2 quick q's

[Superdave]

I've recently been informed that my placement of the o2 sensor on my headers is a bit too far away from the engine to get an accurate reading... You can see the wire hanging down in this pic, it's right on the other side of the old bung. The primaries are 28" long so the o2 is almost 40" away from the rear exhaust ports...



I am using a 4 wire in there and TMK it was working fine... but you never know... maybe the engine ran lean, got hot and that's why i spun the bearing.. it was having a hard time starting after the 25 minute drive to the strip. but the temp sensor never showed it getting warm...

Either way, i was thinking about using (2) 4 wire sensors after i fix the engine, one on the front primary and one onthe rear. Obviously i'll have to build a circuit to combine the 2 outputs for a useable signal by the computer, no problem there... but i was just curious if anyone had done this before? or if anyone had any thoughts on the placement of the sensor...


Thanks for any info,
Dave

[geoffinbc]

I dont see a pic but it can be placed in either collector or in the Y pipe. The important thing is that you keep it hot if it is non self heated. Do this by insulating the pipes. If it is a heated sensor you are ok. Ehaust leaks will throw the sensor way out.

[IsaacHayes]

I'd put it in one of the collectors closest to the engine. It may not get the whole motor's exhaust fumes, so if you go lean/rich on one bank it may not catch it, but if that's the case something is more wrong than what the computer can compensate for. At least it'd be close to the heat.

[95naSTA]

I'm with betterthanyou. Since it's a 91 I'm guessing its not a heated O2. Maybe wire up a heater circuit and just use a 4 wire sensor?

[AaronGTR]

Four wire sensors should be heated. I wasn't aware they were using them yet in '91. I guess a few cars might have, but most were two or three wire and relied on exhaust gas to warm up. Most three wires were actually heated too, but the heater grounded to the exhaust instead of having a fourth wire for dedicated ground for the heater.

You could probably use a four wire heated O2 on any car as long as you supply an ignition power source and ground to the heater wires, then just hook up the proper signal and ground. Only difference would be in the newer cars the O2 heater is computer controlled and monitored so it knows when it's hot enough to get a correct reading. If you were using a four wire with a pcm that doesn't do that, then it could think the O2 is hot enough when it isn't and adjust the fuel off of an incorrect signal. Thats my theory anyway.

I'd say move it forward if you can. It can't hurt and it will only help. I just moved my wideband O2 a couple inches further up just to make sure it was getting a good flow across the tip and to warm up faster.

[treeZ24]

guys you can add heated o2 sensors, its really not that big of a deal. so to answer the confusion, no gm was not using them as of 91. its been modified from the single wire.

the only thing dave i would be concerned about is the position of it. o2 sensors are supposed to be placed above the horizontal to make sure no moisture sits in it and ruins the unit. the only other thing is that the signal might be delayed slightly since it is so far back, but i cannot see this being a major issue.

edit* i've also thought about using the 2 sensors and combining the signals. it is doable but i think you'll have a hard time with timing and cancellation.

[IsaacHayes]

yeah I don't know about how the sensor sends a signal back, if it's just voltage, or whate measured. And combining two could be problematic..

[Superdave]

It is a 4 wire heated sensor, it came off of my 3400 Rather simple to wire in too..

So you guys are saying that even though it's so far away it will still give an accurate reading as far as rich/lean?

[IsaacHayes]

Dunno, I guess if it's hot enough then it should, the repsonse might be delayed being downstream, and it might be wet being down there is probably the main issue.

[sharkey]

i ran a single wire with my headers (22" from the head) and the only issue was when i sat for more than a few minutes the o2 sensor would not send a signal as it was too cold. i put a heated o2 sensor in and it fixed that problem. i run mine at about a 30 degree angle to the ground without issues, it was the only way to do it.

thought about running 2 signals, but to so it right, you need to run 2 signals right to the ecm and the ecm run 2 seperate banks based on the 2 o2 inputs.

[SappySE107]

Haven't tried it with a heated O2 but running a single wire is no good at that distance. The DOHC I scanned with headers (long tube) had the O2 on 1 bank, which was the front bank but clear down the downpipe. Way too long, the sensor never even warmed up other than WOT part way through 2nd gear or so.

I would be concerned with the delay at low RPMs but all you can do is try and see how the tune works. If it doesn't tune well then something will have to be changed.

The O2 switches voltage rich/lean. I have no idea how you would combine 2 O2s since they will both be switching back and forth. On a wide band I could understand it cause its straight voltage. Perhaps going wideband with a narrow band output would be the best bet.