O2 sensors on custom exhaust (1 Viewer)

[ElZilcho]

Greetings all,

I have been lurking here for a while and have found answers to every 80-series issue I've encountered, until now... Short version:

I am building a custom exhaust for my 1993 FZJ80. I am switching to a single cat and would like to replace the two pre-cat O2 sensors with a single sensor (less money, fewer things to fail) and wire the single sensor to be read by both inputs. Does any one know if this is possible or does the OBD1 ECU require input from two separate sensors?

Long version: When I bought the truck I looked into fixing the CEL so I could try to get the truck into closed loop and save some fuel. When I read the codes it showed Throttle Position Sensor fault (fixed) and O2 sensor fault (still broken). Both O2 sensors were surrounded by so much rust I couldn't see where the stud, the nut, the flange, and the pipe began or ended and decided to leave it alone. A few months ago I heard an exhaust leak and took it to a muffler shop only to have them tell me they can't get parts and quoted me a(n unacceptable) price to fab something up. I figured I would live with it. Fast forward to now. This week I was pulling a trailer up a 7% grade and when the truck downshifted into second at WOT... BAMTHUD... my exhaust leak turned into an EXHAUST LEAK!! Turns out I had fired an O2 sensor out the side of the pipe. As such, the exhaust repair won't wait any longer. Additionally, over the winter I will be replacing the turbos on my car and will have two turbos (perfectly sized for a 4.5L engine w/ a 5000rpm redline) sitting around doing nothing, so, ya know... twin turbo Toyota come spring time (I'll start a separate thread for that). Any exhaust work I do now is lead work for that.

 

[dogfishlake]

I'm not so sure you can combine the two, but you can fool the rear sensor. The front sensor dictates all fueling changes that any o2 sensor does and that needs to work properly. The rear sensor will throw codes but you can do a resistor mod to it to make it believe everything is good. There is a YouTube video about it. I did this on my 80 with a cx header and 3" exhaust and codes are gone.

 

[ajax1]

93 94 does two sensor one on each side not front and rear, if I were you I'd remove the pair system then move both o2s up to the manifolds the sensor wires will stretch to let you put them there and they fit perfect. Then cut old exhaust out and have local shop weld in new pipe and a single cat

 

[dogfishlake]

Ah, yes. I slid right past the fact that you have a 93. That is pretty cool that you can move the sensors.

Don't be shy with sharing your turbo build progress!

 

[ElZilcho]

Thank you both.

@ajax1 after you mentioned the PAIR system I found this thread 93 with one O2 Sensor?? which is pretty much the exact same question I had and the exact same solution you proposed. Thanks for your help, that is what I will do.

@dogfishlake Trust me, I wont be shy, and I will have lots of questions because I've never tuned an OBD1 system. Thanks for the help.

 

[ElZilcho]

Had some roof damage in a storm and some holiday distractions so it has been a bit tight on time and money. Here is what i made.

I decided to go with two O2 sensors (there is only one in the photo but two are installed on the truck). After thinking about my dilemma I felt that the problem with a single sensor reading both banks would be if, say, a leaking injector was causing a rich condition on a cylinder in one bank, the ECU would correct and lean out both banks mistakenly. I felt it was better to maintain independent monitoring.

Some one had done some work on this truck prior to my owning it. The catback system is custom and a bit of a hack job. One of the OEM O2 sensors had been hard wired and the other had a wire broken right at the edge of the connector negating a repair. Because the header pipes, y pipe, and flanges on the original cat were all damaged by rust, I fabricated a new single piece out of stainless and moved the y pipe pre cat. Cat is a 3-way, sized for a 5.9L engine and should adequately replace the two factory cats. Switched to 18mm threaded O2 bungs (I know this is sacrilege) so I could use cheaper more common Denso's. Truck fires, runs, and sounds good. The new cat fits up tighter and I shaved a bit of weight too. Good news is... CEL is gone!!!

Bad news is... now with everything back together the truck starts up immediately but once it starts to warm up it hesitates badly. It is almost un-driveable at times. If I floor it, the engine revs fine and the truck takes off. Once the engine is at operating temp the problem subsides significantly but doesn't fully go away. It doesn't seem like it is going to die and idles fine, if a bit low at times (4-500rpm on the tach). It almost seems like sometimes the engine just forgets that it is supposed to be an engine and doesn't respond to throttle input until you goose it at it remembers, "oh yeah, I'm an engine." I pulled plugs expecting to see them looking rich after running with a CEL for so long, but it looked like it was running lean if anything. I suspected vacuum leak and have searched high and low bit can't find anything. Next step is to test VAFM. FSM is in the mail.

Does anyone have any other ideas or recommendations?

If it's not a vacuum leak or the VAFM could it be that I placed the O2 sensors too close to the junction on the y-pipe? Does the ECU just need more time to relearn lambda? Any suggestions welcome.

 

[ElZilcho]

Also, truck runs fine if I unplug the O2 sensors. I just get the CEL again.

 

[ppc]

Check the wiring harness that passes next to the EGR. Lots of threads discuss this.