Pulsing Turbo

[PradoTDI]

Hi all! Relatively new here, and definitely new to TDI tuning. I've got an ALH swapped Toyota LJ78 LandCruiser that is currently running a Malone Stage 1 tune and GT1749VA turbo from a BHW. According to Malone I should be getting 18psi with this setup, but right now I only ever see around 16psi except for short spikes when I really step on it. At highway speeds the boost pulses up and down, usually by about 2psi about once a second. The turbo has a new CHRA and has been thoroughly cleaned out about 1000 miles ago, the vanes operate smoothly. What would be causing the pulsing? Is it something to be concerned about?

Thanks in advance!

 

[CableJockey]

I would be concerned for turbo longevity. Not sustaining boost seems to indicate a leak somewhere, and leaking can cause a turbo to over-spool as it struggles to hit boost.
Are you running with EGR in your setup? I've heard of similar pulsing occurring in trucks with leaky EGR valves.
What kind of boost are you seeing at cruising speeds? I believe a stock ALH in VW applications typically hovers around 4-5psi...

 

[PradoTDI]

I don't believe there is a leak, I haven't seen any other indications of one. EGR has been disabled since before I got the engine. I went ahead and shortened the actuator arm on the turbo a couple of turns and that seems to have helped, I can now hit 20psi if I floor it. Boost is usually around 10-13psi when I'm cruising at 110km/h. I think that the engine just needs less boost than I thought it would pushing a 4600lb brick with a rooftop tent on it I still have pulsing boost, especially when pulling hills.

 

[CableJockey]

Have you checked vacuum lines and n75 (I assume you are still using one)? A vacuum leak could cause strange turbo issues...
20psi is rather high for your turbo. 18psi is considered the max it can safely sustain.
Do you have VCDS? Running logs under boost will help indicate what is actually going on.

 

[PradoTDI]

I haven't checked the N75 vacuum lines, thats a good idea. I tested the N75 last summer by swapping it with a known working one, and it seemed to make no difference. I'm in the process of getting a Stage 4 tune from Malone, they said that 22psi was considered the safe limit for the GT1749VA. I have a VCDS, but it is currently 300 miles away at my brother's shop, I should be able to get it in a couple week's time and will do some data logging then.

 

[CableJockey]

My mistake: I overlooked your turbo being a VA variant. Those are listed as 21-22psi max.

Using a handheld vacuum pump (ie mityvac or equivalent) you should be able to test everything in the vacuum system. A stable 20+in/hg at idle near the cam-mounted pump and roughly the same at the turbo.
N75 can be tested for a leaky valve, but specifics escape me.

Read this: https://pics.tdiclub.com/data/517/N75_Pierburg_valve_repair.pdf

It explains the operation of the N75, with some creativity one can determine whether it is actually sealing the way it should. (If the atmosphere vent isn't filtered, or a hose is cracked, it can suck crap into the valve seat and prevent sealing)