I put on a manual boost controller in the boost reference line, spun all the way clockwise, I get my stock red painted spring's boost of about 5psi. When I crank the valve all the way counter-clockwise, my boost comes on REAL strong up to about 9psi and as I accelerate, my boost drops down to about 0-1psi over about 3 to 4 seconds. I cannot figure out why... Anyone ever experience this? I will call STS again on Monday to see if they have any heads up. I can tell the wastegate is definately where it is being released, but I am not sure I understand why.. Could 9psi start forcing the valve open under pressure as the engine goes thru it's RPM range?

 

Do you have the vacuum reference off the intake?I'm guessing not.

 

yea off manifold. I am thinking that I have a split in a silicone somewhere that is leaking when I hit high boost. There should be no way the wastegate spring would allow me to go below 5psi of boost.

 

went through almost all the silicones and cannot find anything that has a split in it or a leak.. I'm stumped.. What could cause you to lose all your boost after reaching your peak (9-10psi)? Something is venting my boost shortly after it reaches it's max. When I turn down my boost to the spring's default (5-6psi) it does not vent, it maintains 5-6psi...

 

Thats weird.Maybe the blow off valve is opening? A/F -timing way off? Valve springs floating?

 

No blow of valve installed and the Nelson tune is nearly perfect AF wise (reading O2's). I turned down the boost to about 7-8psi and went out for a couple runs, it boosted right up to 7-8 with hard pulling then started creeping back down to 0-1. Driving me nuts! I can only think of one or two things, broken silicone that only leaks at high pressure then opens up, or a break in a weld on a pipe or in the intercooler. Perhaps a throttle body leak? Also maybe the wastegate is haywire. I pulled the spring out to check th diaphram, but did not notice anything. Maybe I should pull the wastegate out and check it's full function and maybe clean it.

On one run, I boosted it to 7-8 then as it dropped down I let off the gas and then slammed it again, sure enough it hammered out 7-8 psi of boost again.

 

Stock cam?

 

Yea, bone stock engine. The truck as a whole only has 13k miles on it. I got it from my father in law who died earlier this year, turns out he was sitting on this beauty since new, and i got it with just a few miles over 10k! Whole truck is pristine! Have the STS kit installed and Nelson tuned. Being a 1500HD it comes with the 6.0L, 4L80E and I put in 4:11 10.5" rearend from a wrecked 2005 2500hd. When boost hits, it is like a rocket! It just starts dropping after peaking around 9psi. There can only be a few things that would vent your boost to zero, a faulty wastegate and pressure being leaked along the intake plumbing somewhere eh? Anything else?
When the boost creeps to zero, the truck is accelerating under normal engine power. Which is not bad, just definately NOT boosted..

 

Valve float, overlap, a couple of other things that don't seem likely because of how it's running: vac leak, vac line blown off -- both would keep you from making the 9psi in the first place and the line off option would sound horrible. On stock springs, you might be floating the valves at those PSIs, but it seems unlikely.

The wastegate is going to stay closed until you make your boost and then open to keep you at boost. If you had a leaky diaphragm in there the reverse of your symptoms would happen (max boost). If the spring were broken it wouldn't close after it opened the first time (and any amount of boost would push it open).

Here's what I would do first:

1) go out and tighten every single silicon couple clamp using a socket wrench instead of a screw driver (if you are using the stock STS clamps -- worm gear hose clamps -- it's an 8mm socket -- hmmm maybe 7 lol). Retest
2) with the truck idling slide under and feel around all the exhaust welds -- don't touch, just hold your hand 2-3" away and slide it all around the pipe as far as possible. A big leak will feel like someone huffing and puffing on your hand, and pin hole will feel like a small puff. I doubt it's this. The symptoms are all wrong. Do the same thing around your WG gaskets.
3) Go out and log or have a passenger watch the RPM gauge and note exactly when the boost starts to drop off.
5) turn it down to 5psi and see if it does it. If not, go up 1psi at a time until you get the boost loss again. See if the RPMs match from #4


Are you sure your boost gauge is accurately reporting the initial psi (meaning it's really at 9psi and not 32 )

 

wonder if a cracked weld on a pipe or in the intercooler could open enough to vent under boost? I went through nearly all the silicones. I skipped one or two, but should probably actually take them off and inspect them. If it were an exhaust leak, why would it build nice boost THEN drop down? And why would a drop of the throttle for an instant then mashing it down again send it into good boost again? Really bizzare...