Magda

Thanks for great suggestion. But how do I estimate real rpms to drop the needle right? With the speed it seems easier. I am not sure what is the correct procedure for calibration will look around on forums.

Yamae

The calibration needs an equipment such as a pulse counter or an oscilloscope and your skill. But it's only the first step. After the confirmation of the rpm and the pulse numbers, you need to fix the unit using your skill.

In case of the unit for a 93-94, the problem is mostly caused by the low voltage regulator circuits but a 90-92 is not that simple and it is not easy to fix for an average DIYers.

Magda

Maybe i should just try to see if there is a discrepancy between actual rpms and the ones appearing on the Dashboard to make sure that the problem is solely the Dashboard and then let it be... I probably have not enough skill to deal with it properly without damaging something else. Thanks for your help

Yamae

If your DVM can read the frequency, the discrepancy can be checked quickly. If not, ask someone to use a pulse/frequency counter or an oscilloscope.

The 1UZ-FE engine fires 4 times per revolution. The revolution frequency is measured in seconds, but RPM is measured in minutes, so the frequency of 1000rpm is calculated as follows for an example.

Divided by 60 and multiplied by 4, which will be 66,667Hz. At 2000rpm, you will have 133.333Hz. But there are two banks and if you read the frequency at one side of the igniter, you can get only a half of total pulses and you will read 33.333Hz at 1000rpm. The signal cab be obtained at the signal IGT or IGF. Either will do. The IGT is the driving signal from the ECU and the IGT is the feed back signal to the ECU.

Judging from your past posts, I know you are a special girl and you can do more than those high end DIYers.

Magda

Yamae, I will try to test it when I get some time and then write an update Thanks a lot for initial critical mass of priming knowledge

Brunif20

Whenever you put in drive or reverse, surging goes away. So since it only acts up when the car is idling, I'd still be looking at the IACV. Does it feel springy? Does it move without any resistance?
Also if you unhooked the battery, the ECU might have to relearn the idle position. If you haven't already, drive it around for 15-20 mins and get it up to operating temperature. Then let it idle in the drive way and switch between P,R,D.
I had this same problem when I replaced my IAC and this got rid of the issue. Good luck!

Magda

I am again reviving this forum. Since my last post the car kind of died... I don't remember the exact symptoms but it stalled and had strange symptoms. For example it would stop driving when I push gas pedal while it would drive only while idling up and down in drive while I would not push the gas pedal. Later the symptoms would change. I stopped working on the car as I was busy with other stuff and I got frustrated and close to junking it....


Last month I decided to tow the car to car mechanic. They told me that the IGNITION IGNITERS may be bad. When I asked why do they think so, they could not tell... So I decided to revisit the challenge as I got some new energy.

I decided to REPLACE CAPS of ECU following Yamae's thread "all my crazy Lexus issues resolved..."
I got capacitors, I spent substantial amount of time to learn how to desoldering and solder.

Here are some pics:

1. My schematics and pics that show which capacitors I got as replacements:





2. My ECU opened up:


3. Backlit back side of 2 boards that form ECU. One with long connector and the other (I call it top one) with short connector:



4. Desoldering and soldering new capacitors
Observations: the old capacitors looked more or less intact with no leaks. The board seem not to have any substantial corrosion.
I replaced all old caps.




5. Last picture: I cleaned solder flux after soldering . The weird thing is that I got this white film / residue after cleanup with 90% isopropanol. I cleaned with lint free kimwipe tissue soaked with isopropanol... It looks like it tends to dissolve some kind of film present on original board. Not sure what it means for the quality of the ECU job.
I think I will close ECU up and reinstall it in LS400 and see if it improves anything...

Hope it solves some problems. Otherwise I probably will junk this veteran of the highway with heavy heart.
When I begun this project it was supposed to be just valve cover gaskets and maybe tuneup. If you look back in this thread you will see that there were many steps where I could break something on top of many existing problems of my beloved ls400 with its 300,000 miles.

If you have any idea on this white residue and if it's dangerous for the ECU board.

Regards and keep in touch with fellow Lexus Fanatics

Magda

Also, quick question to soldering experts:
-
1. I didn't manage to clean all the solder flux after soldering new capacitors in place. And final cleaning of portions of motherboard with 91% isopropanol left motherboard with white residue. Is that ok?...
2. I realized now that because the board is two sided one needs to make sure that solder joint gets to the other side of the board. I soldered only from one side (bottom of the board). How do I make sure that the soldered lead is sufficiently soldered on capacitor side? It is hard to see if the solder joint is sufficient on the other side as it is often covered by capacitor body and other component.

I did my work by myself but am not an expert in soldering.

jaaa

There is a conformal coating on the original board you are dealing with. If you made a good faith attempt to clean off the board with 91 percent isopropyl alcohol, I think you are okay. Did you use something like a SN 63 solder? As long as you didn't lift a pad, you probably got good flow(assuming flux was used). If it were me, I'd put it back together and try it out. Since you are saying you are close to junking it, you don't have much to lose. Good luck!

Magda

Well I put ECU back into the car.
Connected battery.
Put key in On/ Run position to recharge caps.
Waited 5 min.
Turned the car on.
It turned on at 900rpm or so and after a few minutes RPMs gradually got reduced to 300 or so , which also corresponding in engineer quieting down substantially.

I didn't drive the car further than parking lot as it reminds symptoms from just before ECU CAP rejuvenation where it would eventually stall in the middle of the road and die.
I did all the work so far.

My mechanic suggests it may be ignition igniters. I am not sure why he thinks so -but maybe someone here has better idea why would it be ignition igniter.

All the best and thanks.

This car is closer to the junkyard than ever. All I kept doing to improve its condition seems to end in failure. No successes...

dicer

91 percent isopropyl, and not isopropanol and 70% would be fine, ultra sonic cleaners use a very high concentration of distilled H2o so.
I don't think there are hidden connections for solder to flow to. It will either be one side of the board or the other. One discovery actually from someone else that posted about it awhile ago, is the bearings go bad in that idle air control stepper motor, then there also can be problems with the egr valve. I had put a junk yard IAC motor and valve on my one and when I started it, it was stuck on 2000 rpms. I have not had the gumption to play with it for awhile now, but I have taken one of my spare IAC motors apart and the bearings feel dry and not so good. So that can be an issue with idle problems.

pishta

Not reading 6 pages of this but I had my 92 SC400 idle up to 1700 rpm and the thing would idle at 60 on the freeway! braking was exciting to say the least. My issue was the IAC stepper motor bearing was locked up tight. I think the IAC was actually spinning the race on the top bearing. I took the IAC apart and the skateboard bearing on top was frozen solid. I chucked it in a vice and worked it out while oiling it and it took a good 5 minutes of twisting to get it cleared. The bottom bearing was not as bad but still was rough. I got both spinning and reinstalled and the idle was rick solid at 600. Its never run better! Im not sure how these sealed bearings got so bad as there is nothing in this IAC that would allow moisture or even heat into the bearing area. Now Im battling lack of EGR as my SC finally failed smog her in California with high NOx, 531ppm when the average is 70 and passing limit is 487. It actually passed at the 15mph load but failed at 25mph load test phase. Not sure how that is even possible as the RPMs were 40 lower at 25 MPH. Lack of flow?