Boost Dips & Slow RPM Rise
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Senior Member
Joined: 06-13-07
Posts: 1,161
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From: Woodbridge, ON
Boost Dips & Slow RPM Rise
hey guys,
just to clarify...
- no funny-sounding exhaust (sti cobalt)
- no smoke coming from exhaust
- no milky substance in oil cap or dip stick
- coolant reservoir is top-notch
how is it that if i try to put my foot on the gas, the rpms rise very slowly...and the boost pressure just the same (slow like my rpms)...
also, as soon as i let off on the gas, the boost drops very fast, dips down past -22, then equalizes at around a steady -18.
i can hear my sc scream...its whining like the boost pressure should be built up more than what it is...im thinking theres a leak in my intake manifold...what do you guys think? would that affect my rpm acceleration as well?
doing a compression test in the next few minutes...ill let you guys know the results...your speculations until then would be appreciated
just to clarify...
- no funny-sounding exhaust (sti cobalt)
- no smoke coming from exhaust
- no milky substance in oil cap or dip stick
- coolant reservoir is top-notch
how is it that if i try to put my foot on the gas, the rpms rise very slowly...and the boost pressure just the same (slow like my rpms)...
also, as soon as i let off on the gas, the boost drops very fast, dips down past -22, then equalizes at around a steady -18.
i can hear my sc scream...its whining like the boost pressure should be built up more than what it is...im thinking theres a leak in my intake manifold...what do you guys think? would that affect my rpm acceleration as well?
doing a compression test in the next few minutes...ill let you guys know the results...your speculations until then would be appreciated
to many cobalts droppin like fly's 
well it sounds like a possible clogged cat, do you even have one i didnt look at your mods i know you have meth and that causes you to run rich if your not tuned for it, causing cat failure. unbolt the down pipe not all the way just loosen the bolts creating a large gap then drive and see if its better and let us know
Senior Member
Joined: 06-28-06
Posts: 2,212
Likes: 2
From: Ontario,Canada
Sounds like a boost solenoid problem
thats why it goes up slowly with rpm because you are strictly relying on the reduced vacuum to allow the boost to go up instead of the boost solenoid assisting it.
Read below for your self.
The PCM controls boost pressure by using the boost control solenoid. The boost control solenoid is normally an open valve. Under most conditions, the PCM commands the boost control solenoid to operate at a 99-100 percent duty cycle. This keeps the solenoid valve closed and allows only inlet vacuum to control the position of the bypass valve. At idle, engine vacuum is applied to the upper side of the bypass valve actuator, counteracting spring tension to hold the bypass valve open. As engine load is increased, engine vacuum is decreased, causing the spring in the bypass valve actuator to overcome the applied vacuum, closing the bypass valve and allowing the boost pressure to increase. The bypass valve starts to close when the vacuum measures 250 mm Hg (10 in Hg) and is fully closed at 90 mm Hg (3.5 in Hg). When reduced boost pressure is desired, the PCM commands the boost control solenoid to operate at a 0 percent duty cycle. This opens the solenoid valve and allows boost pressure to enter the bypass valve actuator at the lower side to counteract the spring tension, opening the bypass valve and recirculating excess boost pressure back into the supercharger inlet.
thats why it goes up slowly with rpm because you are strictly relying on the reduced vacuum to allow the boost to go up instead of the boost solenoid assisting it.
Read below for your self.
The PCM controls boost pressure by using the boost control solenoid. The boost control solenoid is normally an open valve. Under most conditions, the PCM commands the boost control solenoid to operate at a 99-100 percent duty cycle. This keeps the solenoid valve closed and allows only inlet vacuum to control the position of the bypass valve. At idle, engine vacuum is applied to the upper side of the bypass valve actuator, counteracting spring tension to hold the bypass valve open. As engine load is increased, engine vacuum is decreased, causing the spring in the bypass valve actuator to overcome the applied vacuum, closing the bypass valve and allowing the boost pressure to increase. The bypass valve starts to close when the vacuum measures 250 mm Hg (10 in Hg) and is fully closed at 90 mm Hg (3.5 in Hg). When reduced boost pressure is desired, the PCM commands the boost control solenoid to operate at a 0 percent duty cycle. This opens the solenoid valve and allows boost pressure to enter the bypass valve actuator at the lower side to counteract the spring tension, opening the bypass valve and recirculating excess boost pressure back into the supercharger inlet.
