cyfi66

Has anyone else experienced this? Seems to happen more and more often and more so when cold outside.

Accelerating normally/lightly from a stop, around 20 to 25mph with foot on the throttle steady the car feels like I just let go of the gas pedal and loses power. Once I push my foot further into the throttle it accelerates normally again. Its like from 0-25mph the throttle takes very little input to accelerate and then switches to another mode that requires more throttle input for the same amount of acceleration. The problem is the car feels like its dieing out when it "switches". If I remain on the throttle steady even after it switches the car just cruises at the same speed without accelerating.

I tried connecting to Techstream and monitor the TPS to see if there was a dead spot anywhere but signal was good throughout the range. It only happens occasionally and usually before fully warmed up.

1999 LS400

YODAONE

What is mileage or kilometers on your 1999 LS400?

What is service history?

Have you cleaned your throttle body and air mix paths?

Are your fuel pump and fuel filters (one external filter, second in fuel tank) original?

MAF sensor is original? Perhaps cleaning required. (MAF sensor cleaner)

What is condition of your alternator, battery, positive terminals and negative grounds?

Oxygen sensors?

Yamae

Sounds like an initial state of the soldering crack problem of an APPS (Accelerator Pedal Position Sensor) to me. Click the link below and see the right side of the photo and see soldering cracks indicated by arrows. At the initial state, error codes are not stored because of the too short time length.
https://cdn.snsimg.carview.co.jp/min...t=e535dc9d853f

If so, you will have P112X error code as you drive more when cold days. I had this problem and it was needed to confirm using a plastic hammer and an oscilloscope. Hitting the APPS using a plastic hammer, I found that the waveforms of both pin #1 and #2 below showed glitches synchronizing the shock .

It is an often case that there happens a soldering crack among printed circuit boards with connectors that has a different CTE (coefficient of thermal expansion) when boards had heat-cycled many times. The left photo in the link is another example of this. Toyota's APPSs used at that time (late 20th century) have this type of problem commonly when aged.

Replacing an APPS fixes the problem but there are some people who fix it just resoldering cracked points although it's not very simple. Because you can't expose the soldering points easily. You usually need to brake the plastic housing to do so. There is a simpler way to fix but it's not for everyone. Only soldering skilled people can do it.

bradland

Listen to Yamae, he's rarely wrong.
Also, check for any slack in the throttle cable. After 20 years the cable will certainly stretch. See pic, any excess slack by the red line should be addressed. The adjusting nuts are between the bright green dots.

cyfi66

Throttle body was cleaned about 10k miles ago (wasnt that dirty) and the top cover was removed so I could clean the air mix paths as well - also not really dirty. Fuel filter is probably original but I do have the service history I could check. Is this a "lifetime" filter according to Lexus? If so they probably wouldnt have performed that replacement.

Good idea on the MAF, worth cleaning. Alternator and battery are both pretty new and all the terminals are clean. I could do a volt drop check on the engine block to ground though.

The car has 250,000 miles.

Excellent info. Can you elaborate on how you hooked up the scope? Was it between pins 4-2 and 4-1? I am assuming pin 4 (or 3) is a reference voltage. I have a scope I can use to check that.

Any experience with aftermarket or is OEM the way to go with this part?

Superfast1

Cify66- He hooked up the scope probes directly at the pins. You can leave the ground clips on the scope probes hanging (open) or clip them to any metal ground nearby. If you have two scope probes, you can probe one at pin 2 & the other at pin 4 then tap on the sensor lightly to create the mechanical shock to the sensor. If there is/are solder crack(s) at any of the pins, you'll see the signals breaking up on the scope's display.

Yamae- Based on the schematic you've shown, the APPS consists of 2 potentiometers (variable resistors) in parallel with the 2 center taps at pin 2 & 4. Does the accelerator pedal ties directly to these 2 pins? If so, how? Also, is this sensor only used on 98 to 00 LS since I've never heard of it being used on 97 or older model?

Yamae

Pin 3 is connected to the ECU's signal ground. I always use nickel plated sawing needles to pick up signals inserting them to the insulated wire. After the measurement, I remove the needle and apply some silicon RTV there to avoid the moisture. When I monitored signals, 2 oscilloscope proves were connected to pin 1 and 2. The ground tip was connected pin 3. After confirming the quick voltage drop and up, I stopped the engine and removed the connector to measure resistances between pins. The resistance between pin 3 and 4 was 2.2k ohms but as I hit the APPS, it varied some and it was not stable. But the resistance between pin 1 and pin2 was stable. This simply meant that the soldering point of pin 3 or pin 4 is not OK and I thought that I needed to replace the APPS. Problem was it was not available quickly and also it was rather expensive.

I have variety of soldering irons and the biggest one is a 200W one. My idea was to resolder those 4 pins one by one applying the heat to them each without braking the APPS's plastic housing. I had a similar failed unit with this APPS presented by a local dealer manager and I did some experiment whether it is possible to resolder connecting points without exposing them. I applied a copper tape to each pins not to solder them by the 200W soldering iron. The copper tape is used only to use pins to protect not to be wet by the solder but the heat of the iron is good enough to conduct to the board and melt the solder there. The result of the experiment showed about 50 seconds to start to melt the solder. With this result, I tried to resolder my 98 Celsior's APPS.

It was quite successful. After that, there was no more glitches at pin 1 and 2. About 3 months have past since then and I don't have any more problem. My Celsior is running beautifully. With this result, I didn't need to buy a new APPS and I don't know about the replacement unit.

There is a guy who fixed the APPS braking the plastic. This APPS is for a GS/IS 300 and is a bit different from a one for LS400 98-00.

cyfi66

Connected up the scope and checked all the signals. I didn't really see any changes at first when tapping the mounting screws with a hammer. I did see some weird signal noise every now and then on pin 2 when rotating the throttle and then when tapping with the hammer the signal did look like it was dropping briefly but it wasn't consistent and only now and then. I used my high powered soldering gun to heat pin 2 for about 45 seconds. It seemed to burn the coating off the terminal and was starting to melt the plastic so I didn't go any longer. I didn't see any more noise or dropping out on that pin anymore after that. Didn't feel any loss of power coming to work this morning but will keep driving for a few more days and give an update.

Losiracer2

I would replace the coolant temp. sensor, it'll make a big difference in driveability if you still have the 20+ yr old one in there. They can be purchased at Oreilly (Import Direct brand) for $17-$25 or at the dealer for $40-50.

cyfi66

Well ever since I resoldered the one questionable pin in the APP I haven't had this issue happen at all. It would normally happen every morning. Fingers crossed!

Yamae

Thanks for the update. Seemed like you had the same problem with me. If the problems were completely gone, it would be the soldering issue.

In my case, the problem stated last October when the weather became cold. After the resoldering, there's no more problem also there's no more glitches when observed using an oscilloscope hitting the APPS by a plastic hammer. For the duplication and the confirmation, I hit the APPS too many times and finally I broke the part of plastic housing some indicated by a red arrow like this. There are happening similar problems at my local shops in this cold winter days and I always try the resoldering agreed by customers and shop owners. So far every of them does not show the problem any more. I begin to think that this is a common problem among old Toyota cars. Those APPSs cost $350 to $600 and the resoldering can help you to save some money.

Superfast1

Cold solder joint is a common issue when it comes to electronics.