Bouncing boost needle at heavy part throttle?

[Momar]

I was curious, what would cause this, or if it is normal. If I am light throttle, up to about 10 lbs of boost, the needle is real steady. If I go up above 10 it bounces back and fourth 2-3 psi. WOT, it spikes, and then settles at about 17 where I have it set, but then slowly falls down to 16. I was just curious what would cause this, and if I should be looking for some kind of problem.

Thanks

Ben

[microELVIS]

Believe it or not, on my car it was a header leak. The leak was on the passenger side header, right at the flange on the first tube. I had the headers welded up, put them back on and had a ticking sound that was a leak. I thought it may have been a lifter first, but saw where it was turning black on the header where the leak was.

[2QUIK6]

If your WG rod is not very tight it can do this also. Needs to at least have some tension on it when at rest.

[pwr.slave6]

My guage is jumping around alot also, when i jump on it.
up in the 20-25 lb. area. its useless to get a read. Any more
trouble shooting ideas? thanks
BTW
I run a ext.gate.
going to check out exhuast manifolds.

[Momar]

Originally posted by 2QUIK6
If your WG rod is not very tight it can do this also. Needs to at least have some tension on it when at rest.

My wastegate rod is plenty tight. It wouldnt supprise me if I have an exhaust leak, but I dont know. Why would it only be at higher part throttle boost, and not lower throttle, and WOT?

Ben

[pwr.slave6]

Good question BEN.

My gauge is right on, until it sees real boost.

[Momar]

I think I might try doing a top end clean soon and see if I can find smoke coming out anywhere it shouldnt. Mine spikes at wot, but doenst bounce like it does at higher boost in part throttle.

Ben

[pwr.slave6]

I plan on doing some nosing around under there over the
weekend. New motor was just installed over the last few
months. Are symptoms sound alike. I may swap out the
guage. In the process of elimination.

[Momar]

I started mine up and listened, and it sounds like I may have an exhuast leak up front. I know I had a pretty bad one on the crossover pipe, when I first got the car, but I thought I had that fixed. I may see about pulling it down and resealing it while when I put the new stall in tomorrow. If that doesnt help, I will use the top end clean like I mentioned to try to find it.

Ben

[Hognose]

I had a simalar problem where my boost needle would shake back and forth. Would never read steady. Tried replacing all the lines thinking it was a vacum leak. Even replaced my actuator thinking that was shot, turned out to be the boost gauge itself. Put a new one on and rock steady needle.

Matthew

[TurboBuickSix]

Do you guys have your wastegate holes in exhaust housing ported out? I had this problem but went away after I ported the WG hole.

[pacecarta]

sounds to me like the seat spring pressure setting on the qboost controller that hold the check ball is doing its job .
i get the same bounce (surge ) with the rjc controller , the spring holds back the pressure signal (to help spool up) with a check ball then once the initial spring pressure is overcome the ball lifts off its seat and it surges the boost (read RJCs controller discription on his site)
if im just cruising around and build boost gradually once I hit 10psi it surges and bucks the needle (would make you think theres something wrong) , the controller smoothes out at high boost and works like its not there below 10 psi ,

is it broke? no, thats the way the grainger valves work

if you take the qboost out of the loop you should see the boost surging at part throttle go away.

[Momar]

I thought about that, I just didnt know why I would see it before wot, but at wot. I guess that makes since. I still want to check for exhaust leaks though.

Ben

[pwr.slave6]

Ben , not sure of the top end clean process, (sorry) but what
exactly is this? how do you perform this type of check.
thanks, GUS

[2QUIK6]

Originally posted by TurboBuickSix
Do you guys have your wastegate holes in exhaust housing ported out? I had this problem but went away after I ported the WG hole.

Thats right also. If you have a free flowing down pipe/exhaust and haven't opened the WG hole up any over the stock size, that's most likely your problem. Reason is because there's less back pressure after the turbo and more air can flow thru the turbine and the small WG hole can't vent enough pressure before the turbo to controll boost so it will spike or flutter.

[pwr.slave6]

Thanks all great info.

[Momar]

Originally posted by pwr.slave6
Ben , not sure of the top end clean process, (sorry) but what
exactly is this? how do you perform this type of check.
thanks, GUS

Basically GM has a cleaner that you can use that will clean carbon out of your intake and heads. You use a vacuum line to suck it into the motor and keep the rpms up. When you do this, the exhaust will smoke like a freight train. I have been told that this is a good way to find leaks, because you will be able to see smoke coming out of them.

Ben

[Momar]

Originally posted by 2QUIK6
Thats right also. If you have a free flowing down pipe/exhaust and haven't opened the WG hole up any over the stock size, that's most likely your problem. Reason is because there's less back pressure after the turbo and more air can flow thru the turbine and the small WG hole can't vent enough pressure before the turbo to controll boost so it will spike or flutter.

How do you go about opening these up? Is there a way to tell how big you can make them without going to big to seal with the puck?

Ben

[2QUIK6]

Originally posted by Momar
How do you go about opening these up? Is there a way to tell how big you can make them without going to big to seal with the puck?

Ben

First you need to mark where the puck contacts the exhaust housing..several ways to do that....the way I do it is take off the downpipe/elbow which has the puck mounted in it...not completely off, just separate from the exhaust housing, then cover the puck with a light coating of grease. Vaseline or wheel bearing grease works well. Then reinstalled the DP/elbow making sure the puck is held up/open from touching the exhaust housing until it is completely mounted....once mounted back on securely, close the puck by moving the puck arm, close it firmly several times, then hold the puck open, or duct tape it open, and remove the DP/elbow again.
The grease will leave a nice imprint of its contact patch to the housing.
Mark with a permanent marker the outline, then when opening up, making certain you don't carve too close to the line you drew.
Remove the turbo and the exhaust housing if possible to open it up to prevent any metal shaving from getting into the turbine housing.
Usually opening the WG hole up to 15/16" is sufficient, don't go bigger than what is needed..too big will cause slower spoolup.
I used a dremmel and a tungston carbide bit which carved it out pretty quick and easy.