1993 Lexus LS400 headache "misfiring"

[tonaka]

Hello everyone, 

I drive a USDM (non-California) 1993 LS400 (UFC10, 96'000 miles which I guess are original...) around here in France, and I am experiencing a "random" misfire that is most likely to happen at highway speed or under load. It creates vibration, loss of power, and makes the car shaky when it occurs at idle, which is much rarer btw. 
The problem never occurred when the car is cold which makes me think the 02 sensors could be the culprits. At low rpm (when they are supposed to be in open loop) the problem doesn't occur either. My guess is that it is not related to a basic ignition or fuel delivery part broken since the car is able to start and run perfectly sometimes.

I drove the car 2 days ago at highway speeds, from 70 to 100 mph and even though the car was still drivable, it developed a random misfire (and correlated loss of power) every once in a while and eventually goes back to normal. It feels like one or two cylinders are missing. Sometimes it runs good for 1 hour straight, sometimes misfires for the same time. The misremember can be slight, moderate or hard randomly. 

At city speeds though, I drove it for 3 hours yesterday mainly in the city, with speeds up to 50 mph, and the car ran flawlessly. It only misfired once at idle if I can remember correctly, which it misfired dozens of times at highway speeds. 

It always starts quickly, doesn't smell like gas, etc...

No check engine light or TRC dash lights whatsoever. 

I've read about every topic on LS400 misfiring and haven't found an accurately similar issue.

So far, the car maintenance is up to date, 8 new factory spec Denso plugs. All the rest of the ignition system has been tested and looks in good condition). 
Coolant temperature sensor is new, MAF seems ok, throttle body is clean, fuel filter is a couple months old, runs with the best gasoline available and already ran injector cleaner...

Recently, the car was guzzling like hell (more than 20l/100km, something like 12 mpg) and this common problem was fixed by replacing the coolant temp sensor. But since the car had been running rich for an undetermined period of time, I suspect the 02 sensors to have suffered from this. 

I ordered all 4 Denso 02 sensors on Rockauto and a couple other bits but I feel like the problem could come from somewhere else since it thrown no 02 sensor code recently. Maybe they are only on the way out...?

I'll read codes again and post them, but in the meantime if you have faced a similar issue or know about it, I'd be glad to hear about it ! 

Thank you very much. Joined a photo of my car. It's been imported in France in 2003. 

 

 

lexus.HEIC

[chc0318]

I suggest you check the ECU first. You can open the ECU and check the capacitors to see any leaking capacitor ? If yes, you should send out to repair.

[paulo57509]

Here's a related thread at CL.

https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/656360-all-my-crazy-lexus-issues-solved-ecu-leaking-capacitor.html

[tonaka]

Thank you, I will check ECU and keep posted. I have looked into this thread already, but I thought the ECU would be the last to check. Let's see 

[Jeremy M.]

If the ECU checks out ok, since you suspect the O2 sensors, you can try checking them with a scan tool that has a live data output- they're expensive, but well worth it imo. If you don't want to shill out $100+, you might find a friendly mechanic that will read out your live data with his tool, and you can look for one or more O2 sensors reading abnormally from there.