Peacebay

I've noticed a ton of these VVTI 1UZ LS400 and 3UZ LS430 are prone to a low idle condition when in D, with all accessories, lights and AC off. My 2005 LS430 had this condition to a degree before I sold it and my current 99 LS400 DEFINITELY has the issue. When fully warmed up, in D, the idle reads ~200 on the tach and there is a non-zero vibration that comes into the cabin -- see above pic (this is AFTER replacing my motor and trans mounts). Actual idle rpm in Drive according to my scanner is 495-515 rpm -- still WAY too low. However when the headlights are turned on, or the AC is turned on, the idle raises to an acceptable RPM (650 rpm on the scanner).


This thread will set to document the solution to this issue that so many people seem to have.

These are the things that I have done:

• ECT sensor -- no change, but still nice to replace after 22 years and 230k miles
• Cleaned MAF -- no change
• Replaced all vacuum hoses & PCV (with new hose) -- no change
• Cleaned throttle body air passages using this method with compressed air and two cans of throttle body cleaner -- this DEFINITELY improved shifting and overall smoothness but did nothing for low idle
• Decided to check the long term fuel trims at idle. Bank 1 is at 5% and Bank 2 is at 11%. Almost assuredly an air leak, somewhere
  • Spraying carb cleaner around the engine with it idling didn't result in anything

This is where I am now, still thinking it's a vacuum leak somewhere. I just ordered a smoke test machine, going to start there.

Yamae

Do you know how to check voltages of VPA, VPA2, VTA and VTA2?

All of these should be within the range when the engine is off but the key is @ON/RUN position. Those voltages should be 0.3-0.9V, 1.8-2.7V, 0.4-1.0V and 2.0-2.9V each. When all of these are within the range, the ECU judges that the accelerator pedal is not pushed and the ECU adjusts the idle rpm targeting 750rpm after all the learnings are completed. Sometimes more than 300km run in the city and highway are needed for the learnings though.

As I help local shop owners, I find that aged throttle position sensors (ThPS) and accelerator pedal position sensors (APPS) are tend to be out of the range and the car does not idle within the targeted idle speed. In some cases, the problem is caused by the worn ThPS or APPS but sometimes mechanically incorrect installations or poor position adjustments are causing the problem. It is a smart idea to measure the voltage first. It would be preferred that voltages are the centre values of the specification.

Stroock639

in drive warmed up with lights and AC off the idle should be ≈500rpm, so if it was at 495-515 that's basically perfect... and in neutral ≈750rpm

years ago my car was doing this kinda badly at one point too until cleaned the air mix path using the Yamae method, that helped a lot and now once everything's warmed up it's basically back to 'LS spec' aka you can't even feel it running lol

at any point have you simply disconnected and reconnected the battery? this seems to help reset some of the things that cause an incorrect idle

Peacebay

Yamae! Cool to see you here, thanks for the advice. Admittedly I had never heard of those pins, but from some research I gather that they are found at the ECU. This is the only diagram I've found, do you happen to know the colors of those wires?



Is that true? Idle in gear should be at 500 rpm? Hmmm.. I still think something is off, and regardless my LTFT shouldn't be as high as 11% at idle on bank 2. And yes, I have reset the system every time I install or change something. It typically "fixes" it for a bit, and then it goes back to the low idle.


Thanks guys

Stroock639

i'm pretty sure it was Yamae himself who posted those numbers that i got the 500 from... as long as it's idling smoothly i'd say there's nothing to worry about, the idle speed can fluctuate a bit and still be considered 'correct'

Yamae

The red arrow indicates the APPS's connector wires, the blue arrow indicates the location of the ThPS which can't be seen because it is behind the throttle motor. Regarding the pink arrow, I explain it at the bottom of this post.


Wires are coloured like these below although it's for a JDM. So I added the English expressions.




The pink arrow at the 1st photo shows a screw which sets up the lower end of the APPS's output voltages. The rotation to CW increases the voltage of VPA and VPA2. I just want you to try to rotate this screw to CW direction some like 1 turn, 2 turns or 3 turns and inform me how the idle speed changes temporarily.

Regarding the idle speed of the VVTi 1UZ-FE is programmed to be 750rpm sharp when at P or N. When at D, it drops about 200-250 rpm depending on the engine's and AT's conditions. Too thick oil or or extra loads often causes more than 250rpm drop. When at the A/C on or headlights on, the ECU increases the rpm some.

CLLEXUSS

I only experienced this once with my 2000, I live in a mostly dry climate year round but the day it happened was slowing to a stop light in the pouring rain, and that was the only time it ever happened and so far hasn't happened since. It was a super low rpm and for a moment there I though my engine shut off it was so low rpm. Come to think of it, it was a while back, but it might have happened a couple of times that day in the rain. I'm wondering if it's going happen again once the rainy season starts lol. Quite the mystery here.

CLLEXUSS

Makes me wonder if some type of high humidity atmospheric condition is triggering some type of sensor somewhere, or maybe this is a conspiracy theory lol

RA40

iTrader: (6)

Weird unexplainable occurrences. I've had a few low idle situations though nothing that can be repeated. I was in town late spring which would be mid 60F temps, had driven about 1 mile to the stop light and was waiting for it to turn green. Pushed the gas pedal to find the car had died silently. Over the years I've seen it idle about 200-250 rpms even after it had warmed up but not to recur again. These have taken place after it had 100K miles on it and had been before and after the caps were replaced too. No real consistency for me to pursue finding what the cause was in those instances.

Might have been some odd EMP situation in the car electronics?

Losiracer2

yeah I had something similar happen in my 97 when it was super humid and foggy out the night before, really really dense fog and mist when I was driving through Ohio in the fall one time. I think condensation got in the distributor caps overnight and I threw CELs for 7 out of the 8 cylinders for misfire, car was idling and driving really weird. I cleared the codes and they didn't come back since.

Peacebay

Been crazy busy with work and a bunch of other stuff so I wasn't able to work on the old girl until recently. But feel like I need to update you guys for some closure on this.

Before I went messing with anything, I first wanted to figure out why my fuel trims were messed up. First, since the fuel trims were especially bad on bank 2, I replaced that upstream o2 sensor hoping that I was just getting faulty readings. No dice.

Then, I did a smoke test on her and found an air leak from a cracked crankcase breather hose (bank 2). Replaced the hose and retested the fuel trims. This time, they were both even (which is good) but still reading almost 10% on both banks at idle (bad).

At this point I'm at a loss. Then I decide what the hell, I'll just replace the MAF. So I bought a new NTK one from Rockauto, slapped it in, and BAM. Dead even fuel trims, smooth idle, and even smoother shifting. The low idle is still there, but apparently it's in spec anyway, and it's super smooth so it doesn't bother me.

YODAONE

Two questions:

1.).How much did you pay for NTK (NGK sparkplug) MAF Sensor?

2.) Did it include the rectangular rubber gasket?

Peacebay

It was around $70. But it didn't come with the gasket unfortunately

cyfi66

Mine always idles low, never could figure it out. Turn on the rear defrost and it comes up to a normal level. Been trying to think of a way to trick it into thinking the defogger is on just to keep the idle up. It idles a little rough when idling low. Also a little annoying that the car barely moves with foot off the brake at idle.

Losiracer2

I replaced my PCV system hose, valve and grommet and that seemed to help the idle bump up a little bit as well. Get the PCV and Grommet at the Toyota dealer and the molded PCV hose at the Lexus dealer to save on markup.