Ok everybody, Long Time Lurker, decided to register and see what others are thinking.

Sometime Aound Oct, My GS400 (1999 - 190K) Went into Limp Mode while heading to lunch. I did a lot of research on the web and found some great info on these forums. I decided to follow advice from different threads and follow through with some of the advice.

So far here is what I have done/replaced:

1) Accelerator Position Sensor - Bought a Lexus part off of Ebay, and swapped it first thing. After swapping it, it ran fine for about 4 minutes and then went back to Limp mode.
2) TPS - Swapped it out and still the same problem. (It's not calibrated but I have messed with it to get it close)
3) Replaced the Throttle Control Motor (Electric Motor that runs the Throttle)

At this point my dad came into town and we tested various impedance on all of the parts. The Original Sensors and motor were all within Spec. The Replacement Parts are all within spec. We tested some of the pins on the ECU and the voltage was low. (Can't remember what pins exactly off the top of my head.

I have purchased a VSC/ABS ECU I plan on testing this weekend. (I know its probably not this, but figured I would try as it was cheap.)

Basically at this point the car runs fine for the first 2-5 minutes (The VSC Lights are on the whole time) and then it goes into Limp Mode. It -Seems- like something is triggering Limp Mode after a few minutes.

I know it could be the MAIN ECU (As the Voltages were not in spec) but It seems weird that it would run fine for a while and then stop. Other interesting things to consider:

The Front Drivers tire wobbles under Braking. I think its a worn rotor, but need to take it apart and check (As well as the ball Joints/etc.) The Brake booster makes a "squeaking" sound audible from inside the cab during normal operations. (I believe that means it is going to need replacing?) I changed to some oversize Rims and Tires. They Rub at full turns. I got them together for a steal and decided to change to a smaller tire profile when these run out. (They don't usually rub during my commute.)

Anyway, I am open to thoughts. Anyone gone through anything similar? Thanks in advance.

 

Out of this long read, I got no info. No mention of symptoms, no codes posted

Can you post what exactly is happening, "limp mode" doesn't say much. And please post codes, there has to be a code if you're replacing so many parts.

 

Ok.....sort of all over the place here. Lets start with the basics......what are the codes?

 

I apologize, it didn't seem all over the place until I went back and read it.

Original Codes before swapping out the APP: P0120 & P1125

Current Codes: P1125

Limp Mode = No pedal until the cable opens the Throttle Body (last 10% of the pedal) Also the Transmission shifts into gear roughly (park to Drive, Park to Reverse) Not so much when the car is moving, just the initial engagement of the Trans.

Currently when I start the car the VSC and VSC OFF light come on quickly. The Pedal will function properly for about 2-3 minutes until Limp Mode kicks in. The CEL is currently on.

I have read through this Thread in its entirety: https://www.clublexus.com/forums/per...p-mode-17.html

I was working through the list located in post #245. Trying to Diagnose it.

When we were testing the ECU Voltages, we were testing Voltage between terminals: VC and E2 and also Voltages between: VPA, VPA2 and E2 on the ECU. I don't remember which one was out of tolerance, But it was only one of the connections. (I will have to do testing again to confirm.)

 

Check your wiring from the the sensors on the harness. Trace them as far back as you can. Maybe bad wiring. To set your TPS. I used a scan tool with throttle position percentage showing. You can find the procedure on the forums here.

https://www.clublexus.com/forums/gs-2nd-gen-1998-2005/618147-98-gs400-idle-speed-adjustment.html

 

It seems as though the throttle motor is on the way out. I would verify the TPS is set properly and then go from there. The key is that it runs fine for a couple of minutes.......

 

I replaced the Throttle Motor, and have tested both the original and the new one. They both were within tolerance for acceptable. That's what throws me for a loop! I also rigged up a connection to make sure the clutch is working and both of them do.

 

OK....do you have access to another ecu to try out? Ive seen something similar and it was a damaged ECU!

 

Check your wire harnesses, all 3 (from the motor, TPS and APPS). Since worse case scenario isn't common, I'm willing to bet a plug at the sensor is damaged and the wires are only making periodic connection. I'd start with the APPS. Its possible that whichever of the 3 you replaced WAS the culprit, but when swapping it, the plug got damaged during the process. Our cars are old and those plugs are very brittle. Also, doesn't hurt to throw some of that contact jelly in there

 

No, I am reluctantly trying to avoid that reality. I figure if the VSC/ABS ECU doesn't fix it, that is the next logical step. If I do end up replacing the ECU and buy one that comes with a master Key, can I eventually reprogram it to work with the 2 original keys I have? Or will I need to replace Those Keys with the New Chip? (I am assuming I can keep the same Laser Cut Portion of the key that fits my ignition and just swap out the head with the chip in it correct? ) I haven't found a clear answer to that question yet.

 

Is that the KY Warming stuff?

 

Yes you can swap out just the chip in the key if needed.

 

+1 on this. The wiring and connectors get so brittle with age. They literally fall apart while removing them. Check the pin openings where it makes contact with the pins on the sensors. Sometimes you can see some are more open than others.

 

I had almost the same issue many years ago, I had to look at my vehicle history to actually remember it. My '99 GS400 went into limp mode with CEL/VSC/VSC OFF illuminating. I was able to limp the car home and take it to the dealer the next day. They replaced the TPS and sent me on my way....but I only got about a block away before the car went into limp mode again. Here was the diagnosis (and eventual fix):

 

Naw. OP changed the only 3 components on the throttle body that need to be changed, TPS APPS and Motor. The dealer ALWAYS goes for changing the entire assembly rather than testing each component, they make more money that way. OP pretty much changed the entire TB. His symptoms are electronic, not mechanical. He could have a hole in his TB, and it would still drive fine. IIRC, the only moving part in the TB is the butterfly, and its moving just fine. In YOUR case, through the laws of causality, it was fixed. If you have a faulty valve in a cylinder head, you can change the valve or you can change the entire head. In both cases it gets fixed, but swapping a valve would be much cheaper, capisci?