Can't Seem to Fix Low Idle!!! *rolls eyes*
 

Hi everyone,

I've successfully resolved many issues on my 1996 LS due to all the information on this forum, however one issue that is driving me nuts is my low idle problem. The car has relatively low miles and this issue began with only about 108,000 on the clock. Car gives me no error codes or anything.

- The car starts from cold fine and idles properly at around 1200-1400 RPM.

- As the engine warms up, the idle decreases in steps to around 650 RPM like its supposed to.

- The longer I drive however, I notice the car begins to idle steadily at 400 RPM or occasionally lower, causing the car to shudder and feel sluggish, especially in parking lots where it even feels like it may stall.

- In order to temporarily alleviate the issue, I turn on the A/C or the headlights, and the idle moves back to where it's supposed to be, though driving around with the A/C on constantly is annoying.

- Sometimes even with the A/C ON I can feel it trying to idle lower as if its fighting itself.

- While driving at slow speed, I feel like the engine is being dragged (not a good feeling), but driving at speed the car seems fine.

So far I have cleaned out the entire throttle body and intake, as well as cleaned the Idle Air Control Valve, which did not appear to be faulty (or even that dirty). I have replaced spark plugs, air filter and cleaned the MAF sensor.

I have also read on multiple threads that some 1995 and 1996 models had ECU issues that would affect the car's idle speed and cause the engine to die and jerk while driving. My car does do the jerk, but has never died (comes close though). The car also jerks pretty aggressively when changing from park to reverse, or park to drive, which isn't the case with other LS400s I've driven. I've checked which ECU I have in the car, and I have part number 89661-50303 which apparently is the one that is supposed to alleviate these issues???

At this point I'm pretty sure I've read every forum thread possible on this topic on multiple Lexus forums and still not managed to track down my specific issue. I'm still a little suspicious of the ECU, but from what I've read I have the right one. My next idea was to go after the throttle position sensor, but I don't really know where to keep looking after that other than to start shelling out for replacement parts (IACV, ECU, etc.).

Sorry to write a whole essay on the topic but it's driving me nuts as the entire car runs perfectly except for this one issue. I also don't want this to potentially wear on the car in any way by not taking care of the issue.

Any suggestions or ideas would be appreciated. At this point I'm stumped.

Thanks,
Ed

good job on tackling the known issues and doing your homework. it shows (and it also adds further frustration I'm sure)!

how you considered your ECT? do this and reset your ECU by unplugging the battery and let the car relearn all the parameters it needs to run from the beginning.
https://www.clublexus.com/forums/ls-...ange-pics.html

The 50303 ECM is just the later flash. The same exact capacitors are used. I have seen many 50303 ECM’s with capacitor failures. If you’re feeling up to spending a couple hours you can remove the ECM and take it apart to check the capacitors for leaking. There are 6. No leaking isn’t a sure sign that they are okay but leaking is a sure sign that they are bad.

One of the things that happens when the capacitors go bad is that engine sensors receive incorrect reference voltages and, in turn, relay bad info back to the ECM.

Thanks for the reply and insight! I have not considered this as a point of issue. Has it been known to negatively affect the idle of the car? It seems as though many in the thread had a positive improvement, so I'll definitely look into it.

A good point. I was actually considering grabbing an equivalent ECU from a local wrecking yard and swapping it out to see if there's a positive improvement. As far as ECU issues go it seems that the only problem I have is the sluggishness at low speed/ very low idle. My diagnostic scanner reads fine (as far as I can tell), and I get no weird hesitations or anything else.

My 1990 would start fine, and then begin to drop the idle, It would sometimes stall when I was pulling into a driveway or at lights. I replaced the MAF and it fixed the issue. If you can find a working used one to test, I would do that.

I too have the same problem for my 99, I've replaced my throttle position senor, idle motor, cleaned the throttle, checked for vac leaks, had my ecu rebuilt thinking that would fix it but it did not. Im out of ideas as well.

Did you swap the afm?

I have not replaced the afm. Do you think that could be it?

If you are talking electrolytic caps, look for a bulging top too. They should be flat and not rounded or split, leaking from top or bottom.

If leaking, the contents can eat and attack copper. Be sure to clean it thoroughly.

