FAQ: Boost Bypass Valve/Boost Control Solenoid Questions (No Boost)

Ok, I will take her out from some runs this evening and record my results.

Codith, are you suggesting a remove the BBV from the vacuum line? So the BBV will be in the down position no matter what?

I will try that next. That is basically the Bypass mod, right? I should have boost all the time when its in that configuration? And if that gives me no boost then where is my problem? The super charger?

charlieRobinson wrote:

Ok, I will take her out from some runs this evening and record my results.

Codith, are you suggesting a remove the BBV from the vacuum line? So the BBV will be in the down position no matter what?

I will try that next. That is basically the Bypass mod, right? I should have boost all the time when its in that configuration? And if that gives me no boost then where is my problem? The super charger?

No, just disconnect the hose between the solenoid and the boost bypass actuator valve and cap off the solenoid side so you don't create a vacuum leak. This will not create boosting all the time. It'll actually be damn near normal operation. The PCM just won't have the ability to actively limit boosting for torque abuse, traction control, reverse, etc.

Roger that. I was reading the beginning of this thread and on page 1:

Buickman104 wrote:

Then it may be the Boost control solenoid. When they go they tend to just dump boost. They leave you with like 2 or 3 pounds of boost and just dump everything else.

So maybe it is the solenoid?

If you have the engine running, and disconnect the vacuum line from the bbv, does it then move?

Theoretically the solenoid is optional. For example, people who swapped a supercharged engine onto a non-SC car don't have the wire that runs the solenoid, so it's not connected at all. AFAIK the solenoid is just a dump valve for the traction control to use or something. If you go to full throttle, you lose engine vacuum, and the BBV should close. It's that simple. If you connect and disconnect the vacuum line with the engine running (vacuum), and the BBV does not move, you either have no vacuum in the line, or a bad BBV. That's why I suggest putting a vacuum pump on the BBV directly.

I got on the highway and did 2 WOT runs. Here are my results. Please excuse the rascal hiphop beats and the enthusiast driving. Thank you.
video deleted

A couple things:

The "boost" is showing a positive value the entire time. For a split sec (at :05) it shows negative. This is confusing, because vacuum should be negative when off the gas, 0 when entering boost, and + when in boost. I think your app is reporting positive as vacuum - any boost then would read as a negative number. This is backwards, and decreases credibility of your app.

Do you have the free version, or did you pay the $5?

Your fuel trim is really high (lean), if the gauge is reading correct. If it's reading a true +9.4 at WOT, you have a bad lean condition. This could be related to a MAF sensor issue, or fuel delivery. It should lock at 0 every time you go WOT. If the app is meaning negative and not showing it, then you are -9.4 (rich). This is still not good, but not as dangerous.

The commanded AFR is reacting normally to your WOT.

A quick search reveals others are having trouble reading boost with this app. I'm not sure if this means it's really a problem with the app, or if it's more of a problem with the users. A scan tool that can be purchased by anyone with $5 and an Android phone is bound to have its share of bad reports. Doesn't mean it has to be a bad app!

http://www.passatworld.com/forums/60-mobile-electronics/328249-android-torque-application.html

Aaron, I think you might be right about the trim not showing the correct 'polarity'... The number goes higher when he's on the gas, then drops when he falls back to cruise state - exactly what you'd expect a negative trim readout to do when the Enrichment Mode drives the A/F ratio down to 11:1 with all that fuel, then leans it back to 14.7

Charlie, did you ever take the Boost Control Solenoid out of the equation as I suggested in December and Cody mentioned above? If not, stick a golf-tee into the "pressure hose", either in the line from the manifold to the BCS or the line from the BCS to the BBV actuator.

AA wrote:

A couple things:

The "boost" is showing a positive value the entire time. For a split sec (at :05) it shows negative. This is confusing, because vacuum should be negative when off the gas, 0 when entering boost, and + when in boost. I think your app is reporting positive as vacuum - any boost then would read as a negative number. This is backwards, and decreases credibility of your app.

Do you have the free version, or did you pay the $5?

Your fuel trim is really high (lean), if the gauge is reading correct. If it's reading a true +9.4 at WOT, you have a bad lean condition. This could be related to a MAF sensor issue, or fuel delivery. It should lock at 0 every time you go WOT. If the app is meaning negative and not showing it, then you are -9.4 (rich). This is still not good, but not as dangerous.

The commanded AFR is reacting normally to your WOT.

