[XAAMOTTOMAAX]

So I did the evap bypass hoping to help the car run better. You know, this one...

Well it worked. The car runs better plain and simple...power is much more consistent. So I checked some data. Before doing the bypass, long term fuel trim was usually around -13% to -16%. Not ideal by any means. Well now after running for a while I checked the data again and it's running around -4% to -6%. Seems a lot more fuel is being pulled in through the evap system than one might think. Mine looks a little different than the picture above as it has a T connector and a little hose connector on the turbo inlet pipe connecting the evap hose, BPC and inlet pipe together. Anyway, thought that was some interesting stuff.

 

[greenmatt]

How did you get those figures?

 

[XAAMOTTOMAAX]

AutoTap hooked up to a car PC.

 

[Chris 9-5]

Ok I'm thick - do your finding suggest this is a good or bad thing to do?

 

[XAAMOTTOMAAX]

good. With the evap circuit in place, the ecu was having to make a fuel correction of minus 13 to 16 percent....in other words, 13 to 16 percent less fuel was needed for a stoich a/f mixture than the ecu would expect to have to provide (aka adaptation). With the evap circuit removed from the intake stream, fuel correction is now much less which means the a/f ratios are closer to where the ecu would expect them to be.

 

[greenmatt]

I used to have a similar setup hooked up to an after market ecu in my old car. What else can you see with the autotap?

 

[XAAMOTTOMAAX]

map pressure, timing advance, intake air temp, coolant temp, engine load, rpm, short term fuel trim...there's a bunch more but that's what I can think of off the top of my head.

 

[greenmatt]

Thats with an autotap reader? The $130 one?

 

[John Z Williams]

Pulling out 13% is not that much, I have seen much worse % in fuel trims and it can become tricky if it gets extreme, but don't feel its much of an advantage. When the weather changes and the fuel octane changes, the % will change again and its there to keep the a/f in check.

I see what you are saying, it had to adjust less, but don't really think its enough of a difference to even mention. If it was 50% and you got it down to 13%, now that would be something worth writing home about...

John

 

[XAAMOTTOMAAX]

I disagree. It's pretty obvious the car is running a lot better just based on feel and how it pulls and a 10% improvement in long term fuel trim accuracy could be huge IMO. Sure the car will run with crazy trim numbers just like you say. It won't even throw a trim code until it gets to more than 25% BUT occassionaly it would throw a catalyst system effeciency outside parameters code (can't remember the actual code) so even though it may not seem like a lot, the issue was significant enough to have a very noticable impact on performance. I've watched it pretty consistantly over the last couple years and seasonal gas changes had no impact on trim levels. They always remained in that 13-16% range.


How would you attempt to explain a very significant and obvious improvement in engine performance after completing this modification? It's also important to point out that the change was not obvious immediately but was obvious once the ecu had enough time to adapt. Since so many people have done the mod and pointed out a significant improvement and the point of the thread was to shine a little light on the actual cause for the improvement, I'm curious to hear your theory since the fuel trim data is nothing to write home about IYO.

 

[Guest]

I disagree. It's pretty obvious the car is running a lot better just based on feel and how it pulls and a 10% improvement in long term fuel trim accuracy could be huge IMO. Sure the car will run with crazy trim numbers just like you say. It won't even throw a trim code until it gets to more than 25% BUT occassionaly it would throw a catalyst system effeciency outside parameters code (can't remember the actual code) so even though it may not seem like a lot, the issue was significant enough to have a very noticable impact on performance. I've watched it pretty consistantly over the last couple years and seasonal gas changes had no impact on trim levels. They always remained in that 13-16% range.


How would you attempt to explain a very significant and obvious improvement in engine performance after completing this modification? It's also important to point out that the change was not obvious immediately but was obvious once the ecu had enough time to adapt. Since so many people have done the mod and pointed out a significant improvement and the point of the thread was to shine a little light on the actual cause for the improvement, I'm curious to hear your theory since the fuel trim data is nothing to write home about IYO.

