Where exactlly was the O2 sensor located on the original set-up on 2.8L 91' Trooper. I just finished my 3.4L swap and all worked out very well, except that I used original exhaust manifolds that came with my 95' Camaro engine. So now I have three O2 sensors to choose from, one on each exhaust manifold and the original one down stream on pipe it self. I'd like to know because my original exhaust set-up looks nothing like the stock one and so I think that the sensor has been moved and not necessarily to the right spot.

The stock 2.8 O2 sensor location is the downstream one.

Yeah, the stock O2 sensor is positioned just before the catalytic converter, in the common area of the 'wye' where the left/right pipes join together.

BTW, did you have any fitment problems using the 3.4 exhaust manifolds?

I understood from seeing posts at other forums that there might be an advantage to using these, because they have larger (2") outlets than the 2.8 manifolds.

Did you have 2" downpipes fabbed up back to a 'wye' into the cat, or do it differently.

Also how does it sound and perform.

First let me say that this whole engine swap has proven to me that as usual there is more than one way to skin a cat, just as long as you're happy and truck is runnig good that's all that matters at the end.
Original 3.4 manifolds were almost new looking where as my old 2.8s were all shot to hell, plus I broke off half the studs trying to get them out. So using ones out of 3.4 was more of a necessity. I had no problem fitting them to the rest of my exhaust system, some time before engine change I put all new 2" pipes all the way back, even with the old 2.8 I noticed a difference right away in engine performance. It has a deeper lower pitch sound to it vs. the original stock. My main issue with the 02 sensor is which one to use, the original because its so far back on the pipe takes quite a bit longer to warm up and as a result it keeps ECM on by-pass loop longer. New sensors are installed right in the manifold it self and have a much better cold start performance. Only issue is where they are placed for oxygen content sensing in relation to Air pump. The original placment of 02 sensor takes into account all of the engines' exhaust gases, where as unit installed in the manifold will take each bank of cylinders as that of a whole engine +/- air pump depending where the sensor is.
I guess at the end I'm going to try get the set-up that yields best fuel economy. Having said that I'm wandering what kind of fuel economy are you guys getting once the engine swap is done and everything is working right?

Yeah, the sensor is a ways back there and I have read that it'll even cool off enough to fall out of Closed Loop if you sit and idle for prolonged periods. One reason why the SFI 3.4 locates them in the exh. man's. You'll see the V8's the same way. Better metering and the sensor stays nice and toasty warm all the time.

Also have read where a few tuners have installed the 3-wire heated O2 sensor in the original position with success.

Although there may be issues with leaving the heater on all the time vs. the usual way mfr's do it, which is to have some sort of timing device on it.

Proably a good question to pose to the Super Tuners at this site:

http://www.60degreev6.com/index.php...NUKESID=7a6076e486e332eda6e50a2fb5eaf509

BTW I wouldn't think it'd work too well to sense off of one side only. I expect GM would have done that if it was beneficial to operation.

Sensing the combined exhaust of both sides (notwitstanding the heating issue) is probably a more accurate way to do it, in the absence of dual sensors.

I just wonder, is the fact that the exhaust gasses are cooled somewhat by the time they reach the O2 sensor a contributing factor in that most 2.8's you see have a black tailpipe from running a bit too rich???? Hmmmmmmm......Just a thought! <img src="/forums/images/graemlins/coolg.gif" alt="" />

I've still got the stock wheezy exhaust so can't attest to ultimate mpg but most relate that it's better than the 2.8, as long as you keep your foot out of it!! <img src="/forums/images/graemlins/smile.gif" alt="" />

For past couple of days I've been playing with using different locations of 02 sensors seeing how they all have identical plug-ins, and it seems that the original one will yield best performance as far as fuel economy goes. I still can't get over how much better my truck feels with the new engine in it.
Aside from having almost twice the power of the fadded old 2.8 it also seems to have fixed all kinds of other running glitches. As for the re-fitted 2" exhaust I would strongly recommended it to compliment 3.4 swap. I just had mine slightly altered today to fit the different style of exhaust manifold and thus also alleviating some of the minor clearance issues that arose after new engine went in, mainly few rattles agains the frame. The whole combo is simply awsome.
On another note while I was putting new power plant in and getting some machine work done to the IM I also made a blanking plate for EGR valve just to try how the engine will run without it. I got to say that EGR it self robs a lot of power from the top end especially when you really 'give her'. Unfortunately that whole EGR thing seems to be critical to proper functioning of ECM as well as some other doubtful benefits it poseses in respect to healthy engine operation.
Too bad about the 02 sensors in the manifold it self, wish I could have used them but the engine will run either too rich or too lean depending on the location. At the end of a day this engine swap has done more for the old beast than I could ever had hoped for so I guess I shouldn't get too gready and try to get it all. <img src="/forums/images/graemlins/cheers.gif" alt="" />