Exhaust Problems

[LoopSoosStroop]

Hi Guys

I need some advice on my 2006 N/A 2.5 Fozzie.

The exhaust looks as follows (schematic):

Recently had a crack on the manifold, and while that was fixed I asked them to take out the two cats as well and mechanically bypass the sensors. Now the car has surges in some parts of the rev ranges. I'm thinking that it is because the sensors are not reading the exhaust flow?

What is the correct way to de-cat a system like this?

 

[Rally]

Best way to bypass the cats is to stick a big bit of steel and hollow them out. But you've past that stage. You will need to get your sensors to do their job and put them back into the exhaust flow. Not everything to do with emissions controls is an evil, and I'm not sure that gutting or removing the cats on a 2.5NA engine will be worth all the money and effort. It seems you've jumped out of the frypan and into the fire on this. Feel free to ask beforehand if you wish. I've done substantial modifications to cars from WRX's to Fords and Chevs and rally cars. I do an awful amount of homework beforehand and if it is new to me I seek out as many views as I can get and also analysing myself where it might all go

 

[LoopSoosStroop]

Best way to bypass the cats is to stick a big bit of steel and hollow them out. But you've past that stage. You will need to get your sensors to do their job and put them back into the exhaust flow. Not everything to do with emissions controls is an evil, and I'm not sure that gutting or removing the cats on a 2.5NA engine will be worth all the money and effort. It seems you've jumped out of the frypan and into the fire on this. Feel free to ask beforehand if you wish. I've done substantial modifications to cars from WRX's to Fords and Chevs and rally cars. I do an awful amount of homework beforehand and if it is new to me I seek out as many views as I can get and also analysing myself where it might all go

Ok, advice taken, but where to from here?

How can I find out what the specific purpose is of each sensor?

I'm guessing that the first sensor analyses the exhaust gas and uses this in a closed loop to control the mixture at any given revs, hence my problems at the moment.

The second sensor is prob the sensor that measures whether the cat is doing its job?

So if I re-install the 1st and bypass the second, I should be ok?

Best way to bypass the cats is to stick a big bit of steel and hollow them out.

I don't understand this bit?

 

[Rally]

One sensor is an O2 sensor, the other is an AFR meter- well it is here anyway and came on from I think about MY01. Put them both back in as the ECU is relying on both of them being there to do it's thing. The concept of gutting the cat by sticking something through it is to get rid of any impediment to gas flow. Having a straight through cat will allow this. But your mufflers will still be the restriction. Even should you change all the mufflers, you'll probably end up running rich and losing power, thus negating to some extent the improvement in the exhaust. I guess the next thing is to ask what you are trying to achieve?

If it is increased performance then that is understandable- these non turbo things are no rocket ships. I have looked into a cheap and easy way of boosting performance on my car but gave up because I was not prepared to spend the money needed knowing that despite doing the normal things one does to boost performance the car would still not be that quick. No bang for your buck.

 

[taza]

If it is increased performance then that is understandable- these non turbo things are no rocket ships. I have looked into a cheap and easy way of boosting performance on my car but gave up because I was not prepared to spend the money needed knowing that despite doing the normal things one does to boost performance the car would still not be that quick. No bang for your buck.

+1 Its not worth pouring money into an N/A if your looking for a performance increase. I was thinking about the same thing for my Forester but in the end I found the best way to keep my car running at its highest it to keep the oils, fluids fresh and changed regulary.
Keep the air filter clean and I run Octane 95 Unleaded fuel 9/10 tanks. I find the car runs smoother, maybe slightly better with Octane 95 over the standard 91 and I get an extra 80+km to the tank around town and an extra 130ish on the highway :biggrin:

 

[LoopSoosStroop]

One sensor is an O2 sensor, the other is an AFR meter- well it is here anyway and came on from I think about MY01. Put them both back in as the ECU is relying on both of them being there to do it's thing. The concept of gutting the cat by sticking something through it is to get rid of any impediment to gas flow. Having a straight through cat will allow this. But your mufflers will still be the restriction. Even should you change all the mufflers, you'll probably end up running rich and losing power, thus negating to some extent the improvement in the exhaust. I guess the next thing is to ask what you are trying to achieve?

If it is increased performance then that is understandable- these non turbo things are no rocket ships. I have looked into a cheap and easy way of boosting performance on my car but gave up because I was not prepared to spend the money needed knowing that despite doing the normal things one does to boost performance the car would still not be that quick. No bang for your buck.

I think we are missing each other here...?

We do not have emissions laws in South Africa. My car is on 130k km. The exhaust manifold developed a crack, causing the engine light to come on and for the cruise control light to flash. Cats only last to about 150k km. As the exhaust was coming off anyway to fix the manifold, I asked them to remove the cats and fool the sensors so that I dont ever have these lights coming on due to emissions, not for performance gains. Was cheap enough.

What is a AFR meter? Air fuel ratio?

Who knows exactly how these sensors fit in with the engine management system?

 

[Rally]

The sensors are not there for emissions so much as for the engine to run properly. The ECU needs to know what is going on. Forget about anything to do with emissions control. ANY engine be it your lawn mower or a Formula 1 has to have a certain mixture of fuel to air. This is controlled by the ECU in your car, and in order to do this the ECU needs to know certain things. Since almost every other market has emission laws, Subaru decided to not develop an ECU specifically for your market, but use something they already had. By removing the sensors, the engine does not know what is going on, so it cannot provide the right AFR (Air Fuel Ratio). You can't fool the sensors or the ECU because the ECU has been programmed in such a way that if it adjusts the AFR's in one direction, it should see a result in the exhaust which it picks up through the sensors. If it does not get the expected result, it keeps changing and suddenly you have the very problem you are experiencing. If the CEL came on, removing the sensors will never fix it with that ECU. If you don't want to run sensors, get a different ECU but I think you are creating a problem with no gain to be had. I think perhaps you are trying to stop the CEL coming on but you are trying the wrong thing to do that. You may just have a faulty sensor.

 

[El_Freddo]

+1 Its not worth pouring money into an N/A if your looking for a performance increase.

If we're talking about working the stock donk to go up against a turbo then yes I agree.

If you're after a few extra berries for your own enjoyment in cruising etc a bit of research about exhaust mods can grab a couple of extra kW but in the grand scheme of things will not turn your NA into a sports car overnight.

I've put a 2 and a 1/4 inch cat back system on my EJ22 - it certainly responds quicker than it did before and happily pulls all the way through the rev range up to the limiter quicker than I remember it doing it before. That said this is not why I got the exhaust made. 2 reasons - one was to tuck it up under the body after the lift kit went in and the other was to open up the subi note

As for your problem it would not be hard to replace those sensors where the previously were. As Rally says - the ECU needs them as they'll be giving variable readings all the time. If a resistor has been used to "trick" the ECU the reading is going to be the same all the time and will not line up with what other sensors are telling the ECU in the grand scheme of things.

All the best with it!

Cheers

Bennie