A quick search reveals others are having trouble reading boost with this app. I'm not sure if this means it's really a problem with the app, or if it's more of a problem with the users. A scan tool that can be purchased by anyone with $5 and an Android phone is bound to have its share of bad reports. Doesn't mean it has to be a bad app!

http://www.passatworld.com/forums/60-mobile-electronics/328249-android-torque-application.html

It's the paid version. I am not sure what that blip was at :05. It's not normal.
what should my fuel trim be?
is the 14.7 fuel to air ratio ok?
Do I more problems than what I thought?

Eldo wrote:

Aaron, I think you might be right about the trim not showing the correct 'polarity'... The number goes higher when he's on the gas, then drops when he falls back to cruise state - exactly what you'd expect a negative trim readout to do when the Enrichment Mode drives the A/F ratio down to 11:1 with all that fuel, then leans it back to 14.7

Charlie, did you ever take the Boost Control Solenoid out of the equation as I suggested in December and Cody mentioned above? If not, stick a golf-tee into the "pressure hose", either in the line from the manifold to the BCS or the line from the BCS to the BBV actuator.

I haven't. So I need to unplug the BBV line that goes to the solenoid, leaving the tube on the BBV and plugging the end of it? What should I expect from doing that? What do I need to look for after I set it up that way?

I think the 'blip' was actually a tiny bit of boost. "0" is no vacuum, and your app is reading vacuum as a positive number, so any negative value is boost. So it needs to be normal, if you want boost.

Fuel trim should lock at 0 every time you go WOT. It should stay near 0 during most driving.

The A/F ratio you're seeing is "commanded A/F ratio", not actual. All it's telling you is what the PCM wants the A/F ratio to be, based on all what it's processing from MAF, MAP, and O2. 14.7 is normal for most idle and cruise driving, but it should drop to ~11.5 when at WOT (which it does in your video).

The Fuel Trim(s) should lock at zero at WOT?
Hmmm... I'll have to look at that next time I go out.

Charlie, to remove the BCS from the equation do whichever is easier, or less likely to break a nipple:
- Either remove hose # 2 from the BBV actuator and plug it, leaving that actuator port open to atmosphere, or
- Remove hose # 4 from the BCS and plug it, leaving the solenoid port open.

Quote :

The Fuel Trim(s) should lock at zero at WOT?

Yes, ideally 0 is the target. If it's close, it's no big deal, but +9.4 is not good. It means the fuel delivery is not optimized, and the PCM is compensating for something irregular. Maybe it's too much air (intake), too little fuel (filter), or some other problem. Sometimes this can be the result of trying to mod too far without PCM tuning. If I go WOT and it's anything other than 0, I start checking things out, if necessary adjust fuel delivery to get LTFT back to where it should be.

AA wrote:

Quote :

The Fuel Trim(s) should lock at zero at WOT?

Yes, ideally 0 is the target. If it's close, it's no big deal, but +9.4 is not good. It means the fuel delivery is not optimized, and the PCM is compensating for something irregular. Maybe it's too much air (intake), too little fuel (filter), or some other problem. Sometimes this can be the result of trying to mod too far without PCM tuning. If I go WOT and it's anything other than 0, I start checking things out, if necessary adjust fuel delivery to get LTFT back to where it should be.

AA, I am freaking out. In what order should I trouble shoot this? New fuel filter.. then? should i look at my fuel injection?

Also, if I remove the vacuum to the BBV I should have boost, always. right? What does it mean if I remove the vacuum and i STILL don't have boost?

I'd solve the boost issue first if it were mine. We don't know for sure if your app is reading +9.4 or -9.4, and until we know that, there's no point in assuming you're rich or lean. Maybe you can research the app's FAQ and find out what we are seeing?

If it turns out the A/F mix is rich, it could very well be a result of your lack of boost. Fixing that may bring you back closer to zero. If you are lean, the added boost should drive the number higher, to max at +16.4. If this happens, you could have a bad/dirty MAF sensor, or fuel delivery problem.

Since there's no KR, you can keep driving the car as-is, but check LTFT again once you solve the boost issue. If it goes up, watch KR, and keep WOT activity to a minimum until you can sort it out.

Thank you Aaron. I just read through your trial with a lean fuel issue:

Quote :

Subject: Re: Code P0171 Lean Condition - Solved: Dirty MAF Sensor Sun Nov 15, 2009
The problem has been solved; turned out to be one of the simplest fixes, although a little tricky to identify. It was the MAF sensor - needed a cleaning.

After unplugging the MAF a few days ago, the result led me to believe the sensor was working, but I didn't know for sure if it was working correctly. I tentatively ruled out the MAF sensor, but I hadn't actually looked at it yet.

