Why does exhaust leak cause lean condition?
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MistaScott
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It is not too easy for gurus to chime in or they would.
There has been no evidence (logical or otherwise) that shows that a header crack causes outside air to enter the exhaust system.
So far, my explanation (above) seems to hold the most logical water. Here it is again in shorter form:
The o2 sensor compares oxygen content in the exhaust stream with oxygen content in the surrounding ambient air.
With a header leak, the o2 does not receive as much oxygen as it should since exhaust (including oxygen) leaks out of the header crack. As a result, the o2 interprets a rich condition and the ECM consequently subtracts fuel from the mixture and lowers the BLM.
However, the mixture in the motor now becomes lean and the likelihood of knock increases.
There has been no evidence (logical or otherwise) that shows that a header crack causes outside air to enter the exhaust system.
So far, my explanation (above) seems to hold the most logical water. Here it is again in shorter form:
The o2 sensor compares oxygen content in the exhaust stream with oxygen content in the surrounding ambient air.
With a header leak, the o2 does not receive as much oxygen as it should since exhaust (including oxygen) leaks out of the header crack. As a result, the o2 interprets a rich condition and the ECM consequently subtracts fuel from the mixture and lowers the BLM.
However, the mixture in the motor now becomes lean and the likelihood of knock increases.
This is what the ratio vapor per volume, or vapor/volume represents. That is what you are contemplating. That a gas is equally distributed among another gas regardless of the volume would be a linear vap/vol. I imagine it can change with differing volumes of gas passing by o2 sensor. And a crack could thro it off
But I got to say, we do know that a lean reading is sensed by o2 sens as a lean (o2 laden) mixture.
That is SUPPOSED to then cause blms to go hi as ecm commands enrichment. BUT THAT ISWHERE IM CONFUSED, as in reality, a crack causes a rich mixture and LOW blms. That is what is backwards...the blms are supposed to go hi , as the ecm should be commanding enrichment.
So were both hung up here. Could it be that a crack cools the o2 sensor which does indeed cause an o2 sensor to think lean? But there again, in reality we know a crack causes lower blms and rich. Again , the opposite of textbook facts.
Comeon gurus, you cant all be driving Cobras can you!misterScott, summon Mr Spock.
But I got to say, we do know that a lean reading is sensed by o2 sens as a lean (o2 laden) mixture.
That is SUPPOSED to then cause blms to go hi as ecm commands enrichment. BUT THAT ISWHERE IM CONFUSED, as in reality, a crack causes a rich mixture and LOW blms. That is what is backwards...the blms are supposed to go hi , as the ecm should be commanding enrichment.
So were both hung up here. Could it be that a crack cools the o2 sensor which does indeed cause an o2 sensor to think lean? But there again, in reality we know a crack causes lower blms and rich. Again , the opposite of textbook facts.
Comeon gurus, you cant all be driving Cobras can you!misterScott, summon Mr Spock.
MistaScott
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If you read what I typed in my last post, it all makes sense:
(1) The BLM goes DOWN. Here is how: The exhaust leak fools the o2 sensor into thinking the motor is rich because LESS AIR reaches the o2 (since it escapes out of the header crack).
(2) The o2 then reports a rich condition (due to seeing less air).
(3) The ECM takes away fueling due to the rich condition.
(4) The motor gets leaner (but the o2 doesn't know it).
That makes sense to me. Case closed in my book until someone can think of something that makes better sense.
(1) The BLM goes DOWN. Here is how: The exhaust leak fools the o2 sensor into thinking the motor is rich because LESS AIR reaches the o2 (since it escapes out of the header crack).
(2) The o2 then reports a rich condition (due to seeing less air).
(3) The ECM takes away fueling due to the rich condition.
(4) The motor gets leaner (but the o2 doesn't know it).
That makes sense to me. Case closed in my book until someone can think of something that makes better sense.
Here's what's wrong with MistaScott's idea. The O2 sensor doesn't measure the QUANTITY of the exhaust. It can only measure the concentration of oxygen, not the amount. So even if some exhaust leaks out, what remains will still have the same concentration of oxygen. On the other hand, if air leaks in, which I can understand happening at idle, or at cruise, then the sensor sees more oxygen. It "thinks" there is not enough fuel, and it adds fuel. But that raises the questions that ThikStik mentioned. Unless you get the high BLMs at idle/cruise, then when you go into a boost mode, there is no inleakage, and then you get the low BLM?
