Michael88

Some of you have seen me asking questions about intermittent starts. No codes, cranks fine, turn the key on and I can hear buzzing sound from the throttle motor. If I unplug the accelerator position sensor, disconnect the battery, reconnect the accelerator position sensor, reconnect the battery; then the computer goes through a reset power up and it will start just fine. I thought this was solved when I fixed a few simple vacuum leaks, O2 sensor, MAF sensor, cleaned the air/ fuel mix path. prior to doing these things it would not start after about 5-6 trips. After doing these things it made about 60 successful starts and trips, until it happened again today. No codes seen and I checked before disconnecting anything. I am leaning toward buying a used throttle body and replacing the whole thing. Any help is appreciated.

bradland

Michael88

1. I am referring to the accelerator position sensor (the spring wheel looking one that the accelerator cable is attached to)
2. I have not replaced the TPS (opposite side of the accelerator position sensor) because there isn;t a TPS code being thrown.

Yamae

Are there any differences of the sound level between when the engine start OK and not?
In other words, does the buzzing sound only loud when you can't start?

Michael88

The buzzing sound is only when it will not start.

Michael88

Purchased a used throttle body and a new transmission mount. Got all for under a hundred bucks, so I figured why not. From my research, both of these can cause starting problems and not throw codes. My LS400 vibrates a bit at start and from 45 mph to 60 mph. This leads me to believe wrong fuel mix (sticky throttle motor) and worn transmission mount ( easy change that should be done considering 203k miles without ever changing it.)

Yamae

The buzzing is the result of the unstable servo control which is organized by 4 major key parts. The throttle body has three of them but other than those, there is one more important part. I hope it's not the last one.

Michael88

What would be the last one? By the way, I had to return the used throttle body. The guy was advertising a 98 ls400 part number, but sent a LX470 throttle body. They aren't compatible. Looked the same though.

bradland

I drove my 99 for 8 years and didn't know the pedal position sensor existed!
Have you checked the throttle body sensors with a multi meter?
If you haven't already seen it check the attached PDF from the Svc Manual

Yamae

It is the one which controls the servo loop = ECU.
The ECU checks other conditions together with sensing signals from the throttle body, you need to know.

Michael88

Accelerator position sensor checked at 5 volts. I didn't check the throttle position sensor, because it is tucked down below. Neither have thrown a code. Thanks for the attachment, I needed that. Wow, this pdf is huge thank you so very much!

Michael88

So, I replaced the rear transmission mount, today. The old one so really bad, and was broken in half (top half and bottom half.) The new mount lifted the transmission at least 3/4 inch to 1 inch higher. There was that much "sag" in the transmission. ALL of my shuttering is gone at start up and at 45-60 mph. The improvement is amazing. On the highway at 80 mph, the steering wheel used to shake like the car needed an alignment. The is NO need for an alignment. The new transmission mount removed ALL steering wheel shake and vibrations. I must share this with the community. 1998 LS400 with 203k miles. I am hoping this was the cause of my intermittent no starts. I am now entering a trial period before buying a used throttle body or ECU.

Michael88

Okay, so I got another intermittent start. I went through the service check above that Bradland posted. Awesome , dude btw. All the Ohms checks were within range, with the throttle motor clutch reading right at the upper limit of 5.2 ohms. Another check was to listen for the throttle motor running with the key on/engine off while turning the the accelerator. You should hear it, but not hear "friction sound." I heard it and it sounded like a heavy winter coat zipper being pulled up and down. I think this would be considered "friction." Also, the car is to idle at 750 rpm +-50 while in neutral. I get 1100 rpms one day, and then 750 rpms that afternoon, then 1050 rpms the next day, and 675 that afternoon. I am now shopping for a used throttle motor body. Please, let me know what you think.

Michael88

I have no Lexus hand held, So, I measured voltage between terminals VPA and E2 of the ECM connector as the instructions show above, and got zero voltage with the accelerator fully open and closed. I don't know what this really means yet, because the instructions do not specify. On a flip note, I tried unplugging the ECM to see If I could reset everything and plugged it back in. Winter is coming and I thought I might be able to do that instead of disconnecting the battery in the cold when I get an intermittent start. I also tried starting the car with the ECM unplugged just to observe the results, and that car would crank but not start. Importantly, this is exactly how the car behaves when I have an intermittent no start. I plugged it back in and the same no start resulted. I disconnected the battery and she fired right up. May not be relevant, but I thought I would share.

Yamae

It is strange that you don't get any voltage when you measure VPA. As long as you measure correctly, you should get some voltage. The voltage source for VPA is also for VPA2, VTA and VTA2 and is the 5V regulated DC from the ECU.

You'd better to measure those rest of three and if those also are 0V, it simply means that there is no power supply. If only VPA shows 0V, you need to check the accelerator position sensor and the wiring.