backfire!
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You do stand corrected. In a lean condition, all the fuel is spent, and the excess air molecules are left... that's why it's called lean. AF ratio goes sky high.
In rich conditions, there is not enough o2 for the fuel, hence, it's ejected into the exhaust system... and as soon as if find oxygen in the exhaust tract, under the right conditions (ignition caused by heat), it will explode. It's not my opinion... it's a fact.
In rich conditions, there is not enough o2 for the fuel, hence, it's ejected into the exhaust system... and as soon as if find oxygen in the exhaust tract, under the right conditions (ignition caused by heat), it will explode. It's not my opinion... it's a fact.
Originally posted by 1999Porsche911
Thanks for being respectful, but I DO NOT stand corrected. It is a LEAN condition that can cause a closed throttle exhaust backfire and NOT a rich condition. Think about what happens when the throttle is slammed closed while at high rpms. Fuel is cut off and the vacuum in the engine jumps up. Since the valves are still opening and closing, air is still being sucked into the cylinders from any intake system leak there might be. Now, I ask you, how can a rich condition exist?
Next, a lean mixture will burn eratically and unpredictable. Most often it does not burn comletely and the unburned fuel is expelled past the exhaust valves into the pipes. What can cause the "pop" is that some of the lean mixture will begin igniting and pass by the exhaust valve while still burning and ignite the fuel that is in the pipes. Extreme exhaust temps can also cause this lean mixture to expode inside the pipes.
The more fuel there is to air, the more BURN there will be. The more air there is to fuel, the MORE explosion there will be. An over rich exhaust will burn (flames) if conditions are right but will not cause backfire or a "pop" with throttle closed on deceleration.
Do not confuse a closed throttle exhaust backfire with a WOT backfire. The causes are very different. You can not have a rich condition when the throttle is closed.
I suggest you look closely for leaks on the intake system. If the backfire IS fuel related, chances are the problem lies there.
Thanks for being respectful, but I DO NOT stand corrected. It is a LEAN condition that can cause a closed throttle exhaust backfire and NOT a rich condition. Think about what happens when the throttle is slammed closed while at high rpms. Fuel is cut off and the vacuum in the engine jumps up. Since the valves are still opening and closing, air is still being sucked into the cylinders from any intake system leak there might be. Now, I ask you, how can a rich condition exist?
Next, a lean mixture will burn eratically and unpredictable. Most often it does not burn comletely and the unburned fuel is expelled past the exhaust valves into the pipes. What can cause the "pop" is that some of the lean mixture will begin igniting and pass by the exhaust valve while still burning and ignite the fuel that is in the pipes. Extreme exhaust temps can also cause this lean mixture to expode inside the pipes.
The more fuel there is to air, the more BURN there will be. The more air there is to fuel, the MORE explosion there will be. An over rich exhaust will burn (flames) if conditions are right but will not cause backfire or a "pop" with throttle closed on deceleration.
Do not confuse a closed throttle exhaust backfire with a WOT backfire. The causes are very different. You can not have a rich condition when the throttle is closed.
I suggest you look closely for leaks on the intake system. If the backfire IS fuel related, chances are the problem lies there.
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PS... when "superfans" posted "backfire" in the topic, what he actually meant, which I assume, was after fire... however, as long as I've been on car forums, most people refer to the burbling on overrun as backfire... so, it could be a matter of terms that we are getting hung up on. Technically, backfire is only on the intake side... but, most people when they refer to backfire actually is referring to the burbling or explosion in the exhaust side, as in "backside" of the car... and the term is commonly accepted in this usage.
Last edited by StephenTi; Jun 23, 2005 at 11:57 PM.
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Originally posted by StephenTi
You do stand corrected. In a lean condition, all the fuel is spent, and the excess air molecules are left... that's why it's called lean. AF ratio goes sky high.
In rich conditions, there is not enough o2 for the fuel, hence, it's ejected into the exhaust system... and as soon as if find oxygen in the exhaust tract, under the right conditions (ignition caused by heat), it will explode. It's not my opinion... it's a fact.
You do stand corrected. In a lean condition, all the fuel is spent, and the excess air molecules are left... that's why it's called lean. AF ratio goes sky high.
