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

charlieRobinson wrote:

so I can narrow it down to it being a great chance the problem is a vacuum leak or a BBV stuck in the open position? I am going to get to the end of this!!! I WILL SOLVE THIS AND POST MY SUCCESS! MARK MY VERY WORDS!!!

How can I be sure it is my BBV? How do I know if it is stuck open? Will I be able to move it with my fingers? Any signs to look for?

You can see the rod move up and down on the BBV when you have the engine running and you rev the engine. The little arm should swing down when you rev it and be verticle when the engine is idleing. I am sure there is another way to test it, so hopefully someone will chime in.

- Yes, with the motor off one should be able to check the BBV actuator with his fingers. It should move freely AND be spring loaded in one direction. If the internal spring were broken, it would not close the bypass butterfly valve.

- A vacuum leak on either side of the BBV would only allow more boost - it is the TB vacuum into the top port that holds the valve into bypass mode, and the BCS allows boost to be reduced by allowing manifold pressure to be applied to the bottom port and push the diaphragm in the same direction that the TV vacuum pulls it.

- Short of the computer mistakenly commanding the BCS off, I think the likeliest candidates are a dead (and therefore open) BCS valve, or an internally damaged BBV vacuum pot.

Charlie, I cleared up a couple things in my posts on page 5, and I'd suggest reading the top half of that page to help clarify the functioning of these 3 circuits (TB vacuum, control backpressure, and electrical): https://rivperformance.editboard.com/t2083p60-faq-boost-bypass-valve-boost-control-solenoid-questions-no-boost#88568

I just thought of the most basic thing that might have been overlooked: Make sure your hoses are routed properly... From what I can see in Charlie's photo things look okay, but here's a typical Series II boost control diagram:



1 = Boost Bypass Valve Actuator
3 = Boost Control Solenoid
4&5 = Supercharger
7 = Boost Bypass Valve
8&11 = Throttle Body

98riv wrote:

charlieRobinson wrote:

so I can narrow it down to it being a great chance the problem is a vacuum leak or a BBV stuck in the open position? I am going to get to the end of this!!! I WILL SOLVE THIS AND POST MY SUCCESS! MARK MY VERY WORDS!!!

How can I be sure it is my BBV? How do I know if it is stuck open? Will I be able to move it with my fingers? Any signs to look for?

You can see the rod move up and down on the BBV when you have the engine running and you rev the engine. The little arm should swing down when you rev it and be verticle when the engine is idleing. I am sure there is another way to test it, so hopefully someone will chime in.

It was definitely moving when I revved the engine. If you look at the picture I posted above at the tubing coming from the vacuum(it says 50psi) those are definitely a shoddy job someone already did to the RIv. I was going to replace the hose with new but when I tried to take that hose off I ended up taking the nipple on the BCA with me! I think I read someone else doing this as well. I dont want to ghetto glue (jb weld) it back on so I am going to look for a new BCA If the new part and the new tube does the trick I will be satisfied. GOTTA HAVE MA BOOST!


I have been confused! I keep saying the problem is the solenoid but I just saw a picture and, in fact, it is the boost control actuator that is the problem now! The yellow circle!





http://www.3800supercharger.net/diagnosing.html
A good resource for those whose destinies bring them here


Is it ok to drive with the vaccum hose detached from the BCA? I assume it is no better or wose than whatever has been stealing my boost so far.. I am now going to blow my engine up because the vacuum line is not attached to my BCA... am I?

Does anyone know a place to get one cheaper than $40?

p.s. I know nothing about cars. ....But I want to

try moradpartscompany.com for a good used one.

