boost/vac question

93thunderbird

Registered User
ok im having a problem AGAIN haha, well i have just changed my dis mod,iac,cam sensor,and the coil pack is about 2 months old (crank sensor is about 7 months old w/ new balancer) so i hope its not the crank sensor... any how, right around 5psi or even entering boost it bucks, the tack shoots up then straight down, if i push in the clutch the tack will go to normal, now if i hold the gas still with the tack bouncing around and the car bucks and it will stall.. sometimes like 3 trys before its fires back up... i can drive the car as long as i dont get under boost... if im in neutral and i slowly hold the gas (boost gauge at 18-20) and i go up too 2 grand is the boost/vac supossed 2 stay at 10 or go back down? lf i hold the rpms at 2-3 grand the vac will drop back 2, 18-20 is that normal i wa sthinking maybe i have a vac leak?
 
Let me address what the boost gage is actually showing and what it "means". When you start the engine and at idle the gage will show a vac. On a unmodified engine 18-20" is about normal. As you begin to accelerate what you are actually seeing is the mismatch between the air going into the engine from the supercharger vs. what it can "use" at that time. So lets say you could go say half throttle and hold the pedal constant. (assuming an auto here with quick shifts so as not to complicate matters) When the throttle Body valve is opened a larger amount of air begins to rush into the engine from the supercharger. There is a large amount of air it can't really use and the vac. drops and you go into boost. This continues as RPM goes up and the output from the superchargers rises. Now the ability of the engine to use all that air also goes up too so if you and you will reach a point where the engine is using the air it is getting and you will see boost drop. If you keep constant throttle forever you will likely reach a point where you go back into vac though you will never get to the idle reading listed above. But the steady state condition is such that the engine is using all the available air from the supercharger and could use even more if it had it. Now what happens when you lift the throttle is the throttle valve shuts and the airflow to the SC goes down big time. The engine could still use all the air it is not getting so you go into vac. big time. The SC Bypass valve opens to further reduce the air actually going into the engine since is is being recirculated to the SC to prevent cavitation, (Note this valve is at least partially open anytime you are in vac. As soon as you go to boost it is shut since there is no vac. to allow the valve to be maintained open since it is actuated by a vac. actuator. ) Essentially the engine is an air pump being fed with an air pump (the sc) Boost basically represents the mismatch between the two. That is why many report there max. boost drop when they upgrade the exhaust system since it allows the engine to breath better and it uses the available air from the sc more effectively with a high efficiency.
 
would a boost/vac leak cause...

would it cause my car 2 be normal untill i entered boost/when i enter boost would a vac leak cause the tack 2 jump around n stall? also the guage seems to go to the "0" faster and stay there,unless down shifting/upshifting, the car dosnt really boost (as if the car knows not 2 lol)
 
What kind of plugs and wires are you running? When was the last time they were changed? When the DIS Module was changed did you use dielectric grease on it's base or heat sink grease? There are a miriad of conditons that could cause what you are descibing.
 
its a new motor...

umm.. the motor isnt even a yr old yet (thats when the wires/spark plus were done) under 9k i dunno there carquest plugs 8mm and sumthing platnum spark plugs haha but thats the thing it hasnt done this b4 so i dont think its them.. i went n bought "heat compound" for the dis
 
Don't be so sure on the plugs and wires. Many folks have had problems with using "off brands" of both. I myself had almost identiacal problems with Jacobs Wires as has been reported by several as well. A bad connection at the coil pack to the wires can develop anytime and cause the problem. Do the wires have clips on the connectors for the coil pack? Any evidence of carbon tracking at the coil pack connections? Sure you don't have a plug wire burnt? Getting any codes or a Check Engine Light?
 
boots n filter

the boots do have clips on them (coil side) i pulled them off and only one had like a "pen tip size" carbon build up.. umm.. (at night u cant see any "arching") i held the boots and by the coil pack no zaps lol ,the fuel filter i just changed not even 500 miles ago (changed it thinking that was my problem) also no cel only when the car stalls (the upshift arrow dosnt stay on if it dosnt turn over and the cel goes off as soon as u try 2 start it)
 
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