Ok you know what. I don't get here you all have a certified mechanic ( myself) telling you all for a fact what happens, and you don't listen. Now if some one is smart enough to call a garage or dealker they will tell you the same Dam thing I told you.
One more thing. Have any of you heard of scavageing. How does it happen. I do not know for sure but I do know that there is vacume in the combustion chamber after the exhuast valve opens. So someone explain this to me and tell me that the same thing can't happen in the exhaust.
One more thing. Have any of you heard of scavageing. How does it happen. I do not know for sure but I do know that there is vacume in the combustion chamber after the exhuast valve opens. So someone explain this to me and tell me that the same thing can't happen in the exhaust.
MistaScott
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I am also ASE certified, doesn't make me any smarter nor does it make me an engineer. It means I can turn some wrenches, big deal.
In fact, even if Jack Roush tells me what is happening and has no logic or theory to back it up, I won't believe him.
Ormand: I appreciate your point about the o2 not being able to measure the quantity of oxygen. However, given an exhaust leak isn't the concentration of oxygen at the o2 sensor depleted?
85SHOT: I will look up scavenging, it is an interesting point and may give some insight.
In fact, even if Jack Roush tells me what is happening and has no logic or theory to back it up, I won't believe him.
Ormand: I appreciate your point about the o2 not being able to measure the quantity of oxygen. However, given an exhaust leak isn't the concentration of oxygen at the o2 sensor depleted?
85SHOT: I will look up scavenging, it is an interesting point and may give some insight.
It is not up for debate that an exh leak causes o2 to come into contact with o2 sensor. And that an o2 sensor sees this as a lean condition. These two things are reality. What is up for grabs is why does ambient air entrain itself into the exh stream. And as I posted prior, why do blm's go lower. The vapor/volume should not change given a leak. Picture a straw with a hole in it and your breathing into it. A sensor downstream would still sense the same percentage of a measured gas regardless of the leak. What we have in the exh system leak is a new gas being introduced.
RE;scavenging. That just has to be what is at play here. I do remember alittle about it. During initial exh valve opening, a strong pressure wave is formed. It shoots thru the exh pipe leaving behind it a low pressure region that pulls exh with it as piston approaches tdc. So a performance headered car should really fall out of tune and draw in outside air more than a low performance or turboed car. But I still dont see why air would entrain into exh system. Thats what this thread is for. And for whoever rated this thread a 1, do you know the answer?!
RE;scavenging. That just has to be what is at play here. I do remember alittle about it. During initial exh valve opening, a strong pressure wave is formed. It shoots thru the exh pipe leaving behind it a low pressure region that pulls exh with it as piston approaches tdc. So a performance headered car should really fall out of tune and draw in outside air more than a low performance or turboed car. But I still dont see why air would entrain into exh system. Thats what this thread is for. And for whoever rated this thread a 1, do you know the answer?!
Scavenging....thats it. During overlap, If exhaust is able to scavenge a cylinder, that means that it has more vacuum than the intake charge has. If only be it a moment. And if this wasnt the case, then exh would pop back thru intake runner.
MistaScott
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Yes, scavenging seems to offer an explanation. Here is an excerpt from a good article on the theory behind it:
"When the exhaust valve opens, a high-pressure pulse of hot, expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a suction or a low pressure rarefaction behind the pulse.
Depending on the engine, the pulse can have a positive pressure of anywhere from 5 to 15 psi with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure (I.E. VACUUM). As this low-pressure rarefaction is several milliseconds behind the initial high pressure pulse, it can be exploited to help suck residual exhaust gases out of the cylinder toward the end of the exhaust stroke as the piston approaches TDC."
So, this rarefaction creates vacuum in the header, which sucks air in from outside, causes the o2 to read a lean condition and the BLM to go higher. The ECM then adds fueling causing a rich motor.
Thanks everyone for helping to figure this out. 85SHOT had this from the beginning but I wasn't aware of how scavenging would cause exhaust to come in until it was more clearly articulated.
See http://www.sportcompactcarweb.com/tech/9912scc_ssbbpart2/
"When the exhaust valve opens, a high-pressure pulse of hot, expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a suction or a low pressure rarefaction behind the pulse.