But before you buy a used one, put a vacuum pump on yours and actuate it manually, just to see if it works. The vacuum lines and the little rubber ends and Ts are very sensitive and crack frequently.

it was working. but when I took the vac hose off I took the nipple with me :/

LOL, that happened to me too

BUT, i was able too "glue it back" with epoxy 5 min epoxy and it has work PERFECTLY! It's even more firmly than ever, and I applied a lil bit more on the other side (Vacuum port to the TB) to make it stronger.... and it has WORKED

Charlie, from what I've seen, $40 for a new one is good, as long as it is indeed the M90 model that fits your hoses & mounting bolts correctly. BTW, just to try to keep these darn terms straight, his part is called either the BBV actuator or BVA, Boost Valve Actuator. 'Control' is only used with regard to the BCS, the electrical Boost Control Solenoid.

As far as driving without the hose connected to the broken BVA, you are correct - it can't be any worse than it is already...
Be sure, though, to plug the hose with something so you don't add a big ol' vacuum leak.

ok, so please help me understand this even better. when the car is running WITH boost the vacuum increases pulling the rod up on the BBV which closes the hatch in the super charger's body so no air can escape and creates boost.

when the car is running with no boost the vacuum isnt strong enough to pull the rod on the BBV up so the hatch in the super charger remains open so air can escape and recycle.

Do i understand the function of the BBV accurately? Or am I way off?

boost on = rod up/spring compressed. boost off = rod down, default position spring uncompressed

You've got the basic functioning correct, you just have it backwards...

Remember, Throttle Body vacuum - what would be called 'manifold vacuum' in a non-s/c engine - is produced at low-load conditions with the throttle mostly closed, when you don't need boost. As the load increases and you open the throttle, the vacuums drops.

Thus, as in the diagram on the previous page, the vacuum is compressing the spring - holding the valve up in the bypass mode and recycling the air... As the vacuum drops along with your right foot, the spring overcomes the vacuum and pushes the valve closed, sending the air to the engine to make boost.

So,
-boost off = rod up/spring compressed.by throttle body vacuum.
-boost on = rod down, default position, spring uncompressed.

Ok, i'm stuck again. Everything looks perfect under the hood. No vacuum leaks and everything is functioning properly. Is there any chance my boost issue has something to do with my CATconverter?

What is the best way to investigate the cat to deem whether or not it's a problem?


p.s. what are rockers?

On my '98 vacuum comes from the hard plastic line which connects to a (delicate) T under the S/C snout. Looks like the '96 may get it from a different place.

Can you connect the line to the other port on the BBV?

Do you have a vacuum gauge? Can you connect/disconnect the vacuum line while the engine is running and see the valve move?

deekster_caddy wrote:

On my '98 vacuum comes from the hard plastic line which connects to a (delicate) T under the S/C snout. Looks like the '96 may get it from a different place.

Can you connect the line to the other port on the BBV?

Do you have a vacuum gauge? Can you connect/disconnect the vacuum line while the engine is running and see the valve move?

I will check under the SC snout for vacuum leaks but I am nearly positive there are none.
yes, when I start the car the BBV rod goes up and when the car is off the rod goes down.
I tried to opening the throttle under the hood to see if the rod would move but I dont think its suppose to move and/or I dont quite understand the functioning of all these parts.

I am maxing out at 1psi @ WOT. Please help me get my boost back.

also, i am pretty sure I am getting -10psi vacuum at idle. Max vacuum is probably around -13psi. Should I be seeing more vacuum?

intake seems fine, BBV seems fine, no vacuum leaks, car runs fine except for occasional slow rocking at a stop. I think it is a minor brake/rotor issue. Super charger seems fine. I am just not feeling it on the pedal. I give it everything and cant hit warp speed.

[quote="charlieRobinson"]

deekster_caddy wrote:

I am maxing out at 1psi @ WOT. Please help me get my boost back.

also, i am pretty sure I am getting -10psi vacuum at idle. Max vacuum is probably around -13psi. Should I be seeing more vacuum?

