no O-2 sensor

anybody know of any potenially bad side-effects of disconnecting this. Any good ones?

The computer needs it to control mixture. No O2 sensor would be bad if it runs at all.

More input - is this RENIX (1987-90,) OBD-I (1991-95,) or OBD-II (1996-2001?) If it's OBD-II, which sensor are you talking about pulling - the one in the exhaust manfold, or the one in the pipe after the catalytic converter?

For the "downstream" HEGO (after the cat, 1996 and up,) that sensor is used to provide feedback as to whether the catalytic converter is running - disconnecting this sensor can eventually result in "throwing a code" - the ECU will tell you that either the downstream HEGO or the cat itself is shot. There are simulators that will fool the OBD-II ECU into thinking that everything down there is just ducky - check with Summit or Jeg's.

For the "upstream" HEGO (the ones in the exhaust manifold, regardless of year,) the ECU will not be able to enter "Closed Loop Operation" - where the fuel inejction system control is augmented by the feedback signal after combustion. If the sensor is unplugged, the effects are unpredictable (but I believe it tends to result in "False LEAN" signalling.) If the sensor is removed and allowed to hang free - but remain connected electrically - you will get a false LEAN everytime the system tried to go into Closed Loop - and you will get a code for HEGO failure (most probably) or Closed Loop Failure (second most likely.)

The false LEAN signals will typically result in decreased fuel efficiency, since the ECU will read excess exhaust oxygen and attempt to compensate by enriching the AFR. In extreme cases, this can result in a loss of power - and the reduced mileage will be readily noticed.

Failure to enter Closed Loop operation will also be likely to cause a code, and will result in decreased power and fuel mileage.

Having said that, why is it everyone lately is asking about disconnecting critical sensors? I can see working to modify a few outputs (CTS and IAT come to mind,) but there is no sensible reason for pulling the upstream HEGO, so why do it?

The point of fooling the ECU in certain instances is to increase power, but pulling the HEGO does not accomplish this. It also throws the system into a tizzy - it can't keep track of what it's doing very well.

If you are looking to modify electronic control systems in order to make more power, it would be a good idea to research such systems before throwing out ideas to modify them - especially since these ideas usually sound silly, and you really don't need to make large modifications to get results! Search around, spend some time at the library, and you'll be able to not only answer your own question - but understand just why these things work the way they do! I'm not a huge fan of electronic controls, but I've made an effort to understand the systems on my own before I start playing about with them.

You can find decent books on the subject of electronic controls in general by Haynes and Chilton (I didn't say they were GOOD - just decent,) and a little research into the publishing lines of Mitchell, Motorbooks, HP Press, Bentley Publishing, and other automotive specialty publishers can get you information that would be invaluable.

I'm not trying to sound like I'm coming down on this idea - at least, not on purpose - but it seems like a lot of silly questions have been coming down lately, and I'm just trying to do my bit to slow them down. There's nothing wrong with asking questions - but you really should do a bit of preparatory research into the subject so you will understand the answers. I don't see a point in answering someone with a question about modifying a system when they make it clear they don't understand enough about how it works to understand why the mod will or won't work - and there is a LOT of information out there!

[/RANT]

5-90

If that is the case, then can you explain how the export models can run with-out it? I believe that dr.dynos' xj is like this. Also, wouldn't the computer fall back into what I think they call "open-loop"? I am tring to learn about EFI systems because I want to be able to adjust or build/modify it on my own instead having to catch up with people that I know that work on this to help me get things to work right. Thanks, Ren

Okay, first off, thank you to 5-90 for the helpful, slightly ranty, information about how the o-2 sensor functions, and why. Fwiw, this is a Renix system. The idea came from just curiosity mostly, and wanting to minimize the elec. junk under the hood. I guess that these sensor stays, along with the IAT, CPS, TPS, and CTS. Are there any other sensors that are needed to operate this sytem ( MAP sensor stays, also)? Secondly, like I typed, I am trying to learn about inj. systems and have had the local library order some bucks for me, but until they get here, I am going to have to use the 'net, so once again, please bear with me, maybe(please), reference some sites for myself and others to start with. You seem to have a good working knowledge of the way EFI works and I am glad that you choose to share it. Maybe one day I will be able to work on this stuff as well as I can on the mech. stuff. That is my goal . Thanks for putting up with my rant, Ren

Why do you want to minimize te electrical components? They have a purpose, its not like the days of vacuum controlled smog reducing carbs- with all that useless junk everwhere. The sensors are used to optimally control the fuel & ignition. Some export models lacked an O2 sensor because of concerns that people in these countries would only have leaded gasoline avaliable. These models have a slightly different computer, it doesn't expect HEGO input so it uses other inputs to control the fuel ratio and probably runs a little richer (slightly fewer mpg) than USDM models. Some things that I would at least try removing & plugging are the exhaust gas recirculation, the air warmer, air pumps and that baffle thats in the air box, if I had any of those components (I don't). Oh and that stupid sensor that says youre out of washer fluid when its 3/4 full.

You mean that the sensor on hte washer botlle was really MEANT to do something. I thought was just one of those stupid things that the factory installed so that they could charge you more money for useless junk.LOL
Seriously though, I have already made my own cold air kit, and am just trying to get the 4.0 to run better before I yank it to replace it with something better (read NASTIER). I appreciate all the help so far. I am used to, as you put it, dealing with the smog-dog junk, and have gotten pretty good at getting it to work with most of the junk still in place. I am just trying to finallly get caught up with everyone else and the EFI stuff. Ren

Map sensor is very important...engine will run very rich without it and you can see the exhaust is black like a diesel without it hooked up right. most of the electronic whiz-dads are important under there. and if you do a lot of off roading or even dusty road driving, i'd say the cold air kit thing isn't a great idea. fwiw...i had one i made on mine for a while with a K&N filter. it helped for a little bit, but it clogged super fast and even when it was clean, my always super-clean oil looked like grandma's old molases. i decided to go back to the factory air box and it cleaned my oil up, and functioned about teh same as a half-dirty cold air setup. I got a K&N drop in filter and i've been running it for about a month and a half and my oil is super clean and i've got way more power than a paper filter or an even semi-dirty cold air filter.

Well, like I said the map is staying because I understand at least that one's function. As for the filter, I am starting to think that maybe you are on to something, but, I will have to make an enclosure for my open style because the space where the factory box goes is now home to other stuff that I am not going to remove or re-locate due to a lack of room under the hood. (Boy does that sound familiar, or what?) Maybe I can find or build something that will work in the space that is left. Ren

yeah that's something I had always wanted to do on mine from the beginning, but I never did it and i pretty much would have had to spend way too much time cleaning the filter and building the deflector that it wasn't worth it and i had friends who were quite happy with their drop ins so i just went back to that style. i do like the amount of space getting rid of that box frees up though. i've always wanted to relocate my battery and get rid of all my A/C junk in the pass side and put stuff over there. anyway, good luck to ya!