Hi everyone, I bought Hedman 68478 mid length headers and am about to install them on my '91 RS (305 with TBI and 5spd). I also plan on using Hedman's 17478 y-pipe, which doesn't have a bung for 02 sensor. I took the exhaust manifolds off but had to cut the wire for the 02 sensor because I couldn't loosen the sensor to get it out.

Will the car run fine if I don't have the 02 sensor installed or should I buy something like this to weld onto y-pipe so I can mount one? Vibrant Performance 1188 Vibrant Performance Standard Oxygen Sensor Fittings | Summit Racing

Link for headers:Hedman Hedders 68478 Hedman Elite Extreme-Duty Headers | Summit Racing
Link for Y-pipe:Hedman Hedders 17478 Hedman Y-Pipes | Summit Racing

 

You will need an o2 sensor for the engine to run correctly, that sensor measures air/fuel ratio and is pretty important on a computer controlled engine. Considering you are swapping to headers and need a new one you may want to consider upgrading to a 3 wire heated o2 sensor, pretty straight forward to do and will keep you in closed loop which you want. Lots of threads on here with part #s etc to guide you if interested.

 

What he said.

The ECM has no way to monitor and adjust the amount of fuel it feeds, other than feedback from the O2 sensor.

Yes you should install a bung somewhere. if there's not already one provided by the various mfrs of your exh system parts. The closer to the engine the better. The sensors themselves are completely universal within the constraints of thread sizes and connectors, kinda like spark plugs. Use a 4-wire one; wire the heater ground directly to the engine block somewhere convenient, and the sensor ground to the blk/wht ground system, that the rest of the sensors are connected to. Keep the 2 grounds separate so that the tiny signal from the sensor to the ECM isn't corrupted by the heater current.

 

Agreed on the 4-wire sensor.

Delco AFS-75 I believe.

 

Oh... I forgot... to power the heater, tap onto the fuel pump relay output.

 

Thanks alot guys. Do you know if installing a 02 bung on the Hedman headers would void warranty? Something tells me it would.

I'm thinking of welding it on the Y-pipe right after it connects to header collector - is that close enough to engine?

 

With a heated O2 you should be ok. Just be aware though that the further downstream you put the O2 sensor, the greater the delay between the cylinder firings and the ECM's ability to know the results of what happened. I know in the $8D code for 90-92 TPI, there are parameters in terms of O2 sensor reaction delay time which you can play around with to adjust for full length headers and stuff. Not sure about the TBI code. But you may not have problems in the end... just have to try it and see.

 

ULTM8Z, I got the ACDelco 02 sensor you recommended and have the headers and Y pipe installed. Thanks to everyone else who replied.

If I install the new heated sensor, will my stock computer/ECM recognize it? The guy at the auto shop told me to just use the stock o2 sensor because the ECM wouldn't work with the newer technology heated sensor until I re-flashed the computer.

 

A afs -74 also works and can use this pigtail.
the standard single wire and heated use the same mv range the only difference is the heating element.


the auto parts guy you spoke to doesn’t know

 

So with the AFS-75, four wire, here's the principle behind that...

1.) The ECM purple wire for the O2 signal on the vehicle harness connects to the signal wire on the O2 sensor. Once you make that connection, the ECM is happy.

2.) On the O2 sensor, find the 12V heater and ground wires. The polarity on this doesn't matter. Its only a resistor heater inside the sensor. It doesn't matter which one is ground or 12V. Connect one of those to 12V and the other to some ground point on the engine or chassis. But, make sure you have the heater wires. Do not inadvertently connect signal ground wire to 12V!!

3.) For the signal ground wire, this one is important. The ECM ordinarily uses a reference ground for what it considers O2 ground. This "O2 sensor ground" wire on the ECM schematic is not actually hooked up to the O2 sensor. On the ECM schematic, the O2 sensor ground wire coming off the ECM actually just gets tied into the main ECM grounds. On the factory setup, there is no such actual ground wire coming off the single wire O2 sensor back to the ECM. The single wire O2 sensor grounds itself to the exhaust metal. So if the O2 sensor ground to the exhaust metal is not good, the ECM will not be seeing what the true O2 signal really is. So...

This signal wire coming off the AFS-75 should be run back to the main ECM ground point. On mine, all of my ECM grounds are collocated on a ground stud on the back of the cylinder head. I ran my AFS-75 signal ground wire to that stud. You'll need to find where your ECM ground point is. Once you do that, NOW you have true O2 sensor ground that the ECM will see. The AFS-74 three-wire sensor doesn't have this signal ground. It also relies on the ECM's ground "reference", which requires a good sensor ground to the exhaust metal. The AFS-74 usually works ok, but I wanted the most accurate possible signal for my setup, so I went with the AFS-75.

