BPV opening up at full boost
 

I looked but couldn't find a thread on this. What is everyone making for boost on their LSJ SC? I make about 17psi and I have a problem with my bypass valve opening at full boost and causes my car to jerk around. I have a 2.8 pulley, 80# injectors, 2.5 catback Borla exhaust, dual pass, 2.5 cai, tuned on e85. Let me know what you think. Thanks!

You sure it's just the valve? Block the line coming from the solenoid on the head to the bypass solenoid, make sure it's not being commanded open.

Then vacuum test the solenoid, it should be fully closed at atmospheric, pull open smoothly as you apply vacuum manually (with a handheld pump) and hold it 100%, no leakage.

If you're still getting boost fluctuations, are you sure it isn't belt slippage? Looks/acts similar.

I can zip tie the bpv closed and it'll pull just fine. Not boost fluctuations at all. And I just put a new bpv in.

Ok so try disconnecting the solenoid that connects to it. If it's normal with the line off, it's the ECU turning the boost down. Probably due to a tune issue or very high IAT2s.

I took the tube off and now it's normal. Idk if it's a tune issue and idk about my IAT2s. The last time I checked they were fine. I have dual pass and my intercooler pump works.

I don't own an LSJ but I have supercharged my L61 and I am fairly certain the bypass valve functions the same. From my understanding the bypass valve is there to prevent the supercharger from trying to suck and blow when the throttle body isn't opened enough to allow sufficient flow. Also, the bypass valve works strictly on vacuum, no solenoids, no ECU control. The actuator is activated when there is a significant vacuum between the rotors of the supercharger and the throttle body. This prevents your supercharger from trying to suck air through a closed throttle. One thing that could be causing your bypass valve to activate when it shouldn't is a significant restriction in your intake before the throttle body, like a really dirty filter. I would check for possible restrictions.

If your actuator isn't working it should keep your BPV closed as that is the inactive state, unless the internal spring is broken/worn out. If it wasn't holding vacuum you should have a problem with it not opening up, and you would have a vacuum leak. You should be able to check the spring tension just by manually trying to open the valve by pushing on the actuator rod with your hand. If the resistance feels weak or non-existent that is probably your problem.

Yeah I just replaced the actuator. It does the same thing my old one did. They both worked perfectly fine up until I hit 17psi. And when I took the tube off the solenoid that connects to the actuator is holds boost with no problems.

So it's the ECU trying to lower boost for some reason. Get the car datalogged and find out why. I'd guess too high IATs most likely, if the car has a tune there should be no normal "boost" limit like the stock tune.

Unless I am missing something there is no way for the ECU to have control over the bypass valve. The computer will adjust ignition timing for high IAT2 Temps but it does not control boost. That is unless LSJ's come with extra hardware I don't know about.

https://cimg5.ibsrv.net/gimg/www.cob...307184229d.jpg

Actuator works with spring and vacuum on diaphragm.


https://cimg1.ibsrv.net/gimg/www.cob...d7204f46e1.jpg

If you look close in the upper right hand corner you can see the hole for the vacuum line.

I don't see how the ECU could possibly have any control over this.

Edit: Unless there is a solenoid that connects to the lower nipple. Then that would make sense and I take everything back I said about ECU-BPV.

If there is solenoid that connects to the lower nipple,and you disconnect it, and it fixes the problem then I agree with Italianjoe.

on an lsj there is a solenoid that hooks to the lower nipple, and that hooks to the top of the manifold. the ecu is able to control boost on the lsj.

Yeah that's what I pretty much realized and feel dumb for opening my mouth before I should have.

Crazy, im having the same issue now. Im running a tvs with a 2.7 and looking over my data logs I see my boost is fluctuating at wot. At the iats and rpm I am at, I have nothing that should be triggering the bpv to open. Thinking it might be belt slip but I have a stage 2 belt and zzp adjustable tensioner. The belt looks and feels pretty tight, but it is about 6 or 7 yrs old. I guess I might as well try a new one.

Edit: looking over an older log with a new standard length belt and adjustable tenisioner I see boost was still fluctuating. So I don't think it was the belt. I did have smaller injectors then, so I was limiting boost on the 2.7 pulley for a while. Maybe the bpv solenoid is the culprit?

Yeah I feel like there's no way it can be belt slippage. Cause I can zip tie the bpv closed and will get full boost with no problems. Or I can disconnect the bpv from the solenoid and get the same results.

do a data log and see what duty cycle the solenoid is being commanded.

Do the vacuum test on the solenoid as mentioned but I believe you will need the key on accessory as the solenoid is open with no power and closed with power. Will need it closed to see if it holds good vacuum. I have a 2.8 pulley w/tune and my car will pull boost between 6800-7000 rpm, I just try to shift before that happens.

Mine will open at 4200 rpm at WOT.

With a proper tune the boost should never drop. Get your datalog looked at, there's something going on with the car that's making it retard boost, or there's a mechanical issue like belt slip. Since you close the valve and it stops, it's not the belt.

I did a run with the solenoid disconnected and it made full boost so it's definitely the solenoid. My boost is limited in first and up to 5k rpm in the others so I'm thinking the solenoid is sticking open when it should go to full boost.

Interesting, I've heard of testing the the bpv that way, but not the solenoid. I thought you tested the solenoid with a power source or with a dmm?

Ok, im a little confused by what everyone is saying about the operation of the bpv and the solenoid. I keep reading that the valve is operated by vacuum on both sides. I thought it was operated by vacuum on the blower side and by pressure on the bypass solenoid side. Hence the V and the P on the different sides of the bpv. If the car is under boost, how would vacuum come out of the little port from the manifold to the solenoid? Wouldn't pressurized air come out of there if the car is under boost?

It's vacuum and boost. When there's boost in the manifold the solenoid can use it to open the bypass valve, yes. But normal vacuum pulls the valve open, as you open the TB and the inlet vacuum goes down the valve closes to build boost.

So could it be that my solenoid is bad? I'm thinking of just ordering a new one and seeing what that does lol

Right, but like I said I thought the vacuum was only on the blower side and the boost pressure was on the solenoid side. Are you saying the solenoid can open to allow boost or vacuum?

You could do that if you want to, or you could test it. I think you can find videos on YouTube that show how to check it. I'm going to do that before I start throwing parts at it.