Low boost, what

[TurboDiverArt]

OK, now that I

[C.K. Piquup]

Are you making black smoke on the hiway?Check you mirrors.It`ll probably belch occasionally.Either way,I`d suspect the turbo boost solenoid(at drivers side/rear on intake),small part w/2 wire plug and 1 mounting bolt.This part is $25-30+/-,installs in 5 minutes,and,if bad,makes all the difference in the world when replaced.Beyond that,I`d suspect a stuck waste-gate.How is power?

[patrick m.]

check for vac at the wastegate actuator engine at idle.
eng off, disconnect the vac hose from the actuator, and move it in and out to check for binding.
if you have vac at idle, and no binding in the actuator, disconect the vac connector at the solenoid and conect the two tubes together and go for a test drive. You dont want to drive it this way for very long, just enough to determine boost level (although some do it with no problem)

If you get a great deal more psi with this set up, you most likely have a solenoid problem. also check for power at the solenoid, the PCM provides the ground in a "pulse width" pattern.

also, check the intake path pre-turbo, just to be sure there are no obstructions.

[gmctd]

Just to enhance the above posts, Art -

At idle - around 25" vacuum at the pump, 15" at the wastegate cannister.

Driving, holding steady speed, should be same.

Kick it hard, wastegate vacuum should drop some, maybe to 10", but not below 5". Zero vacuum is zero boost.
Pump vacuum should never drop.

I would substitute a mechanical pressure gage to test actual boost levels.

[DickWells]

Art: I don't know about a 99, what with some differences in the ECM, but I've been running without a wastegate solenoid for two years now. I expected runaway boost, but I have great boost control through every range. I DO have the Kennedy variable resister. I have it adjusted so that I get up to 14-15 lbs maximum boost under very hard pulling at above 2200 rpm. Most hills, I see 7-11. Towing, with an all-up weight of less than 16000, I see from 3-5 lbs on level to slight uphill at 60 mph.
A new solenoid lasts me just a few days, so I gave up on using it. I just jump the vacuum lines together at the solenoid, thus elliminating it.
I know this bit of info may not help you with your low boost problem, but you can use this with all the really good data that you get from GMCTD and others and come up with a solution, without being afraid of running without the solenoid, just as a test.
Good luck.
Dick

[CoachWagner]

I recently had the same problem when I installed my gauge. I discovered small holes in the rubber end of the vacum line that connects to the actuator. Fixed that and boost was back. I don't know how long it was like that but it seems like a new truck since I fixed it. The gauge paid for discovering the problem and fixing it myself. So, check the lines for holes or cracks. smile.gif