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Post and Pre Cat sensors


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Hi, I have a 1993 or 1994 import WRX, my current exhaust has what appears to be only a pre catalytic converter sensor.  I have a stainless steel turbo back downpipe with a sports cat and a post cat sensor that i want to put on.  Does this make a difference? I have absolutely no idea. I have been told it will mess my fueling up, i have been told i need neither and i cant seem to find solid advice in searches either. The downpipe i bought is off a classic about the same age according to the advert.

Any help would be appreciated!

Cheers

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92 to early 96 have the o2 sensor in the headers late 96 onwards classics have the o2 sensor in the downpipe (after the turbo)

My current cusco downpipe has a old o2 sensor "bunging" the hole (not plugged in) and I use the standard "in header" o2 sensor.

I'm not sure if the early loom plug will reach the downpipe sensor position but if it did it would read a cooler temp and mess with the fuelling .So a o2 position change would need to be mapped in [emoji6]

If it's not broke .....upgrade it 😊

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4 hours ago, savage bulldogs said:

 

92 to early 96 have the o2 sensor in the headers late 96 onwards classics have the o2 sensor in the downpipe (after the turbo)

Well this is part of why im confused, my car is an L reg, and on the import documents says it was first used in 1994. But it has a sensor of some sort, i guess o2, after the turbo and before the cat.  Which goes against what im reading on the net.

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As you face the cam covers, left side towards the front of the car there should be a bit of loom clipped to the chassis leg . That should run to the header o2 sensor on the early scoobs .

V3 late 96 onwards classics have a bung in the headers (where the early sensor is) and have the o2 sensor in the downpipe.

It's posible that someone has fitted later headers or just moved the o2 sensor after the turbo . As although I've never seen it ,some say that subaru moved the o2 sensor to stop debris entering the turbo if the o2 sensor breaks in the pre turbo position .

If it has been moved it should've been mapped in as the temps pre turbo would be much hotter and the ecu would probably adjust mixture /pull timing to correct it .

If it's not broke .....upgrade it 😊

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Thanks for the reply mate,

Ive just got in, gonna have a coffee and let the engine cool a bit before heading back out to check.

There definitely is a wired in sensor pre cat in the downpipe, my mechanic and i saw it when the car was on the lift, the wire is attached to something in the engine, i didnt follow it all the way personally, he followed it with a torch and seemed satisfied that it wasnt just hanging or acting as a bung.

So in my mind, and im more than likely wrong lol,  if the o2 sensor has been moved and not mapped, it could explain why all my power is lower down the rev range or in the first half of accelerator travel.... I know im not describing it properly and it doesnt really make sense as i dont know whats wrong or how to explain it. But if you remember on a previous post i made when i first joined, i said that if i floor it, foot to the floor it accelerates and i can overtake, but when i ease off the throttle back to aprox half throttle i get a surge of power. So i basically only use the first half of the throttle as thats where all the power is. All my other cars as long as you dont redline you have full use of the accelerator pedal. But with this if i want the power i only have half.  Its only really noticeable at higher speeds as its an import gearbox im very rarely in 1st, 2nd or 3rd as they are so short. Put it in 5th at 60 and floor it and I can recreate it every time.

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I can't download the pics as the signals crap where I'm working today but I'll have a look and chuck a couple up of mine this evening.

Up to v2 should only have the o2 sensor pre up pipe and just a bung in the downpipe hole . Later classic have just the 1 in the downpipe after the turbo but pre cat .Whereas newage have at least 2 sensors , posibily 3 on the sti for a lambda halfway through the centre section of the exhaust.

If it's not broke .....upgrade it 😊

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Not the best pics , my bays not standard and I run a apexi fc ecu. Its not exactly in the same place as I run Groupe slip jointed headers but the standard headers have the o2 sensor in roughly in that area .

The same wire in your first pic goes to the o2 sensor which is in the headers just before the up pipe.

Under the alarm box You can just about see a black bit (o2 sensor ) jutting out from the headers and going under the chassis leg .

fee42d081e5e4adc3bed42cf8ada3798.jpg

I'm also running a cusco decat downpipe but

I've just bunged the later classic o2 sensor hole with a spare o2 sensor and have left it unplugged (as there's not anywhere to plug it in ).

686c68e559f3f818e7ca8e785162da9e.jpg

You can see the plug jus swinging there but eventually I'll fit a wideband sensor in its place [emoji6]

If it's not broke .....upgrade it 😊

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Ok, so if i check the wire in the engine bay goes to the manifold and has an o2 sensor there then i dont need one in the downpipe and can just bung it?.

If i check the wire in the engine bay and it doesnt go to the manifold but goes to the downpipe then i need it.

Thats pretty much what your saying isnt it?

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