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

Ed has one for $45...but if I can get for $25 all the better!

However, since there's no illustration...is this the actuator, or the VALVE...or are they the same?

Not the end of the world if I buy it and it's the wrong part....I'll keep it... but what do you guys think?

Oh, and now JB weld, I want a new/used part not a "modified" one

Oh, and THANKS!

The illustration below it shows it as part #11...but that doesn't look like mine (but it's a small drawing too)

Nate,
I think the GM p/n is the correct part.
The Actuator and Valve are the same.

If your in a rush try negotiating the price with Morad's because the new one from monster parts will take a week. That's been my experience with them. They don't have the parts for our car's in stock and have order from Detroit and then ship to you.

Haven't ordered yet Rick, but I agree, I'll see what Ed says. Luckily I now have two cars (my Si and Riv) so I can afford to wait.

I'm just anxious to start the Riv. The new gaskets went on okay..I could do it much faster now that I know what I'm doing. Got some bruises and cuts but they're on and they look great..cleaned up nicely.

I'd been getting two drops of oil in my driveway. I know the valve cover gaskets were leaking and I think I'll need another oil pan gasket (even though I've changed it) at some point.

This is the correct part number (89018120 ) for the bypass valve actuator, which does NOT show up on gmpartsdirect.com site, but putting in that part number alone does bring it up...odd

Moderator, is there a sticky for GM part #'s?

my boost control solenoid stays hot even after shut down and when the cars cold. hot to the touch! has any of you guys had this problem? is this normal?

I thought everything was vacuum controlled.
I don't know what solenoid you are talking about.
Are you sure it's not the EGR valve that you are feeling. That will be hot and it gets heat soaked so it takes a long time to cool down.
Just like the exhaust manifold.

on the series 1 there is an electrical connection going to the boost control solenoid its the one with the strange little sponge filter that falls apart when you change out the thermostat . i dont know if the series2 is different. but it stays hot. it weird. maybe its begining to fail. ill just replace it...

I've never actually stopped to feel the temp of that solenoid. I will let you know what mine is acting like.
Obviously, I would think it shouldn't be hot after the car has been off for a considerable amount of time.
If it indeed is hot to the touch hours after the car has been shut down then that indicates there is still current flowing through the solenoid. I would look at the wiring diagram to see what provides current to it before condemning the solenoid itself.
Although the solenoid should be fried if it has had constant current flowing through it continuously for a while. A solenoid is only designed to accept current long enough to hold the switch open or closed for a relatively short period of time. Still need to look at the diagram and read how that system is supposed to work.
Can't be a very difficult system to troubleshoot.

Look at what Travis has for sale in the post "workbench full of stuff to get rid of"
He has an EVAP valve. Is that what you are looking at on your car?

cool thanks rw!

robotennis61 wrote:

on the series 1 there is an electrical connection going to the boost control solenoid its the one with the strange little sponge filter that falls apart when you change out the thermostat . i dont know if the series2 is different. but it stays hot. it weird. maybe its begining to fail. ill just replace it...

You're so right about that filter! I wrecked mine changing to a 180 stat and had to chase down those crazy part numbers...

I would be worried about the wiring before I replaced the solenoid. That thing should not have power to it when the ignition is off. In a '98 for example, the PCM grounds one side of the solenoid, and the other side is connected to IGN1 Hot through the Oxygen Sensor fuse. When the key's off, the solenoid power should be off...

With a shop manual and a meter/test light, you first need to find out where the power is coming from and fix any damaged wiring, and then put a fresh solenoid in.

robotennis61 wrote:

cool thanks rw!

I believe that is the carbon canister purge valve, not the BCS valve...

Eldo wrote:

I believe that is the carbon canister purge valve, not the BCS valve...

You are correct, it's the EVAP purge valve.

Eldo wrote:

And as a reminder for everyone, the boost Bypass Valve is vacuum-operated, but the vacuum circuit itself is controlled by the Boost Control Solenoid valve.

Having said that, I would be worried about the wiring before I replaced the solenoid. That thing should not have power to it when the ignition is off. In a '98 for example, the PCM grounds one side of the solenoid, and the other side is connected to IGN1 Hot through the Oxygen Sensor fuse. When the key's off, the solenoid power should be off...

With a shop manual and a meter/test light, you first need to find out where the power is coming from and fix any damaged wiring, and then put a fresh solenoid in. PM me if you need a filter for it.

Also, with that solenoid Hot at all times, that means it is commanding 100% boost at all times and that is not good. You'll be getting KR and other problems if you don't find the reason for this to be on at all times and fix it soon.

Rickw wrote:

Also, with that solenoid Hot at all times, that means it is commanding 100% boost at all times and that is not good. You'll be getting KR and other problems if you don't find the reason for this to be on at all times and fix it soon.

Actually, unless the Series I is plumbed & wired totally differently from the Series II, the solenoid is commanded 100% almost all the time anyway in a kind of reversed logic. Besides, if the '95 PCM logic is the same, the computer commands the valve from the ground side anyway...

There are several convolutions that I had to get straight in my head to understand this system...
- First of all, the solenoid (again, this is from my Series II manual) only has two command modes, 0 and 100%. It doesn't do any variable percentage tricks like other PWM devices do.
- Second, contrary to expectations, the valve is held closed when the electrical command is ON, or 100%. and it opens when 0% power is applied. I suppose this is a fail-safe idea: If the part fails you'll never have any boost, so you'll never have overboost...
- Most important, the Boost Control Solenoid switches the "control" backpressure between the blower output and the bottom port on the Bypass Valve actuator, not the "main" vacuum from the throttle body to the top actuator port...

