Hi all,
I have been dealing with this issue for a while and am a little confused at this point as to what to do.

About six months ago my 1993 Lexus sc300 (2jz non vvti) started dropping rpms at low speed and at idle, and occasionally dying. I called a local mechanic and he said possibly the fuel filter, so I went ahead and replaced that. After that didn’t work I did the pump, and finally the fuel pump ECU. At first I took out the old ecu and soldered in new capacitors and that seemed to work but then a week later the same issue arose. At this point I bypassed the fuel pump ecu (soldered the two larger gauge wires together using a pinned guide on this forum). When I turn the key now I hear the fuel pump engage so I do believe the bypass worked.

Some other things I have done:
- new battery
- new distributor cap, rotor, seals
- new spark plugs and wires

I also noted that on occasion the battery terminals are hot (generally the negative) but that is usually after I have cranked it a bunch after it dies on the road.

does anyone have any other suggestions on what to do?

Hello,

How often does this issue occur?
Do you feel it Stutter or Misfire before it Stalls?
Have you ever checked or tested the Main ECU for that car?
Can you force it to stay Running by stepping on the Gas, etc.? If so, how would the car behave?
Are there any External Factors that make the issue occur more or less frequently, like Rain, Heat, Fuel Quality, etc?
You mentioned that you have to Crank it to the point that the Terminals get hot after it Stalls, does it have troubles Starting?

Also, what Brand of Parts did you use, especially for the Cap and Rotor? Wells is probably the only at least more or less reputable aftermarket manufacturer for those parts, unless you got them or OEM, I would probably suggest to put the Original parts back in, that is if they are not worn down to the bones..

The fact that the Idle can drop periodically doesn't really help a who lot, more information will absolutely be needed, if possible, a Video of it Stuttering or Stalling would be quite helpful.

For now, try disconnecting the IAC Valve and going for a drive to see if the issue will persist, those Valves had a Mechanical Backup, the Idle RPM would likely be a little Higher than normal, but the Idle should remain Steady. If the Idle will remain steady, with no drops or stalls, there is a good chance that you have a Damaged or Dirty IAC Valve that is simply in need of maintenance/replacement.

As for the Bypass, I generally suggest to stay away from them, even if just for the convenience of not having to listen to the Fuel Pump whine as if it's last day on earth. Here is a thread where I speculated on the ability to retrofit the Fuel Pump ECU out of a 2nd-gen GS, which appears to be a bit more resilient, but it's still yet to be confirmed whether they are actually compatible.

Hope this helps and best of luck!

Hi,
Thanks for the reply.

Happens every time I drive the car lately, but only once its hot and I've driven just far enough away from my house for it to be inconvenient (lol).
I have not checked the main ECU, is this a common symptom of the capacitors leaking or something?
I don't recall feeling like it was misfiring or stuttering, its incredibly sudden with no warming. I feel like it has never happened when I am pressing the gas firmly- it generally happens when I am coasting down the road in a neighborhood going slow.

no external factors seem to have an affect. its been going on for months rain or shine. it doesn't have trouble starting UNTIL it has happened for the "first" time that drive.
I am able to start the car up once again if I wait for maybe 5 minutes and then crank it again. I have only ever used OEM parts to service the car as well.

I'll give that IAC valve thing a try and I'll go ahead and take a look at the main ECU.

once again I really appreciate it!

For sure the main ECU is suspect. Just replacing some leaking capacitors is not the limit to ECU caused issues. I like SIA since ECU repair and testing/certification is their main focus. They first will test/assess your ECU and recommend the service needed to restore the ECU to factory specs.

https://siaelec.com/product/lexus-sc...repair-return/

I'm having the same issue right now with no clue what it could be. I just went out and tried disconnecting the IACV, shattering some of the 30 year old plastic in the process, but the same thing happens where the car will stall out after running for about two minutes. I may have to start looking at the ECU.

ok, I’ll go ahead and give this a shot!

at this point 190$ isn’t that bad to fix this headache of an issue. I will report back so hopefully I can save someone the time in the future.

thank you!

I ended up sending my ECU to Tanin Auto. They have a couple SCs on site they use to test their ECUs and whatever else you need repaired. It ended up being $200

Did you have a check engine light on? Was it smoking at all? Could be a vacuum leak, you would have a P0171 code. Or it could be worn out O-ring in the vacuum solenoid on the power steering pump. The O-Rings go bad and P/S fluid gets sucked back into the intake manifold and then puddles up there until the situation is right for the engine to suck it up and thatll cause a struggle and usually it’ll smoke at the same time.

Look for a vacuum leak. There are several places it can happen. Has the car ran ok other than at idle? If so, I would rule that out first.

Have you changed plugs lately? It is common to miss the two bottom studs on the throttle body assembly and can be a source of air leak.

Well, I would still suggest to at least take the Covers off of your ECU and inspecting it for any obvious damage, stuff like Leaking Capacitors are usually quite obvious, and would make it blatantly obvious that it is time for a repair, as right now, while a more or less certain, it's still a shot in the dark without direct proof.

Hope this helps and best of luck!

@LucasW Did you ever resolve the issue? Was it the ECU?

Hi sorry,
Ive been busy with work and other vehicles. I'll have more time on Monday to pull the ECU cover and check out some of the other options people have posted- look out for a reply sometime next week.

as for the ECU I have no clue how long it takes them to repair them and send them back (once ive sent it) so that'll be a bit longer.

Thanks!

UPDATE: shipping ECU out tomorrow to Tanin auto tomorrow. Current lead time on the website says 5-7 days so hopefully by next week I can have a real update.

Another thing- I popped the OBD 1 port open and I noticed this goo caked inside. has anyone else experienced this?? It's quite alarming and I am unsure of what it could be. I don't remember seeing this when I bought it (Although it has never thrown a code the check engine light does work) and I keep it in a garage generally.


Let me know if anyone has any ideas,
Thanks

It looks like it is packed with dielectric grease. I believe they came that way from factory to prevent corrosion. I don't have any on any of my electrical connectors but it might be a good thing to have.

If you want to clean it, pick up some electrical parts cleaner and a toothbrush, disconnect the battery, then spray down the stuff with the contact cleaner and scrub. You can reapply grease afterwards.

Did you at least take the Covers off for a quick peek?

Thaat appears to be regular Grease, usually used to protect the Terminals from Corrosion, most of those connectors will have at least some type of Grease in them as there are no other Seals there, even though usually it's the White Fluorine Grease that they used to apply .. yeah .. best of luck trying to find it nowadays..

Well, unless you have magic pixies living in your garage and maintaining your car while you are looking the other way, you will probably have to familiarize yourself with a Mandela Effect..

Hope this helps and best of luck!