Stromtroop

Thanks to this forum I was able to trouble shoot my 1994 LS400, it is now running like a clock. What started out as a gas guzzling lumbering car is now a fuel efficient powerful car.

1. I purchased the car in November 2020 for $2500 CAD, ($2000 USD) it had 140,000 km (86,000 miles). One owner car estate sale with all documented servicing confirming the mileage. It was mint, dry warm storage - but covered in dust with 4 flat tires. The last documented oil change was 2015 with technically still 500 km to go before the next oil change. (I changed the oil and filter on the way home).
2. The seller (grandson) put an old battery in just to get it started - it idled just fine, and while in park it easily revved up as it should with no noticeable stalling, stuttering, shuttering or noises. In my short test drive around the block I also did not notice anything out of the ordinary.
3. The instrument cluster was blacked out and the climate control screens were black - but since this was a 27 year old car I assumed burned out light bulbs - Thanks to this forum I would later learn this is a common problem.
4. While driving it home I filled it up with premium chevron (94) gas, and within a few km's the cluster jumped to life - The service engine light was NOT and all guages seemed to work as they should, it shifter smoothly and seemed okay.
5. After 30 minutes of driving (In heavy rush hour traffic) I noticed by gas gauge had sunk 1/4 tank. This was the first indicator something was very wrong with the car.
6. When I got the freeway and gave the accelerator a push, it lumbered up to speed but it did not perform like a V8 should. Hmmm? I live at the top of a hill (about 5 km of a 15% grade) my LS400 would barely accelerate to 60 km/h (35 MPH). It seemed to take a lot of finessing to get it up to speed. By the time I got home, I concluded something was definitely wrong with the car. (I started to think about all of the things that may cause this... dirty injectors, carbon build up, bad Mass air flow sensor, plugged cat converter etc.)
7. I drove it for most of the next day testing the car under different scenarios. Overdrive on / off, ECT button on / off. No matter how I drove or what I did the car simply had no power. It was under performing in every way and when the day was over I had emptied the tank and had only gone 250 km (150 miles).

I found this forum and started reading.... It became clear to me that bad capacitors in the ECU were a likely culprit. I never would have thought to consider this!

1. I removed the ECU. After removal from the car I removed the cover and examined the capacitors. Sure enough one of the capacitors was leaking (obvious brown liquid at the base) and most others were bent over by 10 degrees. I made decision to send my ECU to Jim Walker (I also removed and sent my climate control and instrument cluster to him) I sent them on December 18 2020, and they arrived back on my doorstep Jan 20 2021 - I re-installed same day. (Total cost of repair and shipping was $972 USD)

After re-installing the now repaired ECU the car had a lot more pull, it was noticeably more powerful and although my fuel economy was better it wasn't great.

2. I had also read a thread on the coolant temp sensor control, suggesting that it had a lot do with fuel economy. I contacted my local Lexus dealer and inquired about this sensor. They asked me which coolant temp sensor I wanted, stating that there were three different coolant temperature sensors on the LS400 - One sensor that sends data to the instrument cluster, one sensor that tells the fans when to come on and a third sensor that provides data to the ECU and is the one that will help with fuel economy. (There words... "help with fuel economy") I said I wanted the coolant temp sensor that will help with fuel economy. It was in stock and cost $40 USD. On the way home I also bought a new battery and air cleaner. ($236 USD)

Test drive number 2 - Wow what a difference! The car feels brand new, it now cruises along at 80 km/h at 1200 rpm, If I touch the gas pedal the car will take off and if I put the accelerator to the floor the tachometer jumps to 5000 rpm and the car launches and continues to accelerates until I let off. Today (Jan 24 2021) I drove for 150 Km (100 miles) and the gas gauge barely moved. I'm estimating I will get 700 km per tank.

My total investment in this car is now about $3200 USD. Thank you all who have contributed to this site, your invaluable info and shared experiences helped me, I hope my feed back helps others. Every car is different with unique problems. If there are any LS400 owner out there experiencing poor performance and bad gas mileage with no obvious symptoms (my car did NOT have a service engine light on, it was not stalling or stuttering, it drove well, and shifted well) consider repairing your ECU with Jim Walker and replacing your coolant temp sensor. These 2 repairs changed my car's performance and mileage to such a marked degree that I have no words to describe it other than amazing.

I hope this helps someone out there, Good luck

Losiracer2

Yes coolant temp sensor is critical in engine operation and fuel economy. I did that on both my 97' and 99' LS and it achieved immediate results in driveability and a good mileage boost. ECU caps would be a necessity for 90-97 cars, my 99' doesn't seem to exhibit any issues, crossing my fingers.