I personally have never had it changed since I bought the car. It's probably due due though.

Could a bad VVT solenoid be causing it? My car seemed to idle a little low before I replaced both.

That could be the case in some later LS400 models however mine is a 1996, and VVT was introduced in 1997 IIRC.

1997 was when the vvti was introduced.

have you made sure that ALL of your vacuum lines are in good shape and aren't hard/brittle and/or cracking causing air leaks? I had to replace almost all the vacuum lines in my 97's engine bay, including the ones underneath the engine cover since they were hard as a rock.

And for the PCV valve, try to source the OEM molded hose, I first tried using a fuel hose, but its at such an angle, where it was cutting off air flow and choking the engine, causing my idle to dip to 400rpm and then fluctuate back up.

That's next on the list at this point. I've replaced a couple, but will go after the rest in the next few days.

Well I've changed out the coolant temperature sensor this morning. While it didn't make any difference to the low idle problem, the car definitely runs more smoothly, especially under normal acceleration.

I was pretty surprised at the condition of the old sensor; it was basically wrecked. Corrosion present around the sensor, and the actual electrical fitting was broken off around the edge. Definitely a good thing to replace.

https://cimg9.ibsrv.net/gimg/www.clu...68e5d56b9e.jpg

Low idle in drive at a stop
 

I have been chasing this problem for almost a year and I have just replaced the entire throttle assembly and it has not fixed my low idle. I hangs out at 400 at a stop in drive. All maintenance is up to date, I have even replaced MAF. No check engine light, runs fine otherwise. I have no idea what it could be, I have taken it to mechanics and they have no idea. Please help!

99 Ls400

That would be because there's technically no problem. Lexus does not have a specified RPM for the car when it is in Drive as it is variable depending on numerous factors. As long as it stays running, it's fine.

The only spec published for idle speed is hot/cold idle speed (with AC on and off, 4 numbers total) for when the car is in Neutral or Park only. IIRC factory spec is ~750rpm +/-50rpm when warm, AC off, in N/P (for the '98-00 cars).

A 99 should idle 750rpm ± 50rpm at no load. 400 rpm at a stop in drive is surely lower than the average. As far as I know, most of 98-00 idle higher than that. Those low idle cars can be mostly fixed cleaning the aix mix paths thoroughly including injectors using solvents and a compressor, in my experience.

Read the post below. Before the very 1st LS400 rolled out of the assembly line in 1990, Toyota's sent their engineers to America, Germany, Italy, England, etc... to learn & bench marking some of the best automobile designs during that time. They came back home & put their hearts & souls into their designs for 6 years in developing a luxury car & the rest as we know it is history. However, even as great as the Lexus LS400's design which puts the company on the map, it still has a few flaws & this is one of them (ECU's programming issue). Due to the degradation as the car aged & parts' variation/tolerance stack up working against each other, some cars are more prone & sensitive than other so what this mod does is it by-passes/isolates the power steering's Air Control Valve (ACV) & use the air for that function to provide more air to the engine intake which will help bringing up the engine rpm so the car won't stall during hard braking and/or at low idle & under load (in gears). The LS400's has such a great but overly design in this case for having the additional power steering's ACV, the driver won't even notice a change in the drive-ability as some (including myself) have elected to permanently delete the ACV when the valve failed & leaked power steering fluid into the intake & caused the lost of fluid & white smoke. Anyway, this mod works but it also has its own flaw due to the additional air going into the intake all the time & caused a slight drive-ability issue which you may need to get some use to as one of the members had mentioned in his post.

https://www.clublexus.com/forums/ls-...d-problem.html

That's great and all aside from the fact '98+ cars don't have air control valves...

I read from the original post that his was a 1996.

Other than that,
Superfast1 wrote:
Which does not address in any way the low idle complaint of the original poster. It is so off-the-mark for this complaint that it looks like a copy/paste from some other thread, and I recommend against this modification for any reason but ATF ending up in the intake tract or a sudden leak that ends up on the alternator.
The Idle-Up Valve on the power steering pump only adds a little additional air to the engine based solely on power steering fluid pressure dogging down the engine at full-lock. It does not help "during hard braking" or "under load (in gears)".

Factory idle speed above ambient 50*F is 650 +/- 50 rpm.
Below ambient 50*F it is 800 +/- 50 rpm

In order of likelihood from the factory manual, a low idle speed can be caused by:
1) water temperature sensor
2) ISC valve
3) horrible-but-true, the neutral safety switch. The NSW terminal is normally a ground path for the ECU in P and N, when it is not a ground path (in R D 2 1) the ECU changes the idle speed program and the mixture
4) EGR valve
5) Fuel system circuit (we have a two stage fuel pump pressure)
6) Injector circuit - another way of saying to be cognizant of a cylinder that is not firing fully due to restricted injector spray

Amskeptic- The link I provided is a workaround, a quick fix to the problem that's why I called it a mod (a modification that is) & the person that came up w/ this mod by-passed the useless power steering's ACV & used the air for that function to bump up the rpm. I've also tried it & it works so I'm sharing with whomever interested, whether they want to implement it or not, that's their choice. We're all signed up to this forum because we love our LS400's & also want to help out one another so there's really no need to dissect someone post. Thank you very much.