[RESOLVED] I don't understand why venting the BOV on a stock car is bad

[Mikael SS]

I do, air is going through the MAF then out of the system, so the ecu is dumping fuel for air that isn't there.

After searching around to fix my surge/vacuum problem, i came on this:

My idle surge was a result of the following situation:

1. The idle switch is depressed (foot off the gas)
2. Too much air is getting through the throttle body (ISC stuck open)
3. O2 sensor registers lean mix (too much air)
4. ECU adds fuel to achieve correct mix
5. RPMs increase due to increased fuel/air supply
6. At 1500 RPM the ECU goes into decel mode and cuts fuel (*)
7. With only air coming into the engine, the RPMs drop rapidly
8. At 1200 RPM the ECU turns the fuel back on
9. Goto 2.

*) In our cars, the ECU cuts the fuel supply completely when the car is decelerating. It determines that you are trying to decelerate because the idle switch is depressed and the RPMs are above 1200. This is done to prevent an over lean condition and to assist in engine braking.

from: Terry's Talon Troubleshooting Tips #3 - Fast Idle (aka. Idle Surge)

This would imply that the car is only going to get excess fuel when the BOV is open and the throttle is not closed (its hard to think of a case where that would happen)

personally, i'm leaning toward the above description being inaccurate or lacking details.

Thoughts?

 

[BoostFrenzy]

You run rich as soon as you shift. If you have a A/F gauge, look at it when you let off. You will see that it lingers in the rich ratio for a bit and then drops off the chart lean. If you are trying to shift fast, then this decreases the spool up time of the turbo since you are not in the optimal A/F ratio.

It is also bad at idle for most BOVs. They can bypass the turbo system when at idle if you have your spring set too soft. The only way to fix this is to tighten down the spring or put in a stiffer spring. But, doing so may cause problems when you blow off. Because the spring is so stiff, you may get compressor surge because the spring is too hard for the air to open. So when it is open just slightly at idle from a spring that is too soft, the air jumps around the whole turbo and intercooler system and goes straight to the UICP. So you go CRAZY lean when you go to idle because the BOV is sucking in un-metered (and also unfiltered) air.

In short, you shouldn't vent unless you have a metering device placed somewhere after the BOV in the intake system. Something like a GM MAF. This both lets you vent properly, and also decreases the effects of a boost leak in the piping before the MAF. (Even though you should always fix your boost leaks because any boost leak will make your turbo run at a higher PSI.

EX: With a GM MAF installed, you are getting a leak in your coupler to a FMIC. The engine isn't as affected as if you were running a stock Mitsubishi MAS, but since you are reading a lower PSI, your wastegate will let the exhaust gas spin the turbine to a higher RPM to try and achieve the preset pressure you are running. So the gauge may say 20PSI, but the turbo could very well be pumping out 25PSI+. This is bad, because it shortens your turbos lifespan.

 

[siceclipse]

I want you dead.

 

[BoostFrenzy]

I want you dead.

WTF who me?

 

[siceclipse]

no the OP

there are enough pages on this to make an online library full.

Here is a quick explanation made by my friend JP made up from a compilation of ALL the "vent BOV" threads out there.

Bov Faq! - DSMtalk Forums

IF you cant see it:

This is a quick of list of answers to FAQ's regarding BOV's in an attempt to prevent any new members from making the mistake of making a thread asking one of the most taboo of questions.

-1- Can I just remove the tube that connects the bov to the intake pipe and plug off the hole on the pipe to make it sound louder?

Simple answer is NO! (please read on for explanation)

-2- Why does my car stall or have really low rpm's when shifting into neutral, or coming to a stop when venting my bov?

Air is metered by the maf, this tells the engine how much air is coming in. When you vent, you vent air that has already been metered, this is air that your engine in expecting to use. By venting you are throwing off the tune of your engine.

-3- My car doesn't stall and runs fine when I vent, I like the sound so whats the big deal?

Not every car will demonstrate the usual side effects of venting a bov incorrectly, but that doesn't mean that it is healthy for your engine. (please read #4)

-4- What harm can becaused by venting incorrectly?

Vented air leaves through the bov when you shift or let off the throttle, that air is accounted for by the ecu so it still adds the fuel for that amount of air. Since the balance is now incorrect it runs rich which can cause problems such as oil wash (leading to thinned oil and spun bearings), fouled plugs, carbon buildup, backfires, stuttering, stalling at idle, if the bov is open at idle (like many are) it can cause lean idle and suck in dirty/unfiltered air. It should also benoted that your car will be running richer and your gas milage will suffer greatly costing you more $ on top of the possible damage to your engine.


-5- What do I need to vent my bov correctly?

There are plenty of methods that allow you to do so in a manner that is safe for your engine. Some of these methods include the following setups:

-MAF-t setup
-AEM EMS
-VCC
-Haltech
-Megasquirt

These are all quite expensive just to attain a louder whoosh sound, and on a side note it is important to understand that with all of these setups, venting is only a "bonus" to all the tuning power they provide, which should be your primary goal.


-6- I have an s-afc so can't I just use that to compensate for venting?

No! Please read #5

-7- I just spent alot of $ and bought a new upgraded bov that doesn't allow me to recirculate. What can I do?

You may have to search around a little but most performance bovs (with the exception of some such as Tial's) offer kits that allow you to recirculate. A few of the more popular brands that offer such kits include Greddy, Blitz, and Synapse. So please do yourself a favor and find a kit that corresponds to your bov.

This was a quick thread created to answer a few of the FAQ's regarding venting your bov. I do not take credit for most of the information since it was created by compiling information posted by various veteran members of this site. However, I figured that it would be a useful thread for those considering venting their bov. I hope this answers any basic questions for those who actually search and to the mods please feel free to correct/add any information that they feel appropriate

 

[talonalex92]

Dumbest thread for the millionth time.

 

[JayRolla]

, people always try to find a way that its ok to vent, just so they can have the cool noise.

 

[dsmsucks]

I want you dead.

 

[wret]

Vent if you want. Convince yourself it's okay. Don't try to convince others.