That said, the actual O2 sensor component within all these sensors is exactly the same, so they all send the same signal to the ECM. So your parts guy doesn't know what he's talking about.

 

Thanks alot ULTM, and Merry Christmas. I got the 02 bung welded in on my y-pipe.

I've also got:

• 4 way flat connector (18 ga wires)
• long length of 18 ga wire to extend wires from 02 sensor to proper termination points

Using what you and Armored91Camaro say here: 4-Wire 02 sensor setup on 8746 ecm - Third Generation F-Body Message Boards I'm slowly understanding where to wire these suckers.

Blue from sensor -> purple stock 02 sensor wire
White from sensor -> ground stud on pass side cylinder head?
Black #1 -> grounded to engine block?
Black #2 -> wired to a 12 volt power source like fuel pump?

Me understanding this wiring is like explaining cloud computing to a Neanderthal - I apologize for the slow uptake!



Planning on using this in between sensor and termination points so I can unscrew sensor easily

There are actually 5 wires coming out of the sensor..I think the 2nd white is spliced into the other one before the wires emerge from sheath near connector

 

Not sure if you have a question in your post. But yeah that basically describes it.

Quote:

White from sensor -> ground stud on pass side cylinder head?

ideally you want to see where the tan "O2 ground" wire is going from the ECM and tie it there. But if all your grounds are good, then what you're proposing would be adequate.

 

Yea, my mistake not making it clear if I was asking a question. I was - mainly wanting to know if what I was planning was right.

I don't know where the tan wire is that you mention is grounded, but I did see some half black/half white wires going into pass side cylinder head grounding stud. Should I ground the 02 sensor signal wire somewhere else?

Here is my plan for wiring it right now:

 

Just to follow up for posterity's sake: I installed the o2 sensor as shown in the hand drawn diagram shown above and it appears to have worked fine. I drove the car from CA to SC last week and didn't notice any problems - not that I could tell whether I was in 'open' or 'closed' loop mode! Notice to 3rd gen amateurs - unless you really want a multi-wire 02 sensor, I recommend sticking to the stock one-wire one. This wiring project was a big pain in the ***, not to mention very confusing.

 

If you don’t have a scanner field service mode will show ol or cl with the mil.

 

The problem with the 1 wire sensor in headers is that it can cool off and cause the ECM to fall out of closed loop.

 

Thanks Tuned and ULTM. Didn't know either of those things.

 

​​​​​​​How so? Enlighten us.

 

http://www.casperselectronics.com/ca...product_id=101

there is a kit but should be straight forward to wire 4 wires.

 

Well, I wouldn't take it for granted that wiring and electrical schematics are intuitive to everyone. What may seem simple to us may not be to others.

This is not a slam against the OP either, or anyone else. It took me several years of playing around with wiring before I got comfortable with it.

 

This is my first 3rd Gen, and the first time I've really tinkered with cars, but to elaborate why I found it so confusing:

1. Figuring out the exact orientation and position of sensor itself wasn't dead simple because the tight quarters and the fact that you can't truly know how it will 'sit' without drilling the hole and installing bung (a serious project in itself requiring a welder which I don't have).
2. Figuring out which wire was which coming out of sensor, and where they connect on the car.
3. Soldering 8 wire connections to and from 4-way flat connector.
4. Positioning/fastening of 4-way flat connector (ended up making a custom bracket and drilling it onto the clutch fork protector thing)
5. Routing the 4 wires to the sensor (making sure they don't touch headers, don't hang too low etc.)
6. Tapping into fuel pump relay wire. I spliced my 18ga wire for sensor into a cut section of the gray fuel pump relay wire. This entailed soldering in an awkward position while my brother held the wires together.
7. Modifying the ring terminal to fit the 3/8" ground stud on pass side cyl head. Couldn't find a ring terminal with a 18ga connection on one side and a 3/8" ths on the other. Had to drill it out to fit.
8. Probably more reasons if I thought about it more, but feel I've made my point clear enough. If it weren't for TGO I'd have thrown my hands up and walked away.

Anyway, I'll take detailed pics next time I'm under the car - hopefully it will help the next guy.

 

I always run the 3 wire O2, sensors, much easier.
Power from ign terminal in fuse box with a inline fuse.