TheBaron

My '98 SC300 has had a bumpy idle for a few months now, and sometimes will idle around 350 RPM, so I went ahead and replaced the IACV thinking that that would do the trick as the car still had the original one with 260K miles on it.

Unfortunately the car basically refuses to idle now. CEL codes P0120, P1125, and P1128. I know these are all TPS-related... Does anyone have any insights as to if this situation is more throttle position sensor related or something like accelerator pedal position related?

I am a relative novice and any insights would be much appreciated!

Duck05

Could be your ECU has failed and the leaky capacitors and failed traces on the ICU boards cause all kinds of issues.

Best to remove the ECU and send it to a rebuilder. Will take a week or so.

The throttle body should be cleaned and the MAF sensor be examined.

TheBaron

First off, thank you Duck for your quick response! I'm so sorry I didn't get back here sooner- the past few months have been a bit hectic with a cross-country move and I just haven't had time to really dive into this.

I can confidently rule out the ECU- it looked great upon visual inspection, and apparently the capacitor issue was resolved by the time the '98s reached the lot. In case you have any doubts, I was able to source an identical replacement ECU, swap the EPROM security chips, and the problem remains the same. The MAF sensor was cleaned using the specialized CRC MAF cleaner (I've read elsewhere that these cars are notorious for MAF problems and they can't be cleaned and simply need to be replaced) and when I took the throttle body off to change the spark plugs (painful how they designed it that way...) I gave it a good cleaning.

Either relatedly or unrelatedly (it's gotta be one of the two!) my speedometer and odometer were going in and out together. To address that, I replaced the Vehicle Speed Sensor (VSS). Funnily enough, that made my idle problems worse and now it idles at literally 50-100 RPM and will stall out as fast as you will allow it to.

Next up was the Throttle Position Sensor- replaced that. A mistake I made when doing the VSS was not disconnecting the battery for ~1 hour to let the ECU do a full reset, so I made sure to do that when doing the TPS. Sadly, the problem remains unchanged.

I'm quickly running out of ideas, and still rocking the P0120, P1125, and P1128 codes. I found a thread about how to test the Pedal Position Sensor (PPS) so I'm going to do that next? But I'm not really holding my breath...

The attached video is from a few months ago when I was able to keep the CEL at bay, and shows how the car loves to 'hiccup':

Ali SC3

iTrader: (10)

98+ sc300 should be vvti, which means the idle control is part of the electronic throttle body unit, so that is likely where the problem is.
I would try and swap a known good one and see if that helps, they are pricey as they are harder to find in good working condition.
People have different problems with them they weren't the best tech ever, look in the IS sections there is probably more info over there and engine is the same vvti version.
The tps and some other sensors are all part of the vvti throttle body, but it could also be an ecm issue although you seem to have ruled that out.

You can clean the 96+ and 98+ mafs with maf cleaner as they are regular hot wire type mafs, just let them dry out completely. The 92-95 you should never use maf cleaner on as they are karman vortex type mafs.

TheBaron

Ali- it is vvti, yeah!

Well that is encouraging- quite literally the only thing left at this point is the throttle body lever sensor (essentially the pedal position sensor) so I now have hope that addressing that will do the trick. There are some videos about how you can fix your existing one by resoldering the 4 traces on the inside, but I also found a genuine Toyota one for ~$180 to my door, which isn't too bad and worth every penny if it fixes the problem.

I will let you know how it goes! Again, thank you!

P.S. and thank you for the tip about the MAF- great to know cleaner can work.

TheBaron

So I replaced the Throttle Body Lever Sensor (what would be the Pedal Position Sensor in another nomenclature) and that did the trick for the worst of my problems- cleared the CEL, idle is back to its "normal" self, and the gas pedal is way sharper and responsive now. So that's good!

What remains are my odometer and speed sensor going in and out together- maybe half the time they will work on a cold start, but inevitably go out after 2-5 minutes of operation. I have confirmed it is not the Vehicle Speed Sensor, which I was really hoping would knock that out.

Lastly, the car still hiccups! My latest theory was that it was an O2 sensor on its way out that was causing a misfire (which I have been referring to as a 'hiccup'). I got out my multimeter and tested the resistances on the 3 O2 sensors whose pigtails I could easily get to*. All of their heater elements had resistances of 13.5 Ohms +/- 0.2, and while that value is higher than the 5-6 I was expecting, the fact all of the readings were so identical make me think they can't be bad. Granted, one of them could be on the way out and I just have yet to catch it in the act of failing. The hiccups are only present on cold starts maybe half of the time, and typically after several minutes will go away entirely. Furthermore, the hiccups have a spectrum of intensity: sometimes they're more of a shudder as you feel the car downshift, sometimes they're so light you can only catch it by staring at the tachometer and in those instances the CEL doesn't even flash (most of the time the CEL does flash during the hiccup).

As always, if anyone has any ideas as to what could be going on here, I am all ears!

*I was able to hunt down the 4th O2 sensor, located in the exhaust about halfway down the passenger side door. The wire for that sensor just disappeared straight up into the frame rail, though, so I have no idea where its distal pigtail end lies and can't test it. If anyone knows where that bundle terminates, I would love to test it!

TheBaron

For reasons unknown to me, the car has once again thrown P0120, P1125, and P1128 codes. "Pedal/Throttle Position Sensor," they say. The problem? The Throttle Position Sensor (TPS), Throttle Body Lever Sensor (basically the PPS), and the Idle Air Control Valve (IACV) are all! brand! new! How can a circuit possibly be bad if every part of the circuit is a new part?! I'm completely flummoxed right now.

Playing devil's advocate against myself here, 1) I did not use genuine Toyota parts for these replacements (I'm not in that tax bracket yet) so I suppose one could argue the use of third-party parts has bitten me in the butt or 2) perhaps a wire has chaffed/worn through and shorted such that even if the sensors are fine, the connections are not so it's throwing the codes? That's the best I can come up with.

I'm definitely at the point of calling in the professionals, but I haven't found a local mechanic that I trust yet...

Duck05

If the ECU was replaced (as indicated above) then it may take an experienced electrical mechanic…

TheBaron

Hi Duck! Unfortunately I've already sourced an identical ECU, swapped the EPROM security chips, and even then the symptoms are still identical. Ergo, I feel confident it isn't the ECU...

Frankn

Check the circuits and see if the power and ground is the same for all those things. I bet they are. Then watch supply voltage while idling on the positive and ground. Should give you a direction to go in. Could just be a dirty ground. If it's the power you could bump things around and see if it makes it worse or better.


What area in CA are you in? I'm in Mendo