the front system stops working. To address that almost all front-wheel-drive cars have a "split-diagonal" hydraulic system. The left front brake and the right rear one are on the same system and the other two brakes are on the other system. That insures you'll still have 50 percent of your brakes when you pop a leak. You don't have to know any of that to drive the car. They're just telling you that in the event of a failure you will not lose all of the brakes, so just because you can still stop, don't assume there's nothing wrong. On some cars, Chryslers in particular, the balance between the two systems is so well perfected you may not even notice a problem except for that red warning light. On most other cars all you'll see is a tiny wobble in the steering wheel when you apply the brakes.

As for pumping or not pumping the brake pedal, that comment only applies if your car has anti-lock brakes. If it does, you'll see a yellow "ABS" warning light turn on for a few seconds each time you start the engine. That system is an add-on system to the regular "base" brakes that all cars have. On cars without ABS, the reason for pumping the brakes was because a skidding tire has no traction so it takes a lot longer to slow the car. More importantly, you can not maintain steering control if both front tires are skidding. That's easy to prove on ice. Push hard enough to lock up the brakes while going slowly through a parking lot and you'll see that the car continues going straight regardless of how far you turn the steering wheel. When you let off the brake pedal, if the tires regain enough traction, you'll be able to steer again. By pumping the brake pedal you alternate between some stopping and some steering control.

Now that they've added anti-lock brakes to the car, that system pumps the brakes automatically for you if the computer sees one or more wheels is locking up and that tire is about to skid. The advantage is it controls each of the four brakes independently of each other, and it can pump each one between 15 and 30 times per second; much faster than what you can do with your foot. The thing is many people don't understand that the computer only does that when you're pressing the brake pedal. If you pump it like we used to do, that computer is going to start and stop doing its thing so it loses its effectiveness. THAT'S why they're telling you to not pump the pedal. Just hold it down as hard as you normally would, then let the system take care of any skidding that is trying to occur. Remember, that only applies to cars with an anti-lock brake system. That's becoming real common but not all cars have it.

Caradiodoc
Nov 2, 2013.
No. I actually don't NEED to apply the brakes to have it do that shaking thing. It also does it while I drive around 50 mph. Or if I'm going slow around town.
AD

Tiny
Beckys
May 23, 2014.
Are you the same person as "Pathaani"? Is this the same car and problem? If you feel a shaking in the steering wheel even when driving slowly, that is almost always caused by a bent wheel or broken tire belt. Some broken belts are easy to identify by yourself but some need to be identified at a tire and alignment shop.

Caradiodoc
May 23, 2014.

AD