audiman09

hey this is totally random and irrelevant to me or my car. but i heard that on the 1.8T a Blow-off valve(BOV) is dangerous for the motor? how is this? and does this apply to the 2.7T S4? just curious as to how it would be dangerous.

AudiMick

Applies to all Audis, makes the car run lean. If you must have the silly noise use a hybrid valve .

600whpA4

It makes the car run slightly rich right as the BOV lets the air out and this happens because the air that should be going back thru the turbo is not there and that extra air has to be replaced by air that gets sucked thru the maf sensor.

The lean condition can happen at idle, but only if the BOV is allowed to be open at that time since unmetered air is being sucked into the IC and this will cause the maf readings to be lower then they should.

AudiMick

I think we are both saying sorta the same thing, a BOV confuses the ECU when the oxygen sensors detect the rich condition. The ECU then adapts the MAP as it adapts to the new condition.Closed loop systems tend to have weird consequences sometimes.

From another site. Pretty good explanation.

http://answers.yahoo.com/question/in...2161623AARemgu

"Motronic 7 is a complex system. The front O2 sensor from a 1.8t is what all the aftermarket companies use for their widebands. Your car is DAMN good at knowing its own mixture and compensating (it has a WIDE fuel trim range. It will actually compensate over problems and you'll never know it happened). So what happens is every time your BOV goes off and the engine makes a rich condition the ECU logs it and leans out the system just a bit to compensate. You keep doing it over and over and it'll keep leaning itself out to compensate. The problem is it doesn't need to be leaned out, it's only too rich when the bov is open. It's still going to trim the whole fuel map. Now you're too lean under boost, too lean at idle, everywhere.
Lean conditions are dangerous. Lean is when you get detonation, piston damage and outright engine failure."

600whpA4

Only the 2001 has a wide band, everything before it for the B5 A4 1.8t has a narrow band 02 sensor.


BTW it is a very split second rich condition. The ecu is self adjusting all the time. So if it is rich for a second it doesn't just pull fuel back and stay there. It self adjusts when the AFR changes again.

So if your AFR goes rich the ecu pulls fuel, but if it then goes lean it then adds fuel again. To think its just going to adjust once and stay lean all the time is just wrong. If you log block 032 you will see that the numbers are always changing.

RallyVermont

essentially, older A4's run with a MAF - which requires metered air to return through the intake to ensure a proper fuel air ratio.. with a BOV this air is dumped to the atmosphere, not making its way back to the MAF, causing the ecu to dump more fuel to compensate for the "lack" of oxogen found through the MAF, which causes you to run very rich and foul up your plugs, O2 sensors etc..

Cars wit MAP and not MAF's are able to utilize BOV's without hurting the system.

Long story short, don't use BOV's on cars that rely on a MAF to regulate air/fuel mix.

600whpA4

All A4's have a MAF, 2000+ also have a MAP sensor and 2001+ have a wide band 02 on top of having both of the others.

The slight rich spike is not a big issue. If you use a BOV go with a dual piston type which is made for cars with a MAF or go with a dual/hybrid which has a BPV outlet and a BOV outlet.

audiman09

thanks guys, i was just curious about the issue. im still learning the technical stuff haha