IS300 Long Term Fuel Trims and Runs Rich while Warming Up - Lexus IS Forum
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post #1 of 5 (permalink) Old 12-02-2018, 11:43 PM Thread Starter
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IS300 Long Term Fuel Trims and Runs Rich while Warming Up

Hello!

I recently purchased a 2005 Lexus IS300 (my first Lexus ever) and it's been phenomenal... until it wasn't. It came mostly stock, except for a few weird and horribly installed electronics. Ripped out all of it and did some maintenance (valve cover gaskets, spark plugs, new radiator, etc) and got the car back to stock, and suddenly I had a crank-no start condition. Long story short, my right side fuse junction box fried itself internally and fried the computer and some other sensors, had no continuity across the ignition fuse to the terminal (not a bad fuse - like the actual pin on the connector to the fuse pin had no continuity). Had the dealer replace the computer and a bunch of sensors, I replaced the fuse panel and some other stuff, and got it running nearly perfectly... except one problem...

My long term fuel trims are terrible. They're super out of whack. Both banks hover around -27% and -28%. Oddly enough, I have no check engine light and no codes (I haven't driven it extensively though).
Warming up, it REEKS of gas. It runs super rich warming up, and kind of rich once warmed up at idle (but enough to notice unless you go sniffing around). No rough idle or anything like that... in fact, it's super smooth at all RPMs and conditions (reverse is a little shaky but I think that's a transmission issue)
Here's the kicker though: I checked all the usual suspects (MAF, vacuum hoses, fuel pump relay / resistor, air intake sensors, etc) and everything is dead on to spec and running as it should. I'm no expert, but the pre O2 sensors seem to be operating correctly as well. The car has always been a CA car, and the engine is super clean. No problems while driving. Always had 91 octane gas.

Anyone have any ideas of what could be causing this? I'm out of ideas and stuff to check.

(Also, thanks to everyone for being super helpful in other threads and posting lots of good information and helping me out a lot, I appreciate it!)

Last edited by christopherc; 12-02-2018 at 11:46 PM. Reason: More information
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post #2 of 5 (permalink) Old 12-03-2018, 12:47 PM
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Originally Posted by christopherc View Post
Hello!

I recently purchased a 2005 Lexus IS300 (my first Lexus ever) and it's been phenomenal... until it wasn't. It came mostly stock, except for a few weird and horribly installed electronics. Ripped out all of it and did some maintenance (valve cover gaskets, spark plugs, new radiator, etc) and got the car back to stock, and suddenly I had a crank-no start condition. Long story short, my right side fuse junction box fried itself internally and fried the computer and some other sensors, had no continuity across the ignition fuse to the terminal (not a bad fuse - like the actual pin on the connector to the fuse pin had no continuity). Had the dealer replace the computer and a bunch of sensors, I replaced the fuse panel and some other stuff, and got it running nearly perfectly... except one problem...

My long term fuel trims are terrible. They're super out of whack. Both banks hover around -27% and -28%. Oddly enough, I have no check engine light and no codes (I haven't driven it extensively though).
Warming up, it REEKS of gas. It runs super rich warming up, and kind of rich once warmed up at idle (but enough to notice unless you go sniffing around). No rough idle or anything like that... in fact, it's super smooth at all RPMs and conditions (reverse is a little shaky but I think that's a transmission issue)
Here's the kicker though: I checked all the usual suspects (MAF, vacuum hoses, fuel pump relay / resistor, air intake sensors, etc) and everything is dead on to spec and running as it should. I'm no expert, but the pre O2 sensors seem to be operating correctly as well. The car has always been a CA car, and the engine is super clean. No problems while driving. Always had 91 octane gas.

Anyone have any ideas of what could be causing this? I'm out of ideas and stuff to check.

(Also, thanks to everyone for being super helpful in other threads and posting lots of good information and helping me out a lot, I appreciate it!)
Just asking. Did they put the right O2 sensors in it? I have heard California models use different ones.
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post #3 of 5 (permalink) Old 12-03-2018, 09:07 PM Thread Starter
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Originally Posted by 01 SolarYellow View Post
Just asking. Did they put the right O2 sensors in it? I have heard California models use different ones.
Good question, I'll definitely take a look once the car cools down. I've hear that Toyota / Lexus motors don't like anything besides Denso for O2 sensors... is that true?

Fun fact: I was able to have someone ride with me and monitor my fuel trims as I drive, and it looks like my long term fuel trims are all over the place while driving. At freeway speeds it's actually pretty good, around -8%. At idle however, it's now around -23% (keep in mind I rarely see the short terms drop below -10% while driving)
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post #4 of 5 (permalink) Old 12-04-2018, 06:10 AM
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Good question, I'll definitely take a look once the car cools down. I've hear that Toyota / Lexus motors don't like anything besides Denso for O2 sensors... is that true?

Fun fact: I was able to have someone ride with me and monitor my fuel trims as I drive, and it looks like my long term fuel trims are all over the place while driving. At freeway speeds it's actually pretty good, around -8%. At idle however, it's now around -23% (keep in mind I rarely see the short terms drop below -10% while driving)
People say only run the Denso. I have run a lot of different. But I have seen on some applications the Denso seems to last longer.

The other thing is. It may be restricted at the Catalytic. But not enough to throw a code.
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post #5 of 5 (permalink) Old 12-04-2018, 11:19 AM
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Could it be that the car has the o2 sensor wiring merged by the previous owner and with that wiring configuration in the stock manifold it will drive high LT and ST fuel trims. I had that same issue on my car and put the o2 wiring back to stock and it's running perfectly. You could verify it at the ecu with a multi-meter checking continuity of the o2 sensor connector pins back to the ECU pins. There are several wiring diagrams posted here that should help you locate which ECU pins are for which o2 sensors. Otherwise I would say it may be likely that the o2 sensors aren't connected properly and the engine is trying to tune using the post cat o2 sensors instead of the pre-cat sensors.

As far as o2 sensor brands making a difference I don't think it does at all. I have a denso and a bosch as primary sensors and the car runs perfectly and gets about 300 miles from full to gas light on.

2004 IS300 8N6 automatic - F&F Coilovers, Eibach sway bars, DIY Intake hardpipe, FR-S LSD swap, buddy club driver seat, Aftermarket HID's and LED Tail lights, Trunk strut mod
.....Soon to be NA-T with CX Racing hard parts, Garrett 60-1 turbo, Tial 50mm BOV, Tial MVR wastegate and FIC 6 piggyback
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