low boost

[Oldman]

My boost is now reading "0" at idle and is only reaching about "4" at full throttle on hard acceleration. I have done a "smoke test" and only discovered a small vacuum leak where the flexible metal hose goes from the intake to the base of the EGR valve. I have repaired this. I have checked and resolved issues with bcm and bcs, no problems there. The supercharger seems to be working fine. What other problems would cause low boost?
The cat has been gutted, so I am unaware of any exhaust issues.

Thanks for your help.

[justdave]

[Oops. Just a little hint: do not post info about disabling any Emissions components; it is a Federal offense, and the way things are today, one cannot be too careful. Once you post something, it's out there forever...]
Low boost is caused by 1. - faulty boost indicator 2. - Faulty mechanical / electronic controls 3. - Intake restrictions (unseen) 4. - exhaust restrictions (unseen) 5. - pressurised intake system leaks (unseen). 6. - worn supercharger 7. - worn valves 8. - worn out rings (poor compression).
Has your acceleration rate decreased noticeably? If your car is still hauling butt, and getting you to 60MPH just as fast as before, start looking at your Gauge. Have you done any work recently that could affect the boost? Or has it been so gradual you haven't noticed it until recently? Mileage?

[Oldman]

Thanks for the warning. I'll correct myself and say that I manufactured a "home made" test pipe to see if my cat was clogged and causing low boost.

I have had a constant P0171 code and have been working to resolve that issue. I have replaced all of the vacuum lines, checked and cleaned the EGR, MAF and in replacing the vacuum lines I checked the BCA and BCS (?). I found that the BCA worked like it is suppsed to and passed the test when the vacuum line was disconnected. However on the other part, there was only a continuity test for the electrical connector and my part did not pass the test. I was told on this forum that if I just plugged the vacuum lines on either side of this part, Commonly called the boost control bypass, that it would have no effect on boost.

My gauge has consistently read around -4 at idle and has read as high as 8 or 9 at hard acceleration. My gas mileage has decreased recently, but that is probably due to the P0171 code.

I recently stumbled across a machine to "smoke" the intake system to look for a vacuum leak and found a small leak where the flexible metal hose goes from the intake to the base of the EGR valve. This has been repaired, but boost has not returned. It also feels that there is not as much boost felt when accelerating.

Thanks for your help!

[00Beast]

Dave: FWIW, you can say whatever you'd like, and for "diagnostic purposes" one could temporarily disable an emissions device to test if it is causing a malfunction. It's anyone else's responsibility to know the laws and follow them. You telling someone to do something doesn't make it your fault that they do it...

[mdrew]

my boost reads in the neg at idle as well, what does one operating properly read?

[00Beast]

-10.

[redzmonte]

8-9psi on accel sounds normal. -4 hgs vac sounds low... sounds to me like you still have a vac leak.

[Oldman]

Sorry if I posted incorrectly. At idle, my boost reads about -8 or -9. At heavy acceleration, my boost only reads about +2 or +3. Again, I have used a smoke machine to pump smoke into the intake system to look for vacuum leaks. I detected one and repaired it. I am pretty sure that there are no vacuum leaks. The actuator is working correctly and the bypass was determined to be bad (electrically tested and found faulty) on this site, I was told to simply block both vacuum lines going to the bypass and my supercharger should work correctly and the boost should read within normal range.
So, I will check for blocked exhause - though I don't think this is the problem. As far as blocked intake, I have no idea what to look for or how to repair.

Please continue to send your ideas, I am tired of this having such slow acceleration.
Also, though the car has over 200,000 miles on it, the motor was replaced with a new GM Goodwrench motor at 135,000 after it spun a bearing. So, I feel confident that the compression, valves and internals are in good shape.

Thanks for your help guys!

[yourgrandma]

An exhaust restriction would cause high boost. I still suspect a bypass valve issue. Either in the actuator or the linkage. Shouldnt be too hard to ziptie the bpv closed to check it out.

[00Beast]

I'd remove the BCA/BCS (2-10 mm bolts on their bracket) and block the linkage, then take it around the block. Does the lever move freely?

If you don't know what I'm talking about: http://www.3800supercharger.net/diagnosing.html under LOW BOOST ISSUES

[Oldman]

Yeah, I've been to this post a thousand times. The BCA works perfectly. The BCS did not pass the ohm meter test. I was told by someone on this site that the BCS basically is an electric switch that prevents boost in park and in neutral. I was told to simply disconnect the two vacuum lines and block them off and the disconnected BCS would not cause boost problems.

If you feel that this information is not correct, please let me know.

Are you suggesting that I should use a zip tie to hold the BCA all the way up and drive it a little to see if the boost is capable of reaching beyond the +3 that it is reaching now?
If there's a problem with the BCA at boost, would this possibly cause the P0171 code.

I found and repaired one small EGR leak when I used the smoke maching.
I erased the codes. I checked with the smoke machine again and found no vacuum leaks.
The code did not return for several days until I drove it hard a little and probably kicked it into boost.
It would be great if the BCA is cusing the lean condition code as well as the low boost.
My local junk yard just took in two supercharged cars that have complete motors with S/C still in tackt, so these parts are available cheap.
That would be great if this part could be causing both of my problems.

What do you think?

[00Beast]

You'd want to hold the BCA down, not up, but yes.

IIRC you used teflon tape on the EGR? I guarantee that's gone. Go get the parts you need from the JY car(s) to fix it properly. Ultra copper RTV would work too, but the proper fix is always the best...

[Oldman]

Can I just remove the vacuum line from the BCA?

[yourgrandma]

Simplest thing would be to remove the actuator and disconnect the shaft from the bypass valve. Leave the vacuum lines hooked up so your FPR and MAP still work right. Then you can throw a zip tie around the leg of the shaft to hold it down.