94V8coupe

Wondering if anyone knows if the TPS (Throttle Position Sensor) has any input or effect on idle speed.
Yep, it's another '95 with low idle etc.

Discovered shortly after buying this car, the "well documented" low idle in drive at operating temperature / off throttle shock / general lack of "*****".
Not knowing how these cars generally behave, clearly I missed this stuff on the test-drive or it was being "good" that day.
The idle issue is pretty obvious, especially when it "vibrates" or "shudders" at idle. And sometimes even up at 600rpm. It will dip as low as 400 (for now).
One guy here described it perfectly. I'm paraphrasing but he said it sounded like he had a subwoofer behind the front fender pumping out a continuous deep bass note. Not pleasant at all. This was what drew my attention to the problem. When it idles properly ... it's dead smooth and dead quiet. The difference is huge.

Thanks to this forum I was able to diagnose it as a computer issue. I was lucky to find a replacement (updated part #) at a yard out of a '95. I'm assuming the guy that owned this car had gone through the same thing and replaced his at some point.
Swapped in the new ECU (that was fun) and voila! Problem(s) solved! Well, almost.
No more "off throttle shock", much more responsive from idle to redline, better general shift quality and rock solid 600 +/- 50 idle. For one drive.
After that, all problems were resolved except the low idle. Still doing it. Maybe even more consistently now.
I could probably live with it if not for that rumble / vibration / shudder thing. Almost feels like the timing is slightly off when that happens or one injector isn't being pulsed properly. Just a way of describing the sensation. I hate it. And I have read about guys cars stalling right out. Has not happened to me yet.

This ain't why you own a Lexus. The beater U-haul moving van I rented a month ago idled better than this.

So ... I was wondering, does the TPS have any effect on idle speed? I presume it must signal the ECU that you are off the gas which would in turn trigger the IACV to take over control of the idle speed.

I ask because I have had to replace the TPS on a few cars I've owned, they often develop "dead spots" just off idle (where they spend most of their time). I swear sometimes there is an ever-so-slight hesitation right off idle. Kind of a textbook sign of a worn TPS having seen that before.

Also, I swear she's using more gas with this ECU than the last one. It's noticeable. Figured I would mention it.

Spent a ton of time scouring this forum (and others) and to be honest I don't think there has ever been one definitive answer to this problem.
Most of the threads just kind of die out with no real resolution.

Arsenii

Hello,

Just to verify, did you check the condition of spark plugs, wires, distributors and coils?

For those cars, TPS has a special "Idle" position, it is basically a switch that detects when a throttle is fully released so that the car can go to idle. With that, the sensor has to be set correctly, here is a thread that describes the procedure.

Other than that, it will be hard to give any other advice without knowing what was already done to troubleshoot the issue. It would be helpful if you can describe all the steps that were made to find the cause, so that it will be easier to come up with something else.

Hope this helps and best of luck!

94V8coupe

Hi there, I hear what you are saying, but this is kind of a known issue and I have yet too see one person solve it by doing a tune-up or replacing ignition coils. Plugs, wires, caps & rotors were done 40 000 kms (25 000 miles) ago. In addition, if you turn on the lights, A/C etc., the car idles up just as it should and the idle is smooth. Tune-up parts and so-forth don't go intermittent, they're either good, or they're not.

I do remember seeing this somewhere else. That being said, maybe this off throttle "switch" could be toast or itself sending a bad (or no) signal. Just a thought.

I mean, in terms of troubleshooting, there are numerous threads here (before this was a known issue and even after) with guys throwing parts at the problem, cleaning throttle bodies, IACV's, replacing IACV's, plugs, wires, caps, rotors ... all of the obvious stuff you would start with and none of it fixed anything other than empty someone's wallet. Like I said, I swapped out the ECM for the updated version and it fixed almost all of the driveability issues, just the idle problem remains.

On my drive in to work this morning, first 10 minutes or so she was idling low, with the rumble/vibration/shudder that goes along with that. Even at the correct speed (600) it will behave this way.

After that, she smoothed out just like the Lexus ad with the stack of wine glasses on the hood. So this is an intermittent thing, more often bad than good. Plugs, wires, caps and rotors do not fail like this. They work or they don't (aside from crossfire between wires which I find highly unlikely here).

I did see an interesting post on another site where a former Lexus dealer mechanic said had replaced a ton of IACV's. He was saying that the magnetic coils inside them just flat out wear out and stop working, so proper idle speed control is lost. Cleaning it will do nothing, it just has to be replaced. What if said coils were oscillating the valve and causing that weird vibration / resonance because it's trying to hold idle speed but it can't?

Thank you

timmy0tool

iTrader: (7)

speaking of IAVC, here is a good link i always post about it:
https://planetsoarer.com/IAC/iac.htm

outside of that, have you looked into cleaning the MAF sensor? i didn't see you mention anything addressing that. it's typically part of a "tune-up".

Arsenii

There would likely be a CEL then, but it's easy enough, so might be worth a check.

Depending on how long you are planning on keeping that car, it might make sense to send your old ECU for repair as well, not as part of a diagnostics, but rather to have a spare unit just in case, those were known to fail..

Not always from my experience.. If there is an issue with the wire seal, corrosion can form on the contacts, causing some intermittent and very annoying issues. Wires also depend a lot on temperature and load when they are old, which can also cause them to work at one moment and not work at the other. The interesting thing is that such partial misfires can be really hard to notice on higher RPM, since there is enough inertia to smooth the vibration down a bit.

With that, now that you wrote about half of the system being replaced relatively recently, I doubt there would be an issue.

There are multiple kinds of IAC valves out there, one, like in MZ engine relies on magnetic coils to turn the shaft just right, here you can find more information about it, and while it is highly unlikely, there is a theoretical chance the coils can get saturated and fail, though I am not sure what you need to do to a car for that to happen. As far as I am aware, they don't fail that often, but please correct if I am being wrong..

In your case, however, the IAC valve is being actuated by a basic stepper motor, the more you turn it, the more you open or close the valve, during the rest of the time, it is sitting unplugged, since the power is only needed to actuate it, not to hold it in position. Below is an instruction on how to check the actuation of the valve by powering its coils in sequence, moving the valve one small step at a time.



Next thing to do would be to find a scan tool and start checking Engine Live Data, those cars should already be equipped with OBD2, so might as well use it. Look for Fuel Trims, misfire counter if it has one, Ignition Timing, as well as MAF sensor data.

Hope this helps and best of luck!

94V8coupe

Excellent post, lots of good info there. Unfortunately mine is of the "non rebuildable" vintage. Does not apply to my car unfortunately.

I do not consider that part of a tune-up. Don't mess with the mass-air meter unless you absolutely have to. Mess it up, and you're parked until you get a new one. That being said ... I may give it a go. After other avenues have been exhausted.

94V8coupe

Good point.
You read my mind. Except the old one has the "old" logic that results in "off throttle shock" etc. I may just get the caps and do the work myself. Just to see if it makes a difference.
Another good point.
Sometimes you just get a bad batch of parts, or it's a bad design to begin with. Happens all the time. Power steering pump ECU LCD displaysEGR pipe
I just found it interesting that (so-called) Lex mechanic said he saw them crap out all the time.
Are we 100% sure of that? I know the design changed in '95.
Looks like it.

Arsenii

That's going to be a tedious one, especially if you have never done it before.. In any case, it all depends on the cost for a shop to do it. Make sure to read through this post before getting your soldering iron out, there is a lot of useful information in there.

Well, sometimes it's simply impossible to make something last a lifetime with a lever of technologies available at the time. Any new technology will come with a risk, but if you don't take that risk, you will never evolve past a horse. With that, said IAC valve is nothing new, it's so simple that there is not a lot to break in the first place..

If you have that big hunk of mental on the front of your intake manifold with a tube leading to the intake and a huge connector for 6 pins on it then yes, I am about 98.7% sure.

Hope this helps and best of luck!

94V8coupe

Already read it. And I am pretty handy with soldering and de-soldering (restoring old stereo gear). This is the early ECU with the problems ... they are a dime a dozen on ebay. Now the "good" one I am currently running, that's another story. Rare and expensive. I've been in there already. It's 6 electrolytic capacitors. Not a big deal. Just awkward. No leaking caps (yet) on either ECU so the traces on the boards are clean.
Yet they do fail as I have replaced them on other vehicles. It happens.
This is what got me thinking about it: https://www.justanswer.com/car/9ol0w...currently.html
You misunderstand. What I'm saying is the design of the IAC changed to a new version in 1995. I am aware of it's presence on the intake.

Arsenii

No, I understood it perfectly the first time, what I was trying to convey is that the core principle behind its operation, as well as the way to check it remained the same until it was eliminated altogether with an introduction of VVT-i in 1998.

Hope this helps this time and best of luck!

94V8coupe

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