I threw a code for a faulty TPS last fall, and my 4Runner wasn't downshifting to merge off of an off-ramp. I always by Toyota parts but needed a TPS fast, so I got one from Oreilly's which solved my issues. A year later I'm thinking it could be causing my 4Runner to have idle issues even though it's not throwing a code, and the throttle percentage reads 10% at idle and 72-75% with the pedal to the floor.

THE ISSUE: It starts and drives fine but when I stop in park sometimes my idle is at 850-900 indicated on the gauge and it won't go down. Sometimes I park and it sits at the normal 725-750 RPM range like it's supposed to with the AC off. Other times, more often after I use AC and turn it off before letting it idle, the RPM's will drop down to 575-650 and it almost sounds like a vacuum leak. After a few seconds of that it will go up to 725-750RPM for about 15-20ish seconds, then fall back down to 575-650RPM. Sometimes it does this sequence whether I us AC or not, but it seems to happen more consistently after I use AC and turn it off before letting it idle. There doesn't seem to be a pattern to the varying idle.

Even thought I'm not throwing a TPS code and it drives fine otherwise, could a non-Toyota TPS still be causing a varying, and random idle situation?

Things that have been replaced:
- It has a brand new Toyota IAC valve that I replaced 6 months ago because it threw a code. The throttle body and butterfly valve are still tight with no slop, or play. I obviously cleaned the throttle body at this point for the second time in 30k miles. A new throttle body gasket was used each time.
- New NGK spark plugs
- New AC Delco plug wires 36k miles ago from the previous owner
- Replaced coil last year around the same time as the Oreilly's TPS
- BG 44K treatment 2000 miles ago that helped cure a rough spot in the idle. Previous owner had all fuel injectors replaced at 201K miles. It has 237K now.
- Cleaned the MAF sensor when I replaced the IAC valve 5k miles ago
- It has clean air filter, factory air intake system
- There are no perceived vacuum leak because it runs fine other wise
- The coolant temp sensor is functioning properly and there are no cooling issues.
- I still average around 17.5-18.5 MPG

What do you guys think? Could another sensor such as a failing oxygen sensor be causing this?

To answer your question can aftermarket sensors not work properly. The answer is yes.

Best to use Toyota sensors. Now if the aftermarket sensor tests perfectly and in proper parameters. The aftermarket sensor should function properly in the rig.

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So try a oem tps and see if it solves your idle issues.

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For $50 I think I just might.

Beyond looking at what percentage the TPS reads at no throttle input, wide open, and everywhere in between, is there anything else I can test the TPS for? At idle does it send any other information besides the 10% position?

Thanks

The factory service manual has a testing procedure to rule out if TPS is good or bad..

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A faulty TPS will cause variable idle speeds.

Is your tps position staying at 10 when the idle changes?

When it’s acting up, if you restart the engine does the problem persist?


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Test it directly at the sensor output signal to PCM. The output signal should rise or fall smoothly with no spikes or weird fluctuations. I had one go bad that the PCM was reading good but was actually bad due to small fluctuations. And yes, the TPS is an IAC input.

I haven't checked the percentage while it was idling high, or while varying up and down. When I checked the throttle percentage a few weeks ago I can't recall if it was idling at the normal 725RPM or not.

Today it was idling high when I got home, and fluctuating between roughly 825-900RPM looking at the tach. It doesn't sound as smooth while idling so I'm wondering if the TPS and IAC are disrupting one another?

We just bought a 2013 Prius that I had to order parts for from McGeorge, so I figured I would just get a new Toyota TPS. Even if it's not the problem it eliminates one more variable and my OCD won't be satisfied until I have a Toyota TPS installed. We shall see...

You should watch it when the idle screws up.

Generally speaking at key on before engine run whatever the pcm/ECU sees from the tps it considers to be idle.

When it acts up you could shut it down and restart it. But you should see what the tps position is doing during your issue.

Edit. My one scanner shows both percent and raw sensor voltage.


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The only other wire coming off the TPS, other than the throttle valve output voltage, is a direct wire to the vapor pressure sensor that is spliced to also send a signal to the ECU. I'd be lying if told you I knew the relationship between the two but it could be something to look into.

Also 10-13% is acceptable at idle. Mine's at 11% because I manually adjusted the throttle valve to be open a hair bit more. The FSM says that any value at full gas pedal over 60% is acceptable. Doesn't make any sense but that is just how it works. Mine also shows a value in the 70% range.

Well I installed the Toyota TPS and my issues are fixed. It idles great and consistent with factory specs. This confirms why I always prefer to go with Toyota parts when I can.

Thanks to all who chimed in on the thread!

This is why most members say OEM or OEM mfg parts only. Look at the time spent because the aftermarket part is not up to spec

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Yep, I get all of my parts from McGeorge. Now that I now live in Virginia they are even closer. At the time my 4runner was just about un-drivable since it wouldn't downshift. Definitely not fun while trying to merge into a highway. I didn't like it, but I went with Oreilly to get it "fixed" quick. I've had zero issues with the factory part over the past few days👍