When I opened the sensor to inspect, I noticed some buildup on the wires. A quick session with CRC electric parts cleaner removed the build-up. Reinstalled, took the car for a long drive, and the scanner eventually showed near LTFT between -1.6 and .8 at cruise, and a lock-in at 0 for WOT. There is no more audible knocking, and KR levels are back down to 1.5º max. I managed a brief blip of 3º, but not sustained. Before I was seeing 5-6º just barely trying to accelerate on the freeway.

Thanks to everyone for your help. My car runs great now, and if you've ever wondered how important it is to clean your MAF sensor, I'll say this experience proves it can be EXTREMELY IMPORTANT. From now on I'm going to clean mine every 10k miles.

I will definitely investigate.

Also, the app is very reliable. It is certainly a positive 9.4. I did a search but I am not coming with anything potent. Where can I find more information about the fuel trim, what it means, what it does, what i should look for, etc.. Thank you!

It might be noteworthy that I am not getting any codes when I scan or a SES light.

Quote :

Also, the app is very reliable. It is certainly a positive 9.4. I did a search but I am not coming with anything potent. Where can I find more information about the fuel trim, what it means, what it does, what i should look for, etc.. Thank you!

It might be noteworthy that I am not getting any codes when I scan or a SES light.

The reason I question the integrity of the app, your vacuum reading is showing positive when it should be negative. In vacuum, the value should be around -20. In boost, it goes + above zero.

There will be no codes until both LTFT and STFT max out. +16.4 is the max in the lean direction. You're not there yet, but if the app is correct, you could go near max when boost is restored.

wooooo weeee! I feel like I just got a major upgrade!!!

How Aggie Got Her Boost Back:

Swapped out boost control solenoid with a different one.
WA---LAH!!! BOOST!

holy moses. it takes barely any gas and I am off! So this is how a supercharged 3800 feels!

I think I still have a lean issue. I went WOT (of course) and i saw 2.8 degrees of knock. and I think the fuel trim was in the %10s.

That is my next issue to tackle, just so happy to have boost back in my life!

Quote :

wooooo weeee! I feel like I just got a major upgrade!!!

Wow, I bet. I was driving the other day and decided to enter the highway with only enough throttle to get "0" boost on the gauge. I thought, this is what Charlie feels when the throttle is floored! Very sad.

See if the LTFT gets any lower over the next few days. 2.8º KR isn't that bad. Fuel trim can take several miles to relearn based on the new level of boost. Maybe tomorrow it will be 9%, then 8...

AA wrote:

Quote :

wooooo weeee! I feel like I just got a major upgrade!!!

Wow, I bet. I was driving the other day and decided to enter the highway with only enough throttle to get "0" boost on the gauge. I thought, this is what Charlie feels when the throttle is floored! Very sad.

See if the LTFT gets any lower over the next few days. 2.8º KR isn't that bad. Fuel trim can take several miles to relearn based on the new level of boost. Maybe tomorrow it will be 9%, then 8...

Thanks, Aaron. And thank you for feeling my pain and empathizing! Where can I read more about fuel trim? What it is, what it does, what it means.. etc. I searched the forum but came up with nothing.

I know earlier you said ideally at WOT it should be 0. But what should it ideally be at the rest of the time?

Also, why is it called LTFT? what does the LT mean?

So, it turns out that the BCS was dead and in the wrong position... Hmmm.
That's the reason I first suggested completely bypassing it back in December, but I surmised that there would have to be some boost created before the pressure could flow back through the open valve and push the BCV actuator back the other way. I'm amazed that it completely cancels it out. Maybe that explains the one, tiny blip of boost you saw in the video...

Fuel Trim is what the computer is doing with the fuel mixture, compared to what is considered normal. LT means Long Term and ST means Short Term, thus STFT will bounce around much more than LTFT. Personally, I've never found a satisfactory answer as to how long is 'long'... It's kind of like reading Instant vs Average MPG on a trip computer: The ST trim is right now, and I'd say the LT trim is anywhere from an ignition cycle to a week.
(I've seen mine go to zero each time I shut the engine off 3 times in a row, and other times had it retain its numbers 3 times in a row...)

A positive number means the PCM is adding more fuel than it expects to, based on the data tables that are programmed into it. A negative FT means it is using less fuel, leaning it out. Numbers in the low single digits, positive or negative, are normal and they have to go well into the double digits before the car thinks its bad enough to throw a code.