Under most conditions, the PCM commands the boost control solenoid to operate at a 100% duty cycle (ON), keeping the solenoid valve closed and allowing only inlet vacuum to control the position of the bypass valve. At idle, full inlet vacuum applied to the top port of the BBV actuator diaphragm counteracts spring tension to hold the bypass valve open. As engine load increases, reduced TB vacuum allows the spring tension in the actuator to close the bypass valve and allow boost pressure to increase.

When reduced boost pressure is desired by the computer (see below), the PCM commands the boost control solenoid to operate at a 0% duty cycle (OFF). This opens the solenoid valve and allows boost pressure from the intake manifold to press on the bottom side of the BBV actuator diaphragm, counteracting the spring tension and the low control vacuum in the top port and forcing the bypass valve to open and recirculate excess boost pressure back into the supercharger inlet.

The PCM "desires" the boost control solenoid to operate at a 0% duty cycle (off and open): A) during rapid deceleration, B) under very high engine load situations, and C) any time Reverse is selected.

I hope this will help other guys when they search for boost bypass.

Thanks Eldo, for looking this up and more important thinking it through.
So back to his original statement that this solenoid is hot to the touch all the time, still means he has a problem. Do you agree.
This solenoid should not receive any current when key is off or during the other events that the PCM commands it to be off.

Rickw wrote:

Thanks Eldo, for looking this up and more important thinking it through.
So back to his original statement that this solenoid is hot to the touch all the time, still means he has a problem. Do you agree.
This solenoid should not receive any current when key is off or during the other events that the PCM commands it to be off.

Quite... He has to find out where that juice is coming from when the key is off.

well it seems the problem went away. for now. i checked all the connections and nothing looks out of order. when this happened,and just before turning on the engine,i turned the key as usual and went under the hood to check the BCS with a cold engine, the fans came on! they don't come on with a cold engine ever! i couldnt figure that one out? the BCS no longer stays hot but the fans activate occasionally on start up. very strange. but thanks guys for all the info. really helps me figure out what the problem is if it comes back....

Quote :

Actually, unless the Series I is plumbed & wired totally differently from the Series II, the solenoid is commanded 100% almost all the time anyway. Besides, if the '95 PCM logic is the same, the computer commands the valve from the ground side anyway...

Are you saying the engine runs under boost (BBV closed/100%) nearly all of the time? I would have to disagree. According to scan logs, my car runs without boost about 90% of the time. Or, are you saying the BBV is closed/100%, but without the engine making boost?

Quote :

The PCM "desires" the boost control solenoid to operate at a 0% duty cycle (off and open): A) during rapid deceleration, B) under very high engine load situations, and C) any time Reverse is selected.

Under very high load situations, the PCM should desire the BBV to be closed, imo. This is when you want to build the most boost.

AA wrote:

Quote :

Actually, unless the Series I is plumbed & wired totally differently from the Series II, the solenoid is commanded 100% almost all the time anyway. Besides, if the '95 PCM logic is the same, the computer commands the valve from the ground side anyway...

Are you saying the engine runs under boost (BBV closed/100%) nearly all of the time? I would have to disagree. According to scan logs, my car runs without boost about 90% of the time. Or, are you saying the BBV is closed/100%, but without the engine making boost?

Under very high load situations, the PCM should desire the BBV to be closed, imo. This is when you want to build the most boost.

As I said, you have to wrap your head around the terminology and the way the system operates... Please reread my post more carefully, especially regarding the names of the valves and their individual functions. The Boost Control Solenoid valve only has two commands, 0% and 100%, it is not variable. Also, it only controls overboost, or undesired boost.

In your post you are confusing the BBV with the BCS... In normal operation (I.e. most of the time) the Bypass Valve actuator is simply running on normal inlet vacuum, controlling the bypass valve in relation to engine load: The more gas you give it, the lower the throttle body vacuum, the more the actuator spring closes the bypass valve to give you your desired boost. There is no feedback to the computer for this, no 'percentage'.

The Boost Control Solenoid stays closed (100% on) all this time. Specifically on Reverse, decel, or very high load, it opens (0%) so that the pressure from the intake manifold can push back against the other side of the Bypass Valve actuator diaphragm and open the bypass/lower the boost... Obviously, in the first 2 of those 3 scenarios, it is more of a precaution than anything. "Very high load" should be interpreted to be "overload", and therefore something that the computer is trying to protect against, no matter how much boost the driver wants...

I suppose the simplest summary, when all the non-intuitive steps are taken to their actual conclusion, is that an open BCS means an open BBV, and that 0% BCS should mean 0% boost. BUT, a 100% closed BCS only means a possible 100% boost, if all other factors are in place, like WOT...

See, I said it was convoluted

It's hard to understand... Let me try.

The 'suction' opening of the supercharger is always accessing air, and compressing it. The bypass valve generally stays OPEN to 'bypass', allowing the compressed air to escape from the lower intake back to the front of the supercharger, to be sucked in and compressed again. A little is fed into the engine as it's running, the rest escapes back to the front of the supercharger. So the bypass valve, when it is 'bypassing', is letting the compressed air back out.

As you step on the throttle harder, you go into 'boost' mode, the bypass valve closes, forcing the compressed air to stay in the lower intake manifold under pressure, with nowhere to escape but into the cylinders.

Mark, I think I understand now. Btw, I did read every word you wrote - about 5 times over. The part that threw me off was this: "And as a reminder for everyone, the boost Bypass Valve is vacuum-operated, but the vacuum circuit itself is controlled by the Boost Control Solenoid valve."

This statement led me to believe the BBV and BCS behave similarly or identically. I never read "overboost" until your last post (that might have helped clarify). But thanks for all the info.