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Hi everyone, I've own my Lexus sc430 for a couple of years now and I've been experiencing some problems. Its a 2002 sc430 with 77k miles. When I drive on the freeway at around 60-70mph the car decelerates. Everything else is working. The car feels like I let off the pedal, but I really didn't. I went to the my mechanic and try to see if any codes came up but nothing. Also hooked it up to the scanner while driving and when the problem happens still no codes. the mechanic cleaned the MAF sensor and throttle body. He said he thinks it might be fuel filter problem. I searched on the forums and read some people have the same symptoms as me, but no one really talks about a solution. I read that the Throttle Position Sensor could be the problem, but when I told my mechanic he said he doesn't think that it could be the problem or else it would throw a code. The mechanic also thought that it could be that the tires are low on air so he filled them up and drove home and had no problems. but I feel its still might happen. any suggestions?

ser67

Check you alternator is working okay and your battery terminals are not corroded - could be an electrical glitch causing deceleration that corrects itself when you 're-boot' the car.

Have had similar, but thankfully not on highway.

Throttle was very unresponsive.

Recharged battery and everything fine.

VVTiBob

Just a question, please define ..."my mechanic". Lexus/Toyota have quite a few proprietary DTCs which do not show up on generic scan tools. Sometimes, not always, rather than guessing, it's worth going to a Lexus/Toyota dealer mechanic to ID the trouble area quickly and succinctly. They have the proper and necessary diagnostic tools.

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@VVTiBob He is a certified mechanic who worked for dealerships before and decided to start his own shop. I've taken all my car to him for years and he a family friend. He told me that he has the same scanner and code readers as lexus and toyota dealerships. He even got his friend who currently works for toyota as a mechanic to look at the car and he couldn't figure out the exact problem.

@ser67 My battery is a year old. My engine and everything else is running while the problem happens. I don't think it the battery or alternator. While driving on the freeway the car feels like i let off the pedal but Im still actually on the pedal and the car doesnt accelerate or maintain speed. it just looses speed. But as soon as I let go of the pedal and press right back on it the car drives fine.

any other idea?

bb430

iTrader: (1)

Maybe you should try to have your fuel line flushed and fuel pump checked, or anything related to fuel pump. Maybe the fuel pump relay is going bad. Just some suggestions.

ser67

As this is a drive-by-wire vehicle and the throttle is connected to a computer that works out how much 'gas' to give - and you have no error code then this could well be a throttle problem ?

I'm not a mechanic, but the noisy pedal operates a rheostat (as we used to call them) and if there is a dodgy bit (like the old analogue volume control on your radio which used to crackle) then it will cause a malfunction and the computer will most likely throttle down.

As its only doing what its told to by the pedal - no error codes.

Just a thought

o0o0o0

@bb430- My mechanic suggested me to use Redling fuel system cleaner for synthetic oil cars. He said if that doesn't work, he suggest changing the fuel pump and fuel filter.

@ser67- My mechanic also checked the throttle body and he said everything was working properly. What do you mean by noisy pedal?

Today I drove the car and experience while stop at a light and when I started to gas it, car car didn't go until i pushed hard or gave it a pump or two. happen once or twice during a 40 min drive.

thanks for the help guys.

JohnnyCake

Given these symptoms, if my mechanic told me it was tire air pressure, I would find a new mechanic.

I think VVTiBob might be on to something. Unless your mechanic is specializing in only Lexus repairs, that's an awful lot of cash to spend to get the Lexus proprietary software.

ser67

When I say noisy pedal I'm meaning the pedal that makes the noise (engine revving)

Its a nickname we sometimes use in the UK - like 'go' pedal

Sorry for any confusion - hope you get your problem sorted soon.

toxic

Do you lose RPM's when the car decelerates? What happens if you floor it when it starts to slow? I would have the battery tested even though it is somewhat new. I would make sure the battery connections are tight and clean. Run a dose of Seafoam through the fuel system. I can't believe that if it were fuel related or electrical it would not throw a code.

TOXIC

barncat

You certainly have some interesting ways of expressing things across the pond!

ser67

Yes we do have some strange words and humour

One of my favs, 'Haunted Fish Tank' or television as you may know it !!

VVTiBob

Getting back to the problem...I am really surprised not to see a P0120 or similar DTC, more specifically, as Ser67 suggested, the pedal position sensor error. But this means your mechanic would need to hook up the car and read freeze frame data to determine throttle valve angles relative to pedal position voltage resistance measurement. ( don't you just love driving by wire )
Correct me if I am wrong...but there also seems to be differing states when the event occurs... What I mean by states is; cold versus warmed up, running or stopped and lean vs rich fuel mixtures. Is that correct? Or has this been happening under relatively the same circumstance?
Depending on the voltage measurements as the pedal is being manipulated, the problem may be isolated in the throttle body sensor, the accelerator pedal sensor or the ECM which controls them. That's why I am surprised that no DTC has been recorded and stored when the aberrant behavior is observed.
And, just to be clear, am I correct in assuming all wiring harnesses ( having to do with the components we're discussing here) are clean, free of moisture and properly seated....so there are no loose connections/shorts....right?????

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@toxic- when the car decelerates(loses power while driving) the rpms do drop. I dont drive the car too often only weekends. But I do charge the battery with a battery charger when I don't drive it for a while ( no more than 2 weeks without a float charge). I take a look at the battery though, thanks

When I say loses power I mean the engine just doesn't rev. the power feels like it drops like I let the foot off the pedal, but the engine is still on and everything else is working.

@VVTiBob- It usually happens at the same state. Car is warmed up. wheather temps are usually average between 60-75 degrees. Always use 91 octane fuel, and usually happens at around 2-3k rpms going 60-65mph. but happens sometime driving slower. I also notice that sometime when I'm stopped and then go when press the gas pedal the car doesn't go, no engine rev either. I have to let go and reapply the gas pedal then it goes. I believe that all connection and wires are clean and connected properly. I'll look into the suggested items above.

I just added some redline fuel system cleaner and it feels that it helped a little. the power loss is not as noticeable. My mechanic said he also suspect the fuel filter.

When Im on the freeway and when the car decelerates(loses power) when I have my foot on the pedal driving, I can press harder on the pedal and it will slowly accelerate. (not normal acceleration.)

Again I thank everyone for all their help and suggestions. I hope I can figure out the problem soon thanks

mandyfig

Mass Air Flow (MAF) sensor...classic symptoms.

Clean it with MAF cleaner. Try it.

Might need a new one though.

Good luck.