Charlie,
How are you measuring boost? I'm assuming you have a boost gauge connected. Are you sure it is correctly connected. The boost gauge has to be teed into the hard vacuum line that goes from under the SC snout, across the back valve cover. If you connect it the wrong vacuum line, you will only read vacuum. I have an Autometer Boost gauge, along with an Oil pressure gauge installed where the ash tray used to be. Vacuum is measured in in. Hg (inches of Mercury), boost is measured in PSI (lbs per square inch). I can tell you this. My engine measures a steady 20" of vacuum at idle, in gear, when it is fully warmed up. Boost maxes out at 8 psi or so. If you are seeing 10-13" of vacuum, that is very low. You are loosing vacuum somewhere. The second picture below is with my car running, in Park, and the engine is cold.

[quote="LARRY70GS"]

charlieRobinson wrote:

deekster_caddy wrote:

I am maxing out at 1psi @ WOT. Please help me get my boost back.

also, i am pretty sure I am getting -10psi vacuum at idle. Max vacuum is probably around -13psi. Should I be seeing more vacuum?

Charlie,
How are you measuring boost? I'm assuming you have a boost gauge connected. Are you sure it is correctly connected. The boost gauge has to be teed into the hard vacuum line that goes from under the SC snout, across the back valve cover. If you connect it the wrong vacuum line, you will only read vacuum. I have an Autometer Boost gauge, along with an Oil pressure gauge installed where the ash tray used to be. Vacuum is measured in in. Hg (inches of Mercury), boost is measured in PSI (lbs per square inch). I can tell you this. My engine measures a steady 20" of vacuum at idle, in gear, when it is fully warmed up. Boost maxes out at 8 psi or so. If you are seeing 10-13" of vacuum, that is very low. You are loosing vacuum somewhere. The second picture below is with my car running, in Park, and the engine is cold.

Thanks for your help, Larry. I converted Hg to PSI. but.. I too, when warmed up, am running at 21.4"Hg of vacuum.

I really do not believe I have a vaccum leak.
I am taking readings right from the ECU using Torque.

There is no vacuum leak if you have 21" of vacuum. That eliminates that. The car must feel like it's really down on power with only 1 psi of boost.

The way I understand it, the engine runs through a by pass when not on boost. When the bypass valve closes, the air must go through the supercharger. If the bypass is closing, you should be building boost.

LARRY70GS wrote:

There is no vacuum leak if you have 21" of vacuum. That eliminates that. The car must feel like it's really down on power with only 1 psi of boost.

The way I understand it, the engine runs through a by pass when not on boost. When the bypass valve closes, the air must go through the supercharger. If the bypass is closing, you should be building boost.

It sure does feel weak. I dont know if its because of the boost tests I am doing WOT or the problem itself but my gas mileage has also been poor.

So to be clear,
- You have about 30" of MAP with the key on,
- You have about 10" of MAP with the engine running,
- and at WOT you only have about 32" of MAP?

I think the rest of us agree that a plugged cat cannot be your problem. Frankly, if it was, you'd REALLY feel bogged down... If your BBV actuator moves when starting & stopping the engine (or gunning it,) and you've already checked your hoses, it is almost impossible to have a problem unless the BCS is somehow going bad when you're actually in gear.

BTW, did you ever 'feel' the vacuum-actuator with the engine off to see if it has a good spring-loading to it? It's probably a long shot, but at this point we're running out of options and the internal spring IS what closes the bypass valve in the supercharger... If I were you, the next thing I'd do is bypass the Boost Control Solenoid and take it for a ride (numbers 2,3 & 4 on that diagram I posted on the previous page.) Without the solenoid, a non-defective BBV actuator has to close the bypass valve...

Eldo wrote:

So to be clear,
- You have about 30" of MAP with the key on,
- You have about 10" of MAP with the engine running,
- and at WOT you only have about 32" of MAP?

I think the rest of us agree that a plugged cat cannot be your problem. Frankly, if it was, you'd REALLY feel bogged down... If your BBV actuator moves when starting & stopping the engine (or gunning it,) and you've already checked your hoses, it is almost impossible to have a problem unless the BCS is somehow going bad when you're actually in gear.

