96 LS400 Sputtering and Dying

[Yakuzacracker]

Hello everyone, I made an account here in the hopes that someone could help me diagnose the strange issues that I've been experiencing with my 96 LS. About a month ago my car was running perfectly then one night I started it up and it sputtered, the rpms were not stable and were very low, and it would rev down to 0rpm after 5-10 seconds before dying. I tried to start the car again and it did the same thing, or it would try to start but not turn over. I got the car towed back home and did some research. I saw that people have experienced similar issues and I immediately thought it could be a dead ECU. I contacted lscowboyls and he walked me through getting the ECU out, I sent it off to him and he refurbished it. I installed the refurbished ECU and the car ran great for 3 days and then I parked it, turned it off,  then when I came out and tried to start it again I experienced the same issued I did before. I got it towed to an independent mechanic who is experienced with my car and they are stumped. It's been with them for almost a week. I suspect that it's a dead fuel pump, but my mechanic is not so sure. The only other thing that I can think of is that I brought my LS into a local Lexus dealership for a free oil change and I wasn't experiencing any of these issues before that. I looked under my oil cap and there's a dry brown sludge, so that might be something worth investigating. If anyone thinks they know what it could be or what I should do please let me know! Thanks in advance!

[Lexus Owners Club]

I don't think the oil change or the sludge would be responsible for the engine cutting out.

However, I would look for fault codes and also carry out grass roots diagnostics (checking battery terminals and battery condition and then carrying on through the car one step at a time).

It's laborious but does throw up interesting faults that would otherwise go undetected.

Check also the fuel quality and drain/refill if suspect

[Yakuzacracker]

Thanks for the response Trevor!

Yeah the sludge has nothing to do with the problems I'm experiencing. The car was with my mechanic for almost a week and they couldn't figure out what was wrong with it, but my battery died. They replaced the battery, which was old anyway, and I picked the car up and it ran fine for about 5 days, until today. I was driving on the highway and I completely lost power, I tried to floor it and the car didn't respond. The revs began to flutter up and down and the car was shaking like it was a misfire. I was able to pull over in the breakdown lane and the revs were jumping between 500 and 1000 while the car was shaking. After a little while the car died and would not start. When I would turn the key the car would try and fire and I have power, but the motor will not turn over. It was pretty scary, it's never died when I have been driving before, up until now I've only experienced this issue when the car was stationary. 

Edit: Also when the car died the check engine late came on, when I tried to start it again and it wouldn't turn over, there was no check engine light.