RedPhoenix

iTrader: (10)

So I keep getting FOUR o2 sensor codes. I have NEW Main O2 sensors, original post cat/rear sensors. I also have gutted cats.

The post o2 sensors see that the cats are not working or something, making the car run really rich.

I have 2 options.

1)
My 2 local friends with MK3/1uz swaps do not have their rear o2 sensors hooked up. It throws a code, but they get sick gas mileage (25+ mpg). I dont want to cut the wires to the sensor, but I want to unplug it to test this to see if it works... Where the HELL is the connector to unplug the rear o2 sensors? I started to pull apart the carpet, but stopped.


2)
The second idea was to space the rear o2 sensors back about 1-2", using the spark plug non fouler trick...
http://forums.evolutionm.net/evo-how...ouler-how.html
It just lets the o2 sensor read less of the exhaust, leaning out the car.


My car is pig riched, it backfires when I left off in a gear at high rpms... it smells like gas, etc etc... and I get 14-18mpg...

Options on the o2 extension and where can I unplug my rear o2 sensors from?

1997Soarer

iTrader: (1)

I have a 97 SC300, and I deleted my rear sensor completely and replaced it with a o2 sim and heater resistor from http://www.o2simulator.com/

the o2 extensions / spacers are getting pretty popular since you can find them cheap on ebay. I know there are other ones that are stainless, and some that are even titanium.
here's one I found on google:
http://www.vexmotorsports.com/T1R-O2...or_p_1304.html

I don't recommend running unplugged sensors with check engine lights. If you do have a REAL problem, you won't know until it's probably too late.

FYI, the rear sensors don't affect fuel economy. They are just there to let you know the cat(s) are working. If your car is running super rich, then the problem is likely somewhere else...

Punky

iTrader: (4)

I dont believe the downstream o2 controls fuel trim. I think its MAF/MAP and upstream o2

RedPhoenix

iTrader: (10)

Well if someone can explain why I have 4 o2 sensor lights on with new sensors, ecu and maf, be my guest... I am in a bind, I am trying everything.

Unless someone can post here who gets good gas mileage with deleted cats on a 1uz with all the sensors in...

What does the ECU do when the rear sensors tell the computer that the cats are not working... does it make it run rich/lean? what?

Punky

iTrader: (4)

I have a decatted 1uz. Getting good MPG and o2 sensors in.

However I doubt my o2 sensors are working well considering all the leaded race gas ive ran previously. No CEL though.

Dohno

iTrader: (2)

With the check engine light on the car goes into "get home mode" and runs a richer fuel band. Thats what i have been told in the past.

t2d2

iTrader: (8)

RedPhoenix, did you ever figure this out? I've been getting similarly awful mpg for city driving (but right at rated mpg for highway, oddly), with the main/upstream O2 sensors replaced, and the rear/downstream O2 sensors untouched and presumably original. I have my original cats and no CELs.

https://www.clublexus.com/forums/per...eting-mpg.html

I found your thread in following up on a suggestion there to change the rear O2 sensors, but like the responses here, I can't think of any reason why they would relate to mpg issues.

Ali SC3

iTrader: (10)

the rear o2's do not affect the tune. they just affect the readiness codes. if you have no codes (also check for pending codes) you shouldn't have any issues with that. even with the code I do not think it affects the tune.

maybe thats just about all the mpg's you are going to get. I find my car even stock got nothing close to what the sticker said for city but does on the highway easily. I also find that my foot is heavier than most other people telling me about mpg's. Also I think people who say a 92-97 sc400 can get more than 14-15mpg in the city are possibly calculating wrong or stretching it, or their city has no traffic like a highway. Most perfectly running sc400's are not going to even see 16 in the city which has some stop and go traffic. its the nature of the v8 and the rich tune. the sc300 only gets a few mpg's better is the saddest part for me actually as a sc3 owner.

what actual gas mileage do you get t2d2?

t2d2

iTrader: (8)

Wow, that's a lot of new info to digest. Thanks, Ali! What do you mean by pending codes?

I've been getting 14-15 mph city and 22+ mph highway. I thought I had started out with better mileage than that, but it could be it was just a byproduct of whatever city/highway mix I was putting in on those tanks. From what you're saying, I'm right about where I should be and have nothing to worry about ... other than the fact that SC mileage sucks by nature.

When I drove the car the most aggressively, testing its off-the-line acceleration with the transmission solenoid bypass, that actually corresponded to the best city mileage I've recorded (18 mpg, give or take whatever percentage of the tank was highway). I guess when it's running pig rich already, it doesn't have to add much more fuel to the mix for higher RPMs.

Ali SC3

iTrader: (10)

yeah nothing to worry about most likely, the sc400 and gas mileage is a mysterious beast that comes down to many factors but for some reason babying these cars doesn't always = best gas mileage.

whenever you start them though remember the o2 sensors do not kick on for like 5 minutes or so depending on how cold the motor/sensors are to start, at least on obd1 sc400's which do not have heated sensors (some cali spec may). so if you make short trips you will always be on the worst end of the spectrum as the car hasn't even gone into closed loop where it uses the o2 sensors. even if you have city traffic, if you average longer trips, thats more time when the o2 sensor is actually working so you will save gas compared to someone who turns it on and drives for 15 minutes, then shuts it off. majority of the time is spent in the rich open loop condition, and you are right using the power you have on tap when its warming up can actually save you mpg's in an odd counter intuitive way. that changes as soon as the o2 sensors warm up though and start working, after that when you punch it you will be loosing mpg's certainly.

pending codes applies to 96+ mostly, when you put the scan tool on you may see codes that the ecu is looking at but has not fully tripped yet. its like a temp code.
this can happen on the older versions but unless you have the check engine light on for 92-95 then it should be running ok, especially if you changed the main sensors.

I don't even think I have ever replaced a post cat oxygen sensor, unless it gave no reading at all from a failure you want them to react slower than your main ones if you don't actually have a cat.

tibby12

If its running stupid rich like you say, you should be able to stand at the rear of your car and smell obvious gas fumes and not just exhaust fumes. I think everyone in this thread is on the right page saying that sc4 get ****ty gas mileage and that the rear o2 sensors just monitor the difference of the air/fuel mixture between the upstream sensors and the cat. Pretty much just an emission standard.

As for tricking the o2 sensors to not throw a code. Spark plug foulers have been used for awhile and somewhat work. Hot rod guys have been using them for years to space out the reading of the sensor. Its always a cheap and trick to try and see if you see a difference. My sc4 has foulers on the rear and stock cats. Going to get my cats deleted this weekend and we shall see how that changes things.

It doesnt seem you have a major problem though. I wouldnt too much about it except for the annoying MIL light on the cluster.

t2d2

iTrader: (8)

Aha, that's why I wasn't familiar with the term, being as I'm OBDI.

That wasn't necessarily directed at me, but... I get a pretty strong gas smell at start-up, but I haven't noticed it after that. That fits with what everyone is saying about it just being the SC400 running rich like intended.