Depending on the engine, the pulse can have a positive pressure of anywhere from 5 to 15 psi with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure (I.E. VACUUM). As this low-pressure rarefaction is several milliseconds behind the initial high pressure pulse, it can be exploited to help suck residual exhaust gases out of the cylinder toward the end of the exhaust stroke as the piston approaches TDC."
So, this rarefaction creates vacuum in the header, which sucks air in from outside, causes the o2 to read a lean condition and the BLM to go higher. The ECM then adds fueling causing a rich motor.
Thanks everyone for helping to figure this out. 85SHOT had this from the beginning but I wasn't aware of how scavenging would cause exhaust to come in until it was more clearly articulated.
See http://www.sportcompactcarweb.com/tech/9912scc_ssbbpart2/
MistaScott
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That is impossible if air is entering the header and there are no other issues. No one has said their BLM went lower except the earlier post.
The standard wisdom has always been that a header leak caused BLMs to go up. The BLM can't go down when the o2 senses a lean condition since there is a direct link between o2 and ECM fuel management.
http://www.t6p.com/forums/showthread.php?t=1127 for more info.
The standard wisdom has always been that a header leak caused BLMs to go up. The BLM can't go down when the o2 senses a lean condition since there is a direct link between o2 and ECM fuel management.
http://www.t6p.com/forums/showthread.php?t=1127 for more info.
83ttypecooled
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scavenging
HI guys,
Well, I don't see it that way at all.That's what headers are all about, causing a scavenging effect which increases airflow through the motor.What do you think?
Dale
HI guys,
Well, I don't see it that way at all.That's what headers are all about, causing a scavenging effect which increases airflow through the motor.What do you think?
Dale
MistaScott
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Re: scavenging
That's what we just figured out a couple posts ago...the scavenging pulls air in through the crack.
That's what we just figured out a couple posts ago...the scavenging pulls air in through the crack.
REDS HOT AIR
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<--- quietly steps in
pass this around your brain cells
at Idle when there is no back pressure exhaust going by a crack can pull a vaccum across the crack ( not from vaccum in the header ) which should in turn give the 02 a false lean reading
now on the top side of the scale when under boost no air will go in the crack due to the backpressure in the header but what you'll havent picked up on is the fact that exhaust is excaping under the hood of the car ...
at that point the 02 has less oxygen under the hood to compare to the oxygen in the exhaust therefor you get a false reading again
on another note an antifreeze leak under the hood can affect 02 readings also
<--- quietly steps out
pass this around your brain cells
at Idle when there is no back pressure exhaust going by a crack can pull a vaccum across the crack ( not from vaccum in the header ) which should in turn give the 02 a false lean reading
now on the top side of the scale when under boost no air will go in the crack due to the backpressure in the header but what you'll havent picked up on is the fact that exhaust is excaping under the hood of the car ...
at that point the 02 has less oxygen under the hood to compare to the oxygen in the exhaust therefor you get a false reading again
on another note an antifreeze leak under the hood can affect 02 readings also
<--- quietly steps out
"Exhaust is escaping under the hood", it true, but it is a TINY amount of exhaust gas. I agree that at idle,and at cruise, air can be sucked into the exhaust. At idle, there may even be a little build-up of gas under the hood. But at speed, there is mucho air coming into the engine compartment, through the radiator. The amount of "dilution" from the leak in the exhaust header, which is usually on the opposite side of the engine from the O2 sensor, is not going to amount to anything, IMO. Maybe from a leak on the passenger side, near the O2 sensor, but those leaks are much less common.
MistaScott
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I agree with Ormand. At speed, any exhaust leakage is going to dissipate quite quickly under the hook unless it is massive.
The backpressure of boost raises some interesting questions...so when under boost, the scavenging effect will disappear and the exhaust pulse will not draw in outside air. At idle, the exhaust will draw air into the header. Therefore, the leak shouldn't change the BLM under boost...
And Red, feel free to chime in anytime, the more the merrier in this brain party!
The backpressure of boost raises some interesting questions...so when under boost, the scavenging effect will disappear and the exhaust pulse will not draw in outside air. At idle, the exhaust will draw air into the header. Therefore, the leak shouldn't change the BLM under boost...
And Red, feel free to chime in anytime, the more the merrier in this brain party!
