thebes

My 2000 GS300 is surging while idling, going form about 600 rpm to 2000 rpm every couple of seconds. Anyone experience this with a G2 GS300? I'm about to start replacing throttle body sensors, but I'm not sure which one to start with. Any suggestions?

MedicalDoc

My 2002 GS430 was doing the same, specially going down an incline. Turned out to be the MAF sensor was bad, probably due to a long-standing exhaust leak.

thebes

A few more details might be helpful. Before the idle surge started, I was getting codes p1120 and p1121 and the car was in limp mode. These codes are for the accelerator pedal position sensor (APPS). I checked all of the throttle body sensor resistances - all were within range. Then the idle surge started. I pulled the throttle body, and cleaned the carbon sludge. Checked out the APPS and TPS (throttle position sensor) - they looked ok. I haven't pulled the throttle control motor yet. The throttle body is back in, but the idle surge is still present. I cleared the codes, so no codes present at the moment.

I don't see any big vacuum leaks, but I'm not sure how to measure vacuum with the engine surge. Can't drive the car with the engine surge. I tried to back out of the driveway - left some nice tire tracks. The car is not driveable with the surge. (So maybe it's more than just an idle surge).

My OBDII reader is a simple handheld - just gives codes. Any recommendations for a more thorough OBDII reader (read voltages, throttle valve angles) would be welcome.

MedicalDoc

That's more than a surge!! Remove the MAF and put the multimeter on it.

thebes

MedicalDoc, thanks for the suggestion. I was concerned about the MAF sensor also, so I cleaned with the CRC MAF solution. Didn't make any difference. I haven't checked the resistances on it yet... probably should do that.

This surge is quite severe - it seems like something is telling the throttle to stay open. Then another circuit kicks in to cut the crazy racing of the engine, then it loops back and does it again.

MedicalDoc

BTW, a vacuum leak would make an otherwise quiet car sound like you bought the 'muffler delete package'. Definitely not one of the Lexus' options.

vijayn

Have you checked the signal voltage of the pedal position sensor? It has two sets on sensors in one unit mounted on your acclerator pedal.
Both should rise in voltage smoothly as you press the pedal further down slowly with ignition on. They both start off at different voltages and end at different voltages.

This wiring diagram and pin out data should help you test it.

Also check the throttle position sensor after the APP sensor checks out ok.

vijayn

For a good cheap obd scan tool, try Torque Pro with a good Bluetooth obd2 dongle. Some china made ones work great, just make sure they support the protocol your gs300 uses and is V1.5 NOT V2.1.

You can get live date, codes/ erase. and other functions

thebes

Vijayn, thanks for the suggestions, and that awesome wiring diagram!

Here's my plan. Since the Gen1 cars solve the idle surge by replacing the IACV (idle air control valve), I'm going to replace the throttle control motor on my gen2 car. Gen2 cars don't exactly have an IACV, but the throttle motor performs some (all?) of the same functions. Rockauto even calls this throttle motor an IACV for gen2 cars, but that's not strictly correct (going by the Lexus repair manual). I'll update this thread after replacing the throttle motor.

MedicalDoc

Good luck, Thebes.