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Discussion Starter · #1 ·
Hi guys,
today i decide to use one of the latest sunny days,so that i catch a local indy
to mount my 3"downpipe.
It fits well,during the operation I've noticed that both O2 sensors look poor-rusty and a bit deformed .
In the garage we use some kind of rust removing spray before loosen them.
Later after short trip with new pipe I've noticed that my idle is unstable-not much but revs vary.
My OEM pipe was with cat removed,the new one is also without cat-just SS and bigger.
The car starts and runs with no problem,keeps initially ok idle,but after the first touch of throttle problem appear again.
Is it possible this spray-kind of WD 40 or just refitting procedure to damage
O2 sensors and do they participate in idle control.
I doubt could it be adaptation problem because ECU found new hardware?
I've tried to perform adoption run from 2000 to red line on one gear,and now it is better,but no perfect.
Any ideas please?:eek:
 

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Discussion Starter · #2 ·
It seems that no one had similar problem so it will be mono log
"To be or not to be....."
No wasn't that -As I guess the difference between stock and 3''dp is big enough to be noticed by ECU and after another adoption run it is ok now.
Hope that T7 will use the spool benefits and not discard this hardware-Nordic St3 comes with 3"sport catalyst not downpipe,but effects should be similar.
So another drop in the ocean of Saab tuning now haw .
 

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Hi Doc, I just had the custom made DP installed with a complete 3" double end exhaust to follow next week. I was in UK while my cousin took care of this change and cat was removed. There is no engine check light so far and he said the car feels already more responsive. He didn't mention any problem as you specified and I'm glad now your problem is resolved.
 

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Discussion Starter · #4 ·
Hi and thanks for reply,many nice moments with new adds:cheesy:Just one thing left:
PICTURES PLEASE!
 

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Your 02 sensors actually do quite a bit for engine operation, however, if something were out of whack with your 02 sensors, you'd get a CEL. If you have too little backpressure from not having a catalytic converter, that can also cause a rough idle.

How back pressure works is surprisingly simple, when exhaust gasses come out of the engine, they flow out through the manifold under pressure because they are hot and expanded inside the chamber. In our case, they flow through the turbo, and then into the downpipe and out the exhaust. In our exhausts stock, we have 2 catalytic converters, a turbo, a resonator, and a muffler all creating back pressure on the exhaust evacuating from the engine. When there is a lack of pressure on the hot exhaust air it allows the hot air to expand, cool, and become more dense and flow much slower. This in effect keeps the exhaust gasses from evacuating as efficiently. Hot air will maintain it's speed, keeping it's momentum up, which in turn, creates a vacuum behind it when the valves close. (Remember, on our 4 cylinders an exhaust valve is opened 13 times a second across all cylinders at idle. That's why the exhaust gasses have enough momentum to make a difference, because it's so fast.) That vacuum aids the removal of exhausted air out of the chamber. If all the exhaust air is not evacuated, then your going to have some air in the cylinder that has no oxygen in it, which will richen the a/f ratio without the car even knowing it, especially since at idle there isn't any pressure on the air entering the chamber.

While your idle may be rough, your going to notice bigger gains in higher rpms as the exhaust manifold and turbo heat up, that is probably why after you did a couple 2000 rpm to redline runs, you noticed an improvement. The exhaust gasses are warmer for longer, creating that vacuum effect.

You may be able to smooth out the idle a little bit by heat wrapping the exhaust manifold to keep the heat up a little further into your low-backpressure exhaust. Should also help with turbo spool as well.
 

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Discussion Starter · #6 ·
Thanks a lot for good explanation.The rest of my exhaust is standard new ,MY 02 Aero -2.5"I guess with two mufflers and I suppose that back pressure isn't
too low all the time.
So I guess that after initial ''surprise"ECU will work as usual.
Since bigger downpipe and T5 cams are not included in Nordic Stage3 for Aero may be innovation stress become too much for software.
It seems that I should invest in custom software.
Thanks again and may the Force keeps evil CEL away from us!:D
 

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DoctorPhill said:
Thanks a lot for good explanation.The rest of my exhaust is standard new ,MY 02 Aero -2.5"I guess with two mufflers and I suppose that back pressure isn't
too low all the time.
So I guess that after initial ''surprise"ECU will work as usual.
Since bigger downpipe and T5 cams are not included in Nordic Stage3 for Aero may be innovation stress become too much for software.
It seems that I should invest in custom software.
Thanks again and may the Force keeps evil CEL away from us!:D
The cat-back will actually have less of an effect on back-pressure because it's far enough away from the heat source that the air already cooled significantly. It's the first few feet of exhaust that make the biggest difference. Try heat wrapping your exhaust manifold to keep the temperature up in normal driving conditions.

