LS400Eddo

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

timmy0tool

iTrader: (7)

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

Banshee365

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.

LS400Eddo

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.

LS400Eddo

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.

Moarpower

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.

Banshee365

lexus_ls

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.

Moarpower

Did you swap the afm?

lexus_ls

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

2013FSport

iTrader: (2)

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...

oldskewel

iTrader: (1)

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.

LS400Eddo

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?

LS400Eddo

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.

timmy0tool

iTrader: (7)

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?