Perhaps you can explain exactly what this valve does, as nobody seems to have a straightforward explanation. I'm afraid to commit to blocking the line as many of my friends are skeptics (I think mostly because of its proximity to the wastegate) - but nonetheless it is a valid question.

 

[Chris 9-5]

Perhaps you can explain exactly what this valve does, as nobody seems to have a straightforward explanation. I'm afraid to commit to blocking the line as many of my friends are skeptics (I think mostly because of its proximity to the wastegate) - but nonetheless it is a valid question.

How many times does it need explaining?

Look even has pictures too :roll:

SectorNine50][/b]

Okay I figure I'm going to explain, in detail, how this system works and why it causes boost problems. I even have a pretty picture!

Alright, so as you can see I've circled the check valves that fail. These one-way valves make it so that the vacuum from the throttlebody at lower power levels pulling air from the intake cobra. It also keeps pressurized air from the throttle body from escaping and going back before the turbo since the cobra is at negative pressure from the turbo sucking in air at a rapid rate at high power scenarios. Having the check valves set-up the way they are allows the EVAP system to purge when the engine is vacuuming or boosting.

Anyway, what happens is when the top check valve fails, it allows the charged air from the throttle body (when turbo is boosting) to flow the wrong way through the EVAP purge system's tubes. This pressure flows down through the lower check valve (doesn't matter whether it's good or bad) and runs into the intake cobra and to our boost control valve through that "T." That pressure then pushes open a switch inside the BCV that is meant to resist vacuum (or pressure from inside the BCV) and is controlled by the ECU. After that pressure runs into our BCV and out through the wastegate nipple (combining with another charged air vacuum line which is the bottom nipple) and causes the wastegate to open prematurely.

When the wastegate opens, exhaust gasses bypass the hot-side turbine inside the turbo and in return, the turbo slows down. Because it is opened prematurely, the turbo never has a chance to create full boost, and the turbo gauge reflects that.

My guess is that the reason the top one-way valve fails is because it is under a lot of pressure under WOT, and then straight to vacuum when off the throttle. This is constantly happening even in normal driving conditions. The bottom one-way valve probably lasts much longer because it never sees nearly the same amount of pressure either way.

It is perfectly okay to run the car without replacing that top check valve as the purge system will still operate correctly with one correctly operating valve. However, there is probably minimal performance loss as the pressure inside the throttle body will always be bleeding back into the intake cobra. However, on a stock tune the turbo will be able to compensate for the minor loss. People with aftermarket tunes may like to replace the valves because as we all know, any little bit of boost helps in the higher RPMs when the TD-04 can't hold 21 psi.

But as I have said - about a million times, dont do it unless you want to

 

[Kermit]

But as I have said - about a million times, dont do it unless you want to

Will it work on a Renault?

 

[Chris 9-5]

Will it work on a Renault?

I loved Renault till GM bought them

 

[Guest]

Thanks a lot chris, that was great.

How many times does it need explaining?

Look even has pictures too :roll:

[/i]

But as I have said - about a million times, dont do it unless you want to

 

[Louiep]

Answer to the OP.

I am a noob, as you can see. I hope I am allowed to post on an old thread, I wanted to answer the OP's question.

If the ECU was calling for certain torque (determined by a bunch of factors, the greatest of which are probably the throttle position, transmission temp, and current rpm), it would anticipate certain boost pressures and be ready to provide the matching fuel amount. But if pressure flowing into the BCV from the evap system is opening the wastegate prematurely, the boost will not match the ECU's boost target. The ECU then needs to adjust the fuel amount down, while wondering why your car sucks and cannot produce the boost the original ECU programming says it can.

Make sense? When you disconnect the evap system from the BCV, the wastegate acts like the ECU is expecting, so the turbo makes the pressure expected and the required fuel is thus as expected (+/-), and the trim numbers are closer to 0%.