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Help With Car Accelerating By Itself


marlon08901
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Hello my friends....Let me first start by saying that my 90 LS400 is the best car I ever had and although I have changed a couple of parts here and there...I was never in the position where I would not feel safe in the car until yesterday.

There may be 2 problems not sure if they are related or not. Yesterday morning I put gas in my car (I live in NJ so the gas attendant puts the gas in) a couple of minutes after driving while at a light the car stalled...I tried to restart and it sounded like it was not getting enough gas (I had ran out of gas before in other cars and that's how my lex sounded) after a couple of minutes it started and everything seemed normal until it did it 2 more times later in the day, one of the times while I was driving a bit slow but had not stopped, it stalled and I was able to pull over and park this time it did not start until 30mins later...could the gas attendant putting the wrong fuel cause this problem? (I usually make sure to tell the attendant to put premium fuel in but yesterday I don't think I did)....I had only put 1/2 a tank of gas at the station so I drove to a differ gas station and filled tank up and made sure it was premium fuel...car still gave me the "not enough gas" 2 more times the same day. The car had stalled in the past but it was due to a bad MAF sensor before I had replaced the MAF I had replaced the fuel pump so I don't think it's a fuel pump problem. Anyway, this brings me to my second problem, last night I drove about 20 miles to drop my daughter off at her mother's and everything seemed fine, however, on the way back when I took the ramp to merge in the one of the states' highways I pressed gas pedal and after I hit 60mph I took my foot of the gas but the car continued to increase speed without me pressing the accelerator....needless to say I tried to press on the brakes as hard as I could but the car only seemed to slow down a bit but not stop, that's when I pulled over to the shoulder, put the car in Neutral which made the engine surge all the way to 5000+ rpms...I immediately pulled the key of the ignition...I checked to make suret the floor mat was not pushing the gas pedal and it was not. Then, I waited a couple of minutes, tried to start the car and the engine surged again as if someone was pressing the gas pedal. I turned the car off had it sit for a while (about 20mins) and the car started just fine. Then I was was very careful observing my speed and everything was back to normal...but since it was late at night on the highway and there was no traffic I decided to increase my speed slowly to see if the car would speed up by itself again and as I hit the 60mph mark I felt it start to surge and pick up speed on it's on (my foot was no longer on the accelerator) I pulled over right away...did same procedure as before, at start up engine surged again, pulled key out, waited...turned car back on and drove home as slowly and carefully as I could. In the past I had had issues with the idle going up and down by itself but it was usually in the very low rpm and the problem was solved when I cleaned and replaced the bearing in the idle air control valve. So, knowing that it is not the IACV not the MAF sensor....any ideas what could possibly be? Anyone with a first gen. LS400 had this problem? I started the car up this morning, pressed the gas to see if the engine would surge on it's on but everything seem fine. Any suggestions would be greatly appreciated thank you! Oh...also I forgot to mentioned that I had cleaned the throtle body about 1 yr ago so I know it's in good shape and also checked the cable coming from the accelerator to make sure it was not getting stuck anywhere it seem to be in proper working order.

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Update....so I started the car this morning and slowly drove it to the mechanic and I noticed that whatever speed I drove (did not drive it past 25mph), if I took my foot off the pedal the car would just keep going at the same speed until I applied the brakes.....just like when using the cruise control. The car is already 22 years olds so I figured there could be a possibility that the cruise control module under the driver's side panel is defective and/or perhaps the cruise control mechanism that controls the throttle when the cruise control is engaged is rusted out or somehow getting stuck. I mentioned it to my friend (mechanic) and he mentioned that over the weekend he had work on a car that had the same problem as mine and it was in fact their cruise control malfunctioning. He said he would check the car out today and let me know what he finds...I will keep you guys updated too.

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Update....so I started the car this morning and slowly drove it to the mechanic and I noticed that whatever speed I drove (did not drive it past 25mph), if I took my foot off the pedal the car would just keep going at the same speed until I applied the brakes.....just like when using the cruise control. The car is already 22 years olds so I figured there could be a possibility that the cruise control module under the driver's side panel is defective and/or perhaps the cruise control mechanism that controls the throttle when the cruise control is engaged is rusted out or somehow getting stuck. I mentioned it to my friend (mechanic) and he mentioned that over the weekend he had work on a car that had the same problem as mine and it was in fact their cruise control malfunctioning. He said he would check the car out today and let me know what he finds...I will keep you guys updated too.

Obviously a sticking throttle/...cable.

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  • 3 years later...
  • 4 weeks later...

I think your engine is staying on high idle longer than it should. Remember that the 400's idle at 1600 RPM's at cold start up and they stay on high idle for as long as 10 minutes as the idle slowly lowers to about 600. I have duel burnout marks etched into my garage floor where my wife popped it into reverse on high idle causing both rear tires to loose traction and deposit the rubber.

Now to your problem. I know a lot of people drive around with almost empty fuel tanks all the time. From experience (on the side of the NJ Turnpike & other places) that chronically running your car with low fuel can very easily compromise the life span of your fuel pump as they are only cooled by the gasoline in the tank. When the gas is low, much less cooling and much more stress on the pump. Use Top Tier fuels and keep the tank full and the fuel pump will outlast a car.

I couldn't stand buying gasoline in NJ. I really hate having some pump jockey touching my car. I was yelled at several times for filling my own.

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  • 2 weeks later...

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