Thread Starter
Senior Member
Joined: 06-13-07
Posts: 1,161
Likes: 0
From: Woodbridge, ON
Sounds like a boost solenoid problem
thats why it goes up slowly with rpm because you are strictly relying on the reduced vacuum to allow the boost to go up instead of the boost solenoid assisting it.
Read below for your self.
The PCM controls boost pressure by using the boost control solenoid. The boost control solenoid is normally an open valve. Under most conditions, the PCM commands the boost control solenoid to operate at a 99-100 percent duty cycle. This keeps the solenoid valve closed and allows only inlet vacuum to control the position of the bypass valve. At idle, engine vacuum is applied to the upper side of the bypass valve actuator, counteracting spring tension to hold the bypass valve open. As engine load is increased, engine vacuum is decreased, causing the spring in the bypass valve actuator to overcome the applied vacuum, closing the bypass valve and allowing the boost pressure to increase. The bypass valve starts to close when the vacuum measures 250 mm Hg (10 in Hg) and is fully closed at 90 mm Hg (3.5 in Hg). When reduced boost pressure is desired, the PCM commands the boost control solenoid to operate at a 0 percent duty cycle. This opens the solenoid valve and allows boost pressure to enter the bypass valve actuator at the lower side to counteract the spring tension, opening the bypass valve and recirculating excess boost pressure back into the supercharger inlet.
thats why it goes up slowly with rpm because you are strictly relying on the reduced vacuum to allow the boost to go up instead of the boost solenoid assisting it.
Read below for your self.
The PCM controls boost pressure by using the boost control solenoid. The boost control solenoid is normally an open valve. Under most conditions, the PCM commands the boost control solenoid to operate at a 99-100 percent duty cycle. This keeps the solenoid valve closed and allows only inlet vacuum to control the position of the bypass valve. At idle, engine vacuum is applied to the upper side of the bypass valve actuator, counteracting spring tension to hold the bypass valve open. As engine load is increased, engine vacuum is decreased, causing the spring in the bypass valve actuator to overcome the applied vacuum, closing the bypass valve and allowing the boost pressure to increase. The bypass valve starts to close when the vacuum measures 250 mm Hg (10 in Hg) and is fully closed at 90 mm Hg (3.5 in Hg). When reduced boost pressure is desired, the PCM commands the boost control solenoid to operate at a 0 percent duty cycle. This opens the solenoid valve and allows boost pressure to enter the bypass valve actuator at the lower side to counteract the spring tension, opening the bypass valve and recirculating excess boost pressure back into the supercharger inlet.
hmmmm, so what's the check/ fix for this?
and ya, i had the downpipe off....cleaned the catalytic converter, and threw it back on w/ some copper sealant...stopped my lil leak/tick at least
then i got to my maf on my intake and blew it out just in case it was dirty.
i will say that my car is definately not slow in the rpm range anymore...picks up like it should normally...but my boost is still kinda funny...rises slowly, doesnt hit 16 psi anymore (more like 12-13), and drops very fast when i let off the throttle.
whats my next attack plan to fix this issue?
EDIT-----------------------------------------------------EDIT-------------------------------------------------EDIT
i JUST fixed the problem...thanks to Coblasts for pointing me in the right direction!!!
the pin in my boost actuator/bypass, that allows the valve to close when normally open...THAT F*CKIN pin was jammed- by a vacuum line that was wedged in between that and something else... basically, it wasnt allowing for proper movement of that pin to open and close the valve...which would explain why i was running rich...fuel injectors were dumping in gas because the pcm thought i had more cfm of airflow, when really, my bypass was bleeding it out..
...god im such a clown lol...but hey, at least i got rid of that exhaust leak coming from my newly installed pacesetter header...i guess i killed 2 birds w/ one stone
anyway, my drive to work this morning has been a VERY enjoyable one...boosts like a monkey...so fast and responsive...maybe its because of the new header insall as well, but it also feels a lil quicker...all in all, its going to be a good day
....time to put the bottle back in my car
Last edited by Kibosh3; Jul 10, 2008 at 08:40 AM.
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