Oh I didn't realize you had T5 cams also. They are a longer duration cam and will tend to make the idle a little rougher as well. I bet your getting to the end of the stock ECU's adaptation abilities. I agree, get a custom tune! See if Nordic can't get you a stage 3+ tune that will account for those cams. You'll love the power! :cool:
 

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Discussion Starter · #8 ·
Now the idle is perfect-the problem appear just after dp fitting and disappear
after the 2nd adoption run from 2000 to redline on 4th.
I'm happy with Aero stage 3 as much as with my 1st stage on lpt-it is huge difference,even with winter 16"tyres sometimes I can't take turns as fast as I
want:cheesy:.
Now I'm trying to save some money for Maptun gear kit,so next power upgrade will wait for while.
 

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SectorNine50 said:
Your 02 sensors actually do quite a bit for engine operation, however, if something were out of whack with your 02 sensors, you'd get a CEL. If you have too little backpressure from not having a catalytic converter, that can also cause a rough idle.

How back pressure works is surprisingly simple, when exhaust gasses come out of the engine, they flow out through the manifold under pressure because they are hot and expanded inside the chamber. In our case, they flow through the turbo, and then into the downpipe and out the exhaust. In our exhausts stock, we have 2 catalytic converters, a turbo, a resonator, and a muffler all creating back pressure on the exhaust evacuating from the engine. When there is a lack of pressure on the hot exhaust air it allows the hot air to expand, cool, and become more dense and flow much slower. This in effect keeps the exhaust gasses from evacuating as efficiently. Hot air will maintain it's speed, keeping it's momentum up, which in turn, creates a vacuum behind it when the valves close. (Remember, on our 4 cylinders an exhaust valve is opened 13 times a second across all cylinders at idle. That's why the exhaust gasses have enough momentum to make a difference, because it's so fast.) That vacuum aids the removal of exhausted air out of the chamber. If all the exhaust air is not evacuated, then your going to have some air in the cylinder that has no oxygen in it, which will richen the a/f ratio without the car even knowing it, especially since at idle there isn't any pressure on the air entering the chamber.

While your idle may be rough, your going to notice bigger gains in higher rpms as the exhaust manifold and turbo heat up, that is probably why after you did a couple 2000 rpm to redline runs, you noticed an improvement. The exhaust gasses are warmer for longer, creating that vacuum effect.
You're probably right about the back pressure messing with the O2 sensors a little, but why this causes a poor idle is actually much simpler. The answer is that a nice smooth idle is actually the hardest thing to get an internal combustion engine to do. The reason is that there's a very long time between each combustion (I know 33 milliseconds doesn't sound like much, but it is for an engine) and very little load on the engine. The 9-5 has idle issues stock, add O2 sensors that aren't reading quite right and a change in back pressure and the idle will be weird from time to time.

SectorNine50 said:
You may be able to smooth out the idle a little bit by heat wrapping the exhaust manifold to keep the heat up a little further into your low-backpressure exhaust. Should also help with turbo spool as well.
DO NOT HEAT WRAP THE MANIFOLD. The cast manifold can not handle those temperatures and will overheat and crack. Just leave the standard heat shield in place. Believe it or not, its primary function is to keep heat in the manifold to help spool the turbo, keeping other components cool and protecting people from burns is just a bonus.

Some kind of insulating paint applied to the inside of the manifold would work as well.
 

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Raven18940 said:
DO NOT HEAT WRAP THE MANIFOLD. The cast manifold can not handle those temperatures and will overheat and crack. Just leave the standard heat shield in place. Believe it or not, its primary function is to keep heat in the manifold to help spool the turbo, keeping other components cool and protecting people from burns is just a bonus.

Some kind of insulating paint applied to the inside of the manifold would work as well.
Very good to know, thanks for that information. :cool:
 

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Discussion Starter · #11 ·
I'm now with SS downpipe and will wrap give some performance gain or not?
Also any ideas what for is this small plate just before soft connection with two holes on it? :roll:
 

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Yeah it's just a support point, you'll see when you install it.

It seems Raven may have more information on heat wrapping than I do, however, I could see how it may have a marginal effect on backpressure an performance as it would keep the EGT's up a little bit longer. However, if your not experiencing any problems with running rich or idle problems anymore, than I really don't think you have to worry much about it.
 

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Discussion Starter · #13 ·
SectorNine50 said:
Yeah it's just a support point, you'll see when you install it.
It is installed-that cause the idle disturbance,but nothing to fix this plate around.:roll:
 

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DoctorPhill said:
It is installed-that cause the idle disturbance,but nothing to fix this plate around.:roll:
Oh hrm... Not sure then... :confused:
 

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The plate with two holes in it is for mounting the DP to the car.

And you can put heat wrap around the DP if you want, but again you'll run the risk of overheating the pipes where it connects to the turbo.
 

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A better but more expensive method than wrapping is to get them ceramic coated, also if not stainless it will stop corrosion.
 
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