As I mentioned earlier, when I go WOT the LTFT starts reading richer (more positive) because the system is demanding a ton of fuel, and it really doesn't care about economy, smog, etc... After releasing the pedal and going back to normal cruise, the trim starts going back the other direction. I checked what Aaron said about "locking at zero at WOT", and yes, the STFT does go to zero. However, I think that is simply a temporary "censoring" in the programming as opposed to real data.

As the LTFT rises during WOT, the ST would probably be going bonkers, moving too fast to read in a digital display... I put the LTFT, STFT, Fuel Pump speed and Power Enrichment on the same screen in the scan tool, and as soon as the Power Enrichment mode goes active, the STFT locks at zero like clockwork. I played around with my throttle foot and it was always a direct correlation - as soon as Enrichment mode went INactive, the STFT started reading real numbers again.

Agree with Eldo's post. Another way to explain it, fuel trim is the correction used in the closed loop feedback control, or system, used for delivering air/fuel to the engine. There are two types of fuel trim, short term and long term.

Basically what happens - the PCM sends a certain amount of fuel through the fuel injectors into the LIM, to be mixed with air for combustion. This is mixed per the commanded A/F ratio, based on air/fuel maps built into the PCM's memory. Once the mixture is burned, exhaust passes the O2 sensor, which tells the PCM how much oxygen remains in it. This lets the PCM determine whether the combustion is rich (too much fuel), or too lean (not enough fuel), and how much to adjust the injector pulse widths.

The near instantaneous correction to the A/F based on the O2 sensor's report is STFT. This happens milliseconds after combustion in each cylinder. Observing STFT tells if the PCM is actively correcting fuel delivery, which it does regularly as load to the engine changes, for example going over hills, effects from wind, road conditions, etc.

LTFT is a running average of STFT over a longer period. It is driven by STFT over time, so if your STFT is off by a lot most of the time, it impacts the LTFT, resulting in a value indicating rich or lean. Observing LTFT tells you what your STFT is doing most of the time, and if you are generally running rich or lean.

Ideally, LTFT should be zero at all times, but it isn't possible because of the constantly changing loads present on most trips. Still, it shouldn't change much on a car that is getting the correct amount of air & fuel. Technically, LTFT that is high or low is corrected by the PCM, so it's "okay", but realize that correction takes a small amount of time, which can delay performance. Also, there is a limit to how much correction can be applied. The limits are +16.4 for lean, -30 (I think) for rich. When you reach one of these values, the engine no longer runs with the proper A/F ratio, which is bad.

Reasons fuel trim can be off: 1) bad O2 sensor, 2) vacuum or exhaust leaks, 3) dirty or bad MAF sensor, 4) any modifications to the engine, especially for performance/power.

Ways to correct fuel trims: 1) maintain/replace old sensors, 2) repair leaks, 3) tune PCM to have a fuel delivery curve compatible with engine modifications.

Glad to hear that it's finally fixed!!!

Thanks, Derek.
Aaron, I can never believe how helpful you are. You always take the time to explain things so well. Thank you.

The lean issue has vanished. The numbers on my STFT and LTFT have neutralized. everything seems to be running nicely. I have even seen an improvement with my MPG with the return of boost. This doesnt make much sense to me but I like it.

I have no idea how the fuel injectors interface with the LIM but do they just spray fuel into the LIM at the rate the PCM tells them to? And this is ran by the the fuel pump right? The PCM tells the fuel pump how hard/fast to push the fuel?

I have so many questions right now. Time to research before dinner!

Charlie,
I believe the fuel pump maintains the pressure directed by the fuel pressure regulator in the fuel rail. The PCM simply pulses the injectors for a set amount of time. If the injector is open for longer, the fuel mixture will be richer, shorter, leaner. All this is referenced to the MAP sensor, so that when there is boost, there is fuel enrichment.

It's a combination of fuel pressure from the pump, and the duty cycle ("open" time) of the injectors, but mostly it's the duty cycle, as Larry said. Fuel pressure increases by almost double as MAP increases, which helps to keep the injectors from going static (100% duty cycle) under heavy boost.

finally got the vacuum leak fixed and A-tapped the car. first run was 77 outside 91 intake temp making 24.94 PSI after work 65 87 intake temp 25.04 PSI (- atmospheric pressure) im assuming its around 14.7 making 10.4 LBS of boost. I have a k & N and a fully ported intake and outlet on the rebuilt s/c running 3.4 pulley. Is that a little low for boost? i was thinking my GTP ran around 12 with the same pulley. With the stock S/C i was making 7.9 PSI of boost with a little bit cooler temps (in the 50s) I really see that i need to make a divider for the air filter it is sucking in WaY too much hot air from the engine compartment...