No boost at full throttle

[thor04]

So i put some new gauges on my truck so i can monitor my stuff and my turbo is very on and off.
If i punch it ill get about 8-10 pounds of boost till about 70 km/h then it just falls down to zero the only way it comes back up again is if i let off the pedal completely if i dont let off all the way it will stay at 0 and thats crusing at 120.
Now if i only give it about 3/4 throttle i'll get 9 pounds and once i get to my 120 km/h it will stay at the 4 1/2 pound mark
So is this kinda wierd i just got the timing done on it but only the mechanical part of it they said they couldn't do the computer TDO timing which is at -3.5 now with the GM Tech II which i dont understand that cause if the tech one does it why not the tech two anybody have some advice on this aswell would be great, Thanks

[Robyn]

The issue is most likely in the wastegate system.

Check for codes first.

Make sure the vacuum pump is ablle to deliver a steady 26" HG at idle at the pump.

Next be sure you have full vacuum to the wastegate solenoid (LH Valve cover near the back) Make sure you are getting at least 15"hg to the wastegate actuator on the turbo.

A failed vacuum system, be it hoses, the solenoid or the wastegate actuator itself can all be possibilities.

With gauges installed, a great idea would be to install a turbo master unit from Heath diesel and then the vacuum system will be a moot point.

Set the adjustment so you can get a max boost of about 8-10 PSI with a stock setup. Unless you change the chip, any more boost will set an overboost code.

Keep us posted.

Robyn

[rustyk]

Possibly, too, the air filter, especially if you're getting a lot of black smoke.

[Robyn]

Good point.

Another possible cause is a Plugged CAT.

If the rig has a buttload of miles on it, the cat is likely plugged up.

Under heavy throttle the exhausts system can't flow enough and the turbo can actually stall.

I am more inclined to look at the wastegate controls.

If the filter is not fresh, a new one wont hurt it a bit.

Keep us posted

Missy

[JohnC]

Definitely run all the tests, but this is classic symptoms of a failing wastegate solenoid. It doesn't effectively control the boost, and after 30 seconds or so the PCM just shuts it off. Releasing the throttle momentarily resets it and you're good to go until the next time it cannot control the rising boost...

[DaveBr]

If you post a signature it will help us help you out.

[thor04]

Good Point on the signature, Thanks.
So i got nothing done on the weekend on the truck except for a good wash and polish.

My vacuum gauge is messed up so i got buddy bringing his in the morning.

Getting the codes scanned on tuesday maybe tomorrow

And the Cat idea is a good idea it has to be original cause downpipe is shot i can squeeze and bend it with no effort and the cat and downpipe are a all in one but the rest of the exaust is brand new just got put on before i bought the truck but its only 2 3/4.

Air Cleaner is new 6" diameter 10" long, K&N
Black Smoke I wish the thing doesnt smoke at all maybe a little when i go to pass but thats it

Hopfully should know more by tomorrow Thanks

[arveetek]

Yep, classic wastegate solenoid issues. I dumped mine and installed a Turbo-Master mechanical wastegate controller. No more boost dropping off. Now I have even, consistent, predictable boost throughout all the RPM range.

Casey

[thor04]

Well i checked the Hg at the waste gate its at 18 plus so it looks like a turbo master for me and maybe a new exhaust or at least a new down pipe and cut out the cat

If a guy were to make there own waste gate controler what type of spring would a guy use soft medium hard? i have tons of stuff to make one for free and all the tools to do it then i would have enough money to buy the rest of the exhaust.