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Bouncing Boost Pressure

D

dtreid

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Alberta, Canada
Under moderate to heavy load, my K3500 boost pressure bounces around from say 10-13 psi. It just continually goes back and fourth between the two. It really is quite annoying.

What is causing this and what can I do to fix it? I've tried a different PMD to no avail.
 
PCM limits full Boost to 7-8psi, only spiking higher as the turbo spools up...............
 
Why? How is yours blowing 12psi when factory limitation is 8psi?
 
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I have a 97 Chev K1500 that does the same thing. I plumbed a boost gauge into the port where the IAT sensor is located. Don't know if that makes any difference.
 
What have you done to the factory Boost system to get more than factory Boost? Your sig indicates nothing..................
 
I've done nothing. Maybe my cheap boost gauge is inaccurate. I just put a T on the port the the IAT mount in on the intake manifold and get the boost pressure reading off of there.
 
Ok - you need to go out, pop the hoods and take a look at your turbos, describe what you see on both trucks - better yet, a pic is worth a thou...........well, you know............

For starters, the EGR valve on 1500's is calibrated for that oem 7-8psi Boost limit - Boost in excess of 10psi will blow the valve open, closing as Boost drops, etc, resulting in "flutter"

Also, varying fuel rates can cause that similar flutter

Also, plugged soot trap, crimped exhaust can cause Boost variation
 
The 1500 is an S engine, while the 3500 is an F.
The 1500 will top out at 15psi and then settle down to 10psi.
The 3500 as I said bounces around under heavy load. I've also had it go to limp mode (I guess), where it will only develop 3 or 4psi for awhile and them jump back up and start bouncing around again.
 
That is one of the complaints of the vacuum setup. I'd buy a TurboMaster manual boost controller (great product, excellent quality) or make your own TM's for cheap ... but you need boost gauge first, which can be installed really easy with the nice SS Boost Bolt, from PMDCABLE.COM which you simply remove one of the intake bolts, and replace it with the boost bolt and hook up the gauge... genius.
 
Matt, no stock GM 6.5 will Boost any more than 7-8psi max - he's still not giving the information needed to diagnose his symptoms - what has been done to exceed factory limits?

There is either turbomasters, vacuum bypasses, bailing wire, or lawnmower springs on those turbos - whatever - most of you are here as refugees from wild-ass guessers in service departments and other forums (and we have our unfair share here, also) - same can happen here if we don't know what's on the symptomatic vehicle(s)

Pictures or reliable eyewitness description is what is needed now - I've posted several known causes - which one is this?
 
Gotcha JD. You're right... A picture would show a lot of possibilities. Perhaps a boost fooler wired in, or other clues.

An aftermarket reflash is another possiblity?
 
dtreid said:
Under moderate to heavy load, my K3500 boost pressure bounces around from say 10-13 psi. It just continually goes back and fourth between the two. It really is quite annoying.

What is causing this and what can I do to fix it? I've tried a different PMD to no avail.
Click to expand...

Do you notice this by feel of the truck, or just by looking at the boost gauge? Is it surging?
 
gmctd said:
Matt, no stock GM 6.5 will Boost any more than 7-8psi max - he's still not giving the information needed to diagnose his symptoms - what has been done to exceed factory limits?
There is either turbomasters, vacuum bypasses, bailing wire, or lawnmower springs on those turbos - whatever - most of you are here as refugees from wild-ass guessers in service departments and other forums (and we have our unfair share here, also) - same can happen here if we don't know what's on the symptomatic vehicle(s)

Pictures or reliable eyewitness description is what is needed now - I've posted several known causes - which one is this?
Click to expand...

Snap a pic, your gonna find yourself in the 6.5 "hoosegow" :nono::sad:
 
Here's a pic of how I installed the T for the boost gauge and my unmolested wastgate.
 

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Ok, no baling wire or such - trace your wg vacuum hose back to the source.............

And, that's not a good idea, pulling the IAT sensor outta the intake plenum - now it senses ambient engine bay temps, should be directly in intake plenum to sense compressor outlet temps, very important for PCM to know when to pull fuel at excessive Boost conditions without a charge-air cooler
 
It maybe the boost control solenoid. You can try pulling the vacuum line off at the actuator and spraying some WD40 in the end with motor idling. This may or it may not help if this is the problem but it won't hurt anything.
 
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