System too lean problem... BOV the culprit? [Archive]

flynavy

01-24-2015, 10:49 AM

So I have searched all sorts of forums for the last 3 hours trying to find a good straight answer to no avail. It seems tha the BOV discussion is still alive and well.

I installed an HKS SSQV Blow-Off Valve about 4 years ago. Around that time I noticed that it was holding boost much better than my Greddy Type S DV. My car also started backfiring when I shifted around 3500 RPM, though, and still does. I recently (within the last 2 years) received a system too lean code. I have done almost everything I could think of: check vacuum lines, replace O2 sensor, replace MAF and it still hasn't fixed it. I have been reading more and more about how the BOV throws the ECU off, however I have also read that the HKS is one of the only BOV's that works and is specifically made for the 1.8t engine.

So the question is... Could the BOV be throwing my too lean engine code (even though it's not supposed to)? Everything is hooked up correctly. The TIP hose is plugged, I have the 3mm hose running out of the BOV, etc. However if this is the issue, I'm thinking about going for a Forge since I have heard good things. Do you think this would fix the issue? I would still like to have at least a little bit of the sound that comes along with it, and I'm about to install a Carbonio which I know will help with that.

Any help is greatly appreciated, sorry to beat a dead horse but I'm really at a loss for what the culprit behind this code is. Thanks!

Brandon

sfl_A4

02-25-2015, 07:20 PM

Hi Brandon,

I'm no expert at this, but I've been working on turbo cars for a few years and have been taught a few things so I'll share what I believe.

Let's start with how intake airflow is measured. The first thing to note is on engines equipped with a MAF (mass airflow sensor) the ECU is metering/estimating the volume of air being consumed to determine how much fuel to inject to keep the air/fuel ratio near optimum for the load. There are other sensors that help adjust for air density, such as the Intake Air Temperature sensor (IAT) and so on, but let's just keep it simple for now.

So, under load the turbo compressor spools up and pushes air into the intake pipes. With enough energy, the compressor will push so much air into the intake that it generates pressure (boost) in the intake manifold. As the intake valves open, the pressurized air in the manifold is forced into the cylinder during the intake stroke, the injectors squirt in the appropriate amount of fuel, and then as the piston starts its compression stroke the intake valves close. As the piston approaches TDC in the compression stroke the ECU triggers the spark plug igniting the fuel that then gives birth to the power stroke as the flame front advances throughout the cylinder.

Now, imagine you're racing off from a stop light, you have your foot deep into the go pedal, the throttle plate is wide open, the turbo has the intake pressurized at 20 PSI, and all of a sudden you lift off the gas pedal abruptly, shutting the throttle plate. At that point the intake manifold is still pressurized, the turbine is still spinning at 120,000 rpm driving the compressor on the other end of the shaft, which is still trying to force more air into the intake pipes. Suddenly a pressure wave begins to build in the intake pipes, pushing back against the compressor blades. This is where the diverter or bypass valve comes into play. Sensing the sudden change in pressure in the intake pipes, the diverter valve cracks open, relieving the pressure in the pipes and redirects that pressure back upstream of the compressor behind the MAF. Why behind the MAF? Because that air was already metered by the ECU and by routing it upstream of the compressor it will create a loop and recirculate that air through the compressor to keep the velocity up without adding more pressure.

Ok, so with a diverter valve the air is looping around in the intake pipes and slowly (relatively speaking) bleeds off. The loop actually allows the compressor to keep spinning with minimal reversion. Now suppose you have a BOV vented to atmosphere instead of diverting around the compressor. When you lift off the gas, the BOV opens and lets all that metered air escape into the atmosphere. But the ECU doesn't know the air has escaped, so it orders up enough fuel to match the volume of air it thinks is in the intake system. Well, if that air has escaped and the ECU doesn't know it, it will order up more fuel than it needs. The result is too much fuel and not enough air, a brief overly rich condition occurs. The excess fuel burns very slowly and is only starting to burn when the exhaust valves open and the piston pushes the unburned fuel out into the exhaust. That unburned fuel finds more air in the exhaust system and explodes, which you hear as a back-fire.

Thus, it is my belief that on MAF equipped vehicles you should either have a diverter valve to loop air in the intake, or no valve at all. But I am opposed to using a BOV that lets metered air escape to atmosphere. Additionally, I believe a BOV or a bad diverter valve are more likely to cause a rich condition, rather than a lean condition. A lean condition is when too much air and not enough fuel are ingested, which is typically caused by low fuel pressure, weak or undersized fuel pump, bad injectors, too much boost, poor intake manifold design, etc.

I have a hard time connecting a bad BOV to a lean condition, but if you feel it is running lean the best way to be sure is to use a Wide Band O2 sensor.