1.8t OBD reads boost spikes up to 30 psi w/ rev

[Dirty|Dubs]

Hi guys, I searched around and didn't see anything that really helped my situation. I have an APR stage 1 tuned 1.8t AMB and recently a CEL came on for underboost (P0299). At first, I felt no change in boost even with the CEL, but more recently, I felt a huge drop in power and boost now barely comes on until 3k+ rpm. Bought a new forge DV and a OBD based indo and diagnostic system for android, now I know the DV is good, and I pressure tested for boost leaks at 20psi. The OBD readings show a typical boost of no more than 13psi max and immediately drop down to under 10psi. The weird thing is, they also show sharp boost spikes of up to 30psi when quickly revving in neutral. Curious as to what my problem is, any help would be appreciated. I will attempt to post screen shots of the boost data as well. Thanks.

[Dirty|Dubs]

This is the graph of boost under normal driving, completely nonlinear.


This is the graph of the extreme boost spikes during revving

[diagnosticator]

Both symptoms would occur from sticking and binding of the wastegate, and is consistent with the rough graph of boost pressure and the high spikes. Check out the wastegate for likely sticking and binding. Use a regulated compressed air supply limited to ~10 psig, connected to the wastegate actuator. Vary the air regulator output and observe the motion of the wastegate from closed to fully open. Make notes of the opening pressure and the pressure that the WG is fully open. Vary the regulator pressure slowly.

The graphed boost spikes are actually several bounces of the same boost pressure spike. This could be compressor surge. How is the control hose for the DV routed? Is the N249 active? Is the control pressure hose kinked anywhere? A diaphragm type DV is superior to a piston DV, and I recommend running a diaphragm DV in the reversed flow orientation compared to the OE installed flow direction.

[Dirty|Dubs]

Thanks for the info. I had a feeling it was wastegate related, I was hoping these symptoms might signal a failing n75. I recently actually tested that by removing the actuator line from the n75 and went for a brief drive, still wouldn't reach full boost, so I don't think that's the issue, but I'm hoping. As for the DV, I simply just reconnected stock hoses to the corresponding locations on the new DV (not sure what exactly the control hose is, sorry). The n249 should be active, I haven't started messing with it yet and I don't see any links in the control pressure hose. The DV is a forge 008 with the yellow spring installed, which probably isn't helping anything if I'm only pushing 12 peak psi (the forge DV is piston, I know).

[Dirty|Dubs]

Still curious as to how it manages to show extremely positive boost pressure while revving the engine in park, i.e. the second image

[diagnosticator]

Still curious as to how it manages to show extremely positive boost pressure while revving the engine in park, i.e. the second image

That is an important question. In neutral gear the only load on the engine is the crankshaft power needed to balance the friction losses, pumping loss, and fluid dynamics related losses since there is not enough mass exhaust gas flow and the EGT is to low to drive the turbine with the energy needed to develop the shaft power necessary to achieve the high energy equivalent of a boost spike at ~30 psig. This suggests the apparent boost spikