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AF Learning Cell A at +10


subawang

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I recently replaced the air filter and cleaned the MAF, and reset the ECU and drove for a couple of weeks.

 

Afterwards, I pulled a learning view and found the AF Learning Cell A at +10. B, C and D are all very close to 0.

 

So it seems that I may have a vacuum leak at the manifold? What are the typical things to check for these symptoms?

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Perform a boost leak test and see where you are at. Usually an AF Learning A problem is pre-turbo inlet.

 

I would assume pre-turbo inlet leak would only show up in say C or D when the inlet is under vacuum. But I only have a high value for A.

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A would be a vacuum leak anywhere on the intake tract. Could be the IM gaskets (search for it) or I have also not heard the best think about the Avo TB hose...

 

I think a leak at the TB hose would be in the other direction, since it's always under pressure.

 

But I think it is possible that I have a leak at the IM gaskets, though I was hoping someone would give me another possible cause. They've already been replaced, but maybe need replacing again.

 

And I think I as one of the first forum members to find this problem way back when (http://legacygt.com/forums/showthread.php/dealer-unable-properly-diagnose-problems-81832.html).

 

Any other vacuum hoses that might trigger only cell A?

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The TB hose would still be positive in cell A because it is drawing in unmetered air when the car is in vacuum (you should never hit boost in A). The car would then go rich in boost because you are loosing air as it is being forced out under pressure. These rich trims could have easily been tuned out by your tuner in maf scaling.

 

If it was preturbo you would be lean all over cause everything before the turbo is always under vacuum. But again, B and C could have been tuned out via maf scaling.

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TB hose should almost never be under vacuum. After all, it is upstream of the throttle. And no MAF tuning has ever been done - the car is on stock intake.

 

So still back to the original question, how can A be positive and high, while B, C and D be close to 0. Anything else to check aside from manifold gaskets?

 

Since it's only A, and within the compensation range, I haven't been nagged by it enough to actually do anything. Let me know if you think it's a bigger deal.

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lol wtf, the entire intake track including the TB hose is under vacuum untill you hit boost...

 

I would suggest get a boost leak/vacuum test.

 

Yeah, I'll try a boost leak test sometime. Doesn't seem so urgent, right, since it's only A and is covered by the compensation?

 

And anything between the compressor outlet and throttle is very rarely under vacuum. If you don't trust me, check another reputable source.

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Yeah, I'll try a boost leak test sometime. Doesn't seem so urgent, right, since it's only A and is covered by the compensation?

 

And anything between the compressor outlet and throttle is very rarely under vacuum. If you don't trust me, check another reputable source.

 

Say what? It's under vacuum (relative to the ambient) until you start producing boost... The TB is under MORE vacuum because of the throttle plate creating a restriction, but upstream of the throttle plate is still under vacuum.

 

If you don't believe me or understand why this is the case, trace the vacuum line routing and see if you understand why your BPV/BOV works. If upstream of your TB was "very rarely under vacuum" then your BPV/BOV would be open an awful lot due to the TB vacuum pulling up on the BOV/BPV diaphragm/piston and your alleged lack of vacuum not pulling down on the other end of the diaphragm/piston.

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Say what? It's under vacuum (relative to the ambient) until you start producing boost... The TB is under MORE vacuum because of the throttle plate creating a restriction, but upstream of the throttle plate is still under vacuum.

 

If you don't believe me or understand why this is the case, trace the vacuum line routing and see if you understand why your BPV/BOV works. If upstream of your TB was "very rarely under vacuum" then your BPV/BOV would be open an awful lot due to the TB vacuum pulling up on the BOV/BPV diaphragm/piston and your alleged lack of vacuum not pulling down on the other end of the diaphragm/piston.

 

Your argument doesn't prove that upstream of the throttle is under vacuum. And you do realize there is a spring within the bpv right?

 

Question for you - if you are so sure there is vacuum, where do you think it comes from?

 

No need to further argue in this thread either. We can discuss via pm further if you are interested. I'm merely interested in thoughts on my issue. Let me know if you have any.

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Your argument doesn't prove that upstream of the throttle is under vacuum. And you do realize there is a spring within the bpv right?

 

Question for you - if you are so sure there is vacuum, where do you think it comes from?

 

No need to further argue in this thread either. We can discuss via pm further if you are interested. I'm merely interested in thoughts on my issue. Let me know if you have any.

Of course I know there's a spring in the BPV, and a very strong one in the stock BPV at that. But my point is that if you had an aftermarket BOV, you wouldn't be able to keep the spring as lightly adjusted as you actually are able to if what you were saying is correct and that there is no vacuum at all under the piston.

 

The vacuum comes from the the same place the TB gets its vacuum from. The only difference is that there is a throttle plate (shut) that results in there being a pressure differential. Both pre and post TB are under vacuum - it's just that the manifold is under MORE vacuum. The pressure differential is less (maybe even no difference) when the TP begins opening, but in that case, you can actually just look at your boost gauge which will read vacuum at light throttle.

 

There's a way to confirm this... Remove your BOV/BPV and cover the hole with a very sheet of paper. I bet you it gets pulled in. Still don't believe me?

 

Where do you think the air for your engine's combustion at idle comes from when the TP is closed? You think it comes from through the vacuum lines somehow?

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Of course I know there's a spring in the BPV, and a very strong one in the stock BPV at that. But my point is that if you had an aftermarket BOV, you wouldn't be able to keep the spring as lightly adjusted as you actually are able to if what you were saying is correct and that there is no vacuum at all under the piston.

 

The vacuum comes from the the same place the TB gets its vacuum from. The only difference is that there is a throttle plate (shut) that results in there being a pressure differential. Both pre and post TB are under vacuum - it's just that the manifold is under MORE vacuum. The pressure differential is less (maybe even no difference) when the TP begins opening, but in that case, you can actually just look at your boost gauge which will read vacuum at light throttle.

 

There's a way to confirm this... Remove your BOV/BPV and cover the hole with a very sheet of paper. I bet you it gets pulled in. Still don't believe me?

 

Where do you think the air for your engine's combustion at idle comes from when the TP is closed? You think it comes from through the vacuum lines somehow?

 

On my phone, but I'll try my best to type out a response.

 

Don't know much about aftermarket bpv/bov but that shouldn't influence the present discussion much.

 

My point is exactly that the vacuum does not come from the same place, it comes from the throttle as the compressor generally flows at a PR >= 1.

 

Yes, boost gauge will read vacuum under low loads, because it is reading manifold pressure, which is downstream of the throttle. One test I thought of for you, is to stab the throttle quickly at low loads and you should see the pressure quickly reach atmospheric. Assuming you agree that the turbo cannot spool up so quickly, then this should settle the discussion quickly.

 

And no, I don't plan to replaced bpv with a sheet of paper to prove anything to you (or myself), hopefully that can be accomplished with some basic reasoning.

 

And the air at idle likely comes from the throttle not being fully shut - my guess is that on drive by wire cars, they've done away with idle air control valves. I'm pretty sure I remember logging throttle percentage at idle and saw something like 2%.

 

Again, let me know if you have something to add to the discussion about my AF learning A cell being at +10.

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Maybe a better test would be to cut a hole in your TB hose or just disconnect your bov completely and let it run... Because what you're saying is that everything pre TB is the same pressure as ambient, then you wouldn't suck in any ambient air and your fuel trims would be completely unaffected, right?!
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Maybe a better test would be to cut a hole in your TB hose or just disconnect your bov completely and let it run... Because what you're saying is that everything pre TB is the same pressure as ambient, then you wouldn't suck in any ambient air and your fuel trims would be completely unaffected, right?!

 

Wow, you might want to reread what I wrote. Otherwise that's some poor reading comprehension.

 

Since you earlier post of trying a boost leak test, you haven't contributed anything useful. Feel free to show yourself the door ;)

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And the air at idle likely comes from the throttle not being fully shut - my guess is that on drive by wire cars, they've done away with idle air control valves. I'm pretty sure I remember logging throttle percentage at idle and saw something like 2%.
Yes, that's entirely correct. And if the throttle plate is just a tiny bit open, and there is a strong vacuum in the manifold, what do you think happens to the intake tract that is upstream of the throttle plate?

 

There is vacuum (albeit a small amount relative to the manifold) upstream of the throttle plate at idle and low throttle precisely because the compressor has NOT reached its threshold yet for generating airflow.

 

As far as your test goes, yes, if you notice when you blip the throttle, the amount of vacuum decreases, but it doesn't go all the way to 0. As I said, small amount relative to the manifold. Non-negligble amount relative to atmospheric.

 

Also, as far as your issue goes, in your LV, is your IAM at 1?

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Wow, you might want to reread what I wrote. Otherwise that's some poor reading comprehension.

 

Since you earlier post of trying a boost leak test, you haven't contributed anything useful. Feel free to show yourself the door ;)

 

Yeah and then you told me I was wrong on each subsequent suggestion I made. Oh well I guess I will just stop trying to help someone who obviously knows everything. I hope you blow your motor to pieces.

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Yes, that's entirely correct. And if the throttle plate is just a tiny bit open, and there is a strong vacuum in the manifold, what do you think happens to the intake tract that is upstream of the throttle plate?

 

There is vacuum (albeit a small amount relative to the manifold) upstream of the throttle plate at idle and low throttle precisely because the compressor has NOT reached its threshold yet for generating airflow.

 

As far as your test goes, yes, if you notice when you blip the throttle, the amount of vacuum decreases, but it doesn't go all the way to 0. As I said, small amount relative to the manifold. Non-negligble amount relative to atmospheric.

 

Also, as far as your issue goes, in your LV, is your IAM at 1?

 

I cannot state for sure if the compressor is overblow at idle, meaning a restriction to the engine. My best guess is that it is borderline, meaning still operating at a PR ~ 1.0. So assuming minimal losses due to such small airflow at idle, this would lead to ~1 bar pressure within the intake between the compressor and throttle plate. And gas dynamics state that the upstream gas does not know what is downstream, so a vacuum past the throttle does not translate into a vacuum upstream of the throttle.

 

IAM is at 1. Recent track day didn't impact AF learning or knock one bit.

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Yeah and then you told me I was wrong on each subsequent suggestion I made. Oh well I guess I will just stop trying to help someone who obviously knows everything. I hope you blow your motor to pieces.

 

I definitely don't know everything, which is why I started the thread. But if someone starts giving advice which doesn't make sense, I prefer to discuss. I could have just ignored you, which it now seems like I should have done.

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Infamous tune... as in done by Shamar, or their current tune? I would compare your map to stock. Specifically the AF stuff. I had a Shamar tune, and I can tell you that the MAF scaling was adjusted, despite my intake being completely stock.
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Infamous tune... as in done by Shamar, or their current tune? I would compare your map to stock. Specifically the AF stuff. I had a Shamar tune, and I can tell you that the MAF scaling was adjusted, despite my intake being completely stock.

 

Yup, shamar tune. I thought I did a thorough check on all maps he changed, but I'll have to double check the MAF scaling. But FYI, I've been running this tune (with timing touched up by myself) for ~3 yrs and only recently did this issue arise. So that leads me to question something on the hardware side, like some leak or vacuum hose popping off.

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