Replacement componets can be ordered through Digikey. Match all of the obvious physical parameters and select name brand components with the widest temperature range. Keep in mind a higher voltage can be used in most cases but do not use a lower voltage than what came out.

Obviously soldering skills are a must and be aware that somewhere in the later date range a switch to Lead Free solder could have been made. Meaning, mixing lead and lead free could lead to failed solder joints.

Often times a leaded solder tip will lead to splattering when touching a lead free solder joint and vice versa...

Good luck...

When it's warmed up, at 400 rpm going through the parking lot, is that when you are stopped and in Park, or just idling in drive? Just trying to rule out a problem with the transmission.

Thanks for all the responses everyone; I've got a lot more to look into.

I have a new coolant temp sensor on the way that I'll install and report back. I'll also test the MAF using the usual multimeter test, just in case, and possibly the TPS which I assume can be tested in a similar way to the MAF?

Oldskewel,

It is worth noting that the low idle issue is ONLY in the drive gears (e.g. Reverse, Drive, 3,2,1, and L) it idles at the correct speed in Park and Neutral.

This makes sense since the engine is experiencing more load in gear. since you mentioned it, when was the last time you changed your trans fluid?

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.

The '98 Model Year is when VVTi was introduced, along with a host of other changes resulting in the UCF2x Post Minor Change LS400s.

The confusion stems from the way some countries register cars. For example, an early 98 model year car sold in '97 in AUS is registered as a '97 car, rather than by model year.

The original poster was, the poster I was replying to however (lexus_ls) has a '99.

If original then water temperature sensor should be changed in any LS400.

Am interested in your comment on Neutral Safety Switch...had a really low idle situation with stalling in Drive happen only once...This applies to what years LS400s? 1999?

1999 LS400 dispensed with EGR

I am extremely limited with my "knowledge base" as I have experience only with first generation LS400s. Thus, my list of possible causes includes EGR for the earlier cars.
Colin

Hello, LS400Eddo and Lexus_LS .... regarding the mysterious low idle situation, I wonder if I may share with you my latest discovery : what if your ECM / ECU was rebuilt by a guy who does a lot of Lexus ECM's ... the twist is, he only uses replacement parts that are truly sourced from Japan ... plus he has soldering down to a science .... I was surprised to find all the comments from people that he has done work for ... all is found on his FaceBook page. So if you go to FaceBook, just type in this name : Japanese ECU ECM PCM Repair by Lscowboyls ... As for me, I have an old 95 LS400 and although I am not having drivability issues, once I found out how much your car's performance can be improved with this particular ECM rebuild ... well ... gosh, I think I will for sure give it a shot : ) Read though his FB pages and you will see what I mean ... really amazing comments left by people that I bet will resonate with you. I plan on having him rebuild my unit soon .....

take care and best of luck .. .. .. .. ( I will let you know how it goes after I get my ECU done )

Hi JetCat. I actually had my ECU rebuilt hoping it would solve my low idle issue and it didn't do a thing. No change. My ECU looked great too, but I replaced all the chips.

Same problem
 

Sounds like your injector's lower part is badly clogged. The Lower part means the path for the air to go through indicated by arrows. In order to clean the paths of 8, you need to use at least a can of carbon cleaner and a compressor applying to those 2 larger holes that are connected to 8 injectors air mix paths.

Depending on the clogging level, you sometimes need to clean injectors professionally using a ultra sonic cleaning machine. I also must add that any fuel treatment is not effective to clean those air mix paths because there goes only the air not the gasoline.

https://cimg8.ibsrv.net/gimg/www.clu...dbf53fcc6c.jpg

The drawing below may help you to know how 2 larger holes are connected to 8 injectors to send air. Sorry that non related parts are explained only in Japanese.
https://cimg0.ibsrv.net/gimg/www.clu...92d8040635.jpg

He's just another rebuilder except he's built up this mysterious aura and attempts to claim that he himself discovered the issue. He attempts to build the image that he and only he can "properly" rebuild the ECU, this is simply not the case.

LS400 ECUs have been repaired by countless automotive electricians for more than 20 years, there isn't any particular reason to pay extra for LSCowBoyLS's services. Search this forum for more information on his history, his rep isn't the best.

We're discussing the idle air control valve found on '89-97 LS400s, not the PS idle up valve tha is found on ALL LS400s. Completely different functions.