Pensioner2

My LS 400 may have developed a problem. The car has 368K miles on it of which I have driven 260K of them. To me the car drives fine. No check engine light, 30/31 MPG per tank of gas @ 65mph on a flat dry WI or IL freeways if I behave myself and use the cruse control. The car idles at 600rpm, is carefully maintained and the Denso IK 20 iridium plugs have done 25K miles. The motor uses about a quart of oil between oil changes which are every 25K miles (Amsoil 0W 30 with Amsoil filters).

The issue is this. I drive the car every day and think it is fine, but sensitive. When I start off from a stop sign if I am not very careful the car will leap forward when I start off. I am careful but occasionally passengers comment about the car surging from stationary.

My 25 year old daughter recently drove the car and commented that it leapt uncontrollably from stationary and that she would never drive my Lexus again. She has driven about 5K miles total in her life to date. Her car is a Ford Focus.

I took the car to my local Lexus dealer and explained the issue. Their young tech took it for a test drive with me. He commented that the car was a powerful V8 and that it did not seem to have any problems, in his opinion.

I need the expert input from a person or persons who understand in detail how the engine management system works and exactly what is happening in the ECU when the car changes from a warm idle to start moving and possibly how the torque converter relates to the ECU.

The issue is the same with a warm or cold motor on a -20F or a +100F day.

randal

On the 98, there is a pedal position sensor. I would imagine that at 368k miles it might be a little worn. Maybe a slight dead spot on it that you have to get past as you are pressing then it takes off. Could also be a sticky throttle body butterfly. Just a guess, I am not an expert by any stretch.

Yamae

I know some about the ECU control but I need to confirm some more before I comment more.

You wrote that the car idled at 600rpm. Was it at D or P position of the AT shift lever?
When was the last time when you or some one cleaned the throttle body?
Did you also cleaned the air mix path then?

Models after 98 don't have the IACV and the idle control is all done by the opening of the throttle valve controlled by the ECU. When the throttle valve and the body is badly clogged and it is out of the range, the ECU is confused and behaves as is not expected.

If you have never cleaned those for the past a few years, I suggest you to clean them well first.

BWLS400

The mileage on your car gives me plenty of hope. For what it's worth, if it hasn't been changed, the main temperature sensor on these cars controls a lot of its driving behavior. That's just my $.02 from searching and seeing what differences others have seen by replacing it. I hope you get it all worked out!

Pensioner2

The car idles at 650rpm in P and 620rpm in D. The throttle body was cleaned two yours ago by my local garage. They commented then that it was not dirty. Their cleaning effort had little or no effect. Do you have any instructions on cleaning the air mix path?

Yamae

It is quite unusual to have 650rpm in P. It should be 750 rpm sharp precisely because the ECU controls to be so. Something must be disturbing or blocking the path or the ECU or some others are out of the range. On the other hand, 620rpm in D is strangely a bit higher than normal.

Air mix path should be cleaned all small 8 holes and 2 larger holes by the pressurized solvent when the engine is idling+. You need a full can of engine conditioner to clean 2 larger holes. Sometimes, 2 cans are needed according to my experience. It takes time to clean all 8 passages to injectors and you need totally 10 minutes or more to do this job.

Yamae

The chart below might be useful for you and some others. This is only for models 98-00 though.

Kram

I think that you may have a worn throttle body shaft I feel this way because the problem does not go away or return when engine temp changes to check this problem just spray wd 40 on the throttle body where the spring and wheel are located if the rpm changes you have a worn throttle body and you will need to take apart and pin the housing to shriek the hole where the shaft goes though the body you need to do the pinning with the shaft in place unless you have the proper size drill to hone out the hole to the shaft size and at the same time make sure the shaft is not the problem by removing it and using the unused part of the shaft to check for out of roundness

Pensioner2

Have you seen the throttle body on a 1998 LS 400?

dicer

You need someone with a good scan tool that can verify the conditions at the time this happens. If not everything is pretty much just guess work. I think these cars are flyby or rather drive by wire as far as the throttle goes. Also they are variable valve timing. Any anomalies happening in these and or other systems could cause the problems. Even something like some material flaking off a torque converter lock up clutch, but that would be more pronounced I'd think. I tend to agree with what randal said about the pedal variable resistor if that's what they use, they will wear out and have dead spots like he said. The scan tool would see that. A quick resistance check would also.

Pensioner2

I checked the resistance of the throttle pedal position and the throttle plate position sensors. They were within the specifications in the manual. The throttle pedal position sensor has a pair of pots. Their resistance changed in a linear fashion when I rotated the throttle shaft. My next step is off to the diagnostic scanner.