In rich conditions, there is not enough o2 for the fuel, hence, it's ejected into the exhaust system... and as soon as if find oxygen in the exhaust tract, under the right conditions (ignition caused by heat), it will explode. It's not my opinion... it's a fact.
Bytheway. LEAN does not mean that there is no fuel.
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Fuel can still be delivered in overrun by a leaking injector. Also intake popping has been more commonly called "lean misfire" and can be caused by overheated cylinder/plug tips, or the edge of an intake valve on a lean cylinder and can cause premature ignition,
generally more noticeable on a carbed race motor allready set up lean for power. Correct me if necessary. Jay Putnam
generally more noticeable on a carbed race motor allready set up lean for power. Correct me if necessary. Jay Putnam
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I suspect that he will find it's an air leak in the exhaust or intake system, if in fact it's fuel related. Otherwise it could be numerous other things. Most often, intake backfire only happens when the throttle is open
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Mechanic said there was a leak in the exhaust. However, after fixing the exhaust leak, there is still a little bit of popping when decelerate. Do you guys think that this may be caused by a bad "air injection diverter valve"? What exactly is an "air injection diverter valve"?
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Originally posted by superfans
Mechanic said there was a leak in the exhaust. However, after fixing the exhaust leak, there is still a little bit of popping when decelerate. Do you guys think that this may be caused by a bad "air injection diverter valve"? What exactly is an "air injection diverter valve"?
Mechanic said there was a leak in the exhaust. However, after fixing the exhaust leak, there is still a little bit of popping when decelerate. Do you guys think that this may be caused by a bad "air injection diverter valve"? What exactly is an "air injection diverter valve"?
The turbos pressurize the air as it goes though the piping, intercoolers, and into the throttle body. However, when you close the throttle body, the pressurized air is no longer allowed to flow into the engine. Thus, the pressurized air tries to equalize with the ambient air pressure outside of the car(much like air in a balloon expands and tries to escape). Well, the obvious route for the pressurized air to flow (since it can no longer go through the throttle body) is back to the turbos (the only other way it can go). You do not want to try and flow back through the turbos (spinning the turbos in the opposite direction). That is where the diverter (blow off) valves comes into play. The pressurized air is released through the diverter valve and put back into the intake system. Think of it as a pressure-release valve if you want more generic terminology.
Below is a drawing of a turbo system. The diverter valve is not pictured, but I will explain as to where it is located. As you can see, it is a closed system once air enters into the ambient air intake via the turbo (that is what your air filter intake is hooked up to on the 911 turbo). The compressed air (blue) goes into the Engine Cylinder via the throttle body. When you let off of the gas, the throttle body closes and the "blue" air in the diagram is pressurized and starts to move back towards the turbo (as you can see the arrows flow from right to left in the picture, but the air tries to flow the opposite way when the throttle body closes and the air is pressurized). The diverter valve sits between the engine cylinder and charge air cooler (intercooler). The pressure is released and diverted back to the intake (which pulls it back into the ambient air inlet).
Originally posted by Ruiner
If everything is stock, then you might have a leaking diverter valve or some other component of your system. The reason why I propose this issue (if everything is stock) is because your MAF measures air as it enters the system. If some of that air is lost (leaking diverter valve or hose), then the ECU will squirt in an appropriate amount of fuel based on what it measured. However, air has been lost and the ECU is not aware of that. That will produce a rich condition, especially between shifts. Rich (extra fuel) running into your exhaust could cause the popping easily.
Are you at stock boost? If so, how does your boost look in 3rd/4th gear at WOT? What are the peak hold values? What does it fall down to?
If everything is stock, then you might have a leaking diverter valve or some other component of your system. The reason why I propose this issue (if everything is stock) is because your MAF measures air as it enters the system. If some of that air is lost (leaking diverter valve or hose), then the ECU will squirt in an appropriate amount of fuel based on what it measured. However, air has been lost and the ECU is not aware of that. That will produce a rich condition, especially between shifts. Rich (extra fuel) running into your exhaust could cause the popping easily.
Are you at stock boost? If so, how does your boost look in 3rd/4th gear at WOT? What are the peak hold values? What does it fall down to?
Last edited by Ruiner; Jun 26, 2005 at 11:13 PM.
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