I'm interested to know if anyone else out there has the same crazy rich AFR that I do. Averagely seems to be between 8.7 - 9.1:1 AFR anywhere the foot, load, or RPM seems to be, which to me is hella rich! Maybe a GS thing maybe not. Have done my exhaust but it was the same prior to that as well and I'm questioning if this is right, because in my books it's way overfueled. A scan produces no faults suggesting sensors are within spec, but even so a valid sensor could be responsible for making it be fed too much fuel. I would have expected to see something a little bit more like 14:1 on cruise and idle or better and maybe 12 or 13:1 when I'm pegging it. Just doesn't seem right. I run 100RON and it's pretty stock GS460 JDM except for the exhaust, 1UR-FSE.

What's your typical measured AFR?

 

How old are your sparkplugs? Also when you replaced them did you make sure you replaced them with original oem or legit denso?

I would pop out a sparkplug and see its condition

 

I inspected them when I purchased the car used at 80k, it's just cracked over 100k now and doubt they've ever been changed. When I checked them they looked mint, maybe it's time. Could that be responsible for that much of a terrible AFR though do you think? I guess... What would you expect to see on AFR normally on this?

 

Its possible the previous owner installed fake plugs, and if thats so then yes it will kill your a/f ratio. Its very easy to purchase a set of "oem" or "denso" plugs a whole pack for $20 off ebay or amazon, especially right before selling the vehicle. The originals iridiums (Part # FK20HBR11) denso or oem cost from $11-15 each plug.

Also did you inspect the air filter, that can aswell choke the air intake, causing the car to run richer. Same for cleaning the throttle body and doing a top engine cleaning, such as seafoam or other brands. This will help clean out the intake runners, intake valves, heads etc of built up carbon. Its very easy, with the car warm and engine running pop the engine cover off, located blocked off vacuum line remove that. Spray or vacuum (depending on the brand you use) into the vaccum port while the cars running. You want to barely choke the car, idle will stumble, that way the chemicals get forced into the engine. Then replace the plugs after you finish

 

What are your short fuel trims at idle, and cruising? Ideally you want close to 0, but anything under +/- 10% is good.

Here is a good read on A/F ratios.
https://x-engineer.org/automotive-en...e-performance/

 

The GS460s 1UR-FSE runs rich from the factory, part of its "detuning" from Lexus in order to not only get better emissions, but more importantly, make it SLOWER. Lexus could not allow the GS to outperform the flagship LS.

9:1 isn't unheard of, in fact is pretty normal, however it can be fixed with a tune.