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

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.

I am still stumped by this.
When i lay on the gas it takes FOREVER to reach boost. and when I do i am already maxed out so I only get 1 psi of boost, if that.

o=gas * = boost

how it should be
oooooooooooo*********************MAX

how mine is
oooooooooooooooooooooooooo**MAX

the vacuum sticks around forever until I max out WOT then i see the tiniest bit of boost.
I know the vacuum is engine vacuum so is that part of the problem?
Is my super charger the problem?

I will take a video and show you guys. When I rev the car under the hood the BBV does not move! I can MAX out the RPMs but the vacuum never turns to boost.

I want to squeeze the horn. I want to wake it up. Please help!

Can you put a vacuum pump on the BBV and see if it moves?

Where are you measuring vacuum from? The stock vacuum line does not like to be tampered with. It's very likely you have a broken tee or connector.

It's a very simple vacuum loop - engine running, vacuum pulls the BBV open. Give it throttle, the vacuum goes away and the BBV closes.

deekster_caddy wrote:

Can you put a vacuum pump on the BBV and see if it moves?

Where are you measuring vacuum from? The stock vacuum line does not like to be tampered with. It's very likely you have a broken tee or connector.

It's a very simple vacuum loop - engine running, vacuum pulls the BBV open. Give it throttle, the vacuum goes away and the BBV closes.

Thats just it. The vacuum does not go away so the BBV never closes. I cant get the vacuum to cut off.

Engine vacuum has to go away when you open the throttle and load the motor. That is fact. You may have a leaking or disconnected line, or it is connected wrong. Your vacuum goes away when you lay into it.

It is a very simple thing to check the BBV with a vacuum source. It either works or it does not. Then check vacuum at the BBV to see if it is reading engine vacuum or not.

If the boost gauge is connected wrong, you will only see vacuum, not boost. The boost gauge must be connected to the hard vacuum line running over the rear valve cover.

Please tell me what you make of this. I have no referential experience to make a judgment.

Charlie,
You need to use a cheap analog gauge physically hooked to a vacuum tap. When you open the throttle with no load on the engine, the vacuum will at first increase slightly. If you quickly move the throttle wide open, vacuum will drop to almost 0. I'm not sure the response time on a digital gauge is fast enough to see this. In any case, you need a load on the engine. Buy a cheap vacuum gauge at any auto store, and hook it up with enough rubber hose so that you can monitor it while driving the car. You can even duct tape it to the windshield. That digital read out is very nice, but it doesn't measure boost, does it?

It does measure boost. I used it in Ruby('98 Riv) all the time and the readings were in real time.

I don't see why you can't just read the Torque app while driving in gear. Then couldn't you watch vacuum/boost under load?

charlieRobinson wrote:

It does measure boost. I used it in Ruby('98 Riv) all the time and the readings were in real time.

OK, then you need to hook an analog gauge to vacuum line to the BBV to see what it is seeing. Revving an engine with no load on it isn't the greatest thing to do. I guess you have hooked that gauge up and driven the car, and it barely registers 1 psi?

That actuator should at least blip a little. There doesn't appear to be anything wrong going on with the vac only side of the actuator. I'd check and recheck all vac/boost hoses. Your LTFT looked slightly higher than I'd expect but not vacuum leak territory yet but just make sure. Then see if the boost solenoid is functioning. You can voltage check it but then if there's voltage you have to assume it's bad. If it were me, the first thing i'd do is remove the solenoid from the boost side circuit since it should make boost without it. The solenoid functionally enables boost so if it isn't working, you'll never get boost.