BTW, did you ever 'feel' the vacuum-actuator with the engine off to see if it has a good spring-loading to it? It's probably a long shot, but at this point we're running out of options and the internal spring IS what closes the bypass valve in the supercharger... If I were you, the next thing I'd do is bypass the Boost Control Solenoid and take it for a ride (numbers 2,3 & 4 on that diagram I posted on the previous page.) Without the solenoid, a non-defective BBV actuator has to close the bypass valve...

Eldo, thank you for your suggestions. The BBV does come up when the car is on. And it does have a good spring feel to it.
Here's a question: when the car is running and the vacuum pulls the BBV up.. if i pull on the throttle from under the hood should i see the BBV move? I dont think I should because there is no real load going against the engine. Am I wrong here? Should I see the BBV move when I give it throttle from under the hood?

I am out of ideas. Everything looks fine, and works fine. I just have no boost. 1psi at WOT.

Hi Charlie,

I just went out & fired up my engine and gunned it from under the hood. Any quick 1/2-throttle yank on the bell crank does makes the BBV actuator rod drop...

BTW, did those "inches" estimates of mine correspond to your conversions? I just want to make sure that millimeters, inches, water & mercury aren't causing any confusion.

no confusion on the units, Mark.
I squeeze the throttle and i dont see the BBV move.
I know the vacuum pulls it up when I start the car.
What would cause it from going back down when the throttle is open?

Also, when I was replacing the hoses the BBV nozzle broke off. before I swapped it for another I drove with the hose plugged.
I believe its tube 2 on the explosion diagram. that kept the BBV in its down position the entire time. That means I should have had boost all the time then, right? I did not have boost then. Does this mean the problem lies somewhere else?!

Hmm... All these data just seem to contradict each other.

You have a BBV that moves when you start the car, but not when you gun it. If you had the hoses backward, it would stay down when the engine was started. If the hoses are correct and the BCS was dead and in the wrong position, technically you have to make boost before its "control-hose" can push the BBV actuator back up, so you should see some movement of the rod...

That is the only thing you just said that isn't correct: If hose #2 was broken off the actuator, and you just plugged the hose coming from the manifold, then you should have had a 'pure' TB-vacuum-controlled BBV. However, you are correct when you say that you would have boost available at all times if SOMEthing was keeping the the rod down at all times...

I'd make sure that #1 is connected properly, then try gunning it again with #2 removed from the actuator. After that, I can't think of another scenario...

also, my intake manifold pressure is nowhere near 30. its at like 9

charlieRobinson wrote:

also, my intake manifold pressure is nowhere near 30. its at like 9

Well, I suspect this is one of those cases where all the danged technical terms get complicated - It's not just inches & PSI but also ABSOLUTE pressure vs vacuum.

At sea level, the MAP of a dead engine would be about 30" of mercury - or atmospheric pressure. With the engine idling, a healthy vacuum of 20" would leave a MAP of only 10" or so...

A WOT engine (Wide Open Throttle), WITHOUT a supercharger would also have 0 vacuum and therefore 30" of MAP.
Anything over 30" of MAP would therefore be supercharger boost pressure.

After reading this thread i decided to check the boost valve and solenoid. Interesting find, everything seemed to be ok but I noticed that most of the vacuum and pressure lines looked old and tired. I made the mistake of trying to take the FP line off on the engine and it disintegrated. Two hrs later after finding more bad lines I changed out:
Map sensor to blower.
Engine to T to TB
Other side of T to tranny
Engine to the top throttle body.
Engine to FP regulator
Engine to solenoid.
Solenoid to BBV
BBV to vacuum source.

Connected a boost gauge between engine and FP regulator and went for a spin.
Ist and 2nd gears were 5lbs steady until shift.
3rd gear pulled quickly to 6lbs and shifted to overdrive at 7lbs.

Decided not to do the bypass mod because after replacing all the lines the car felt faster but that might have been that is was cold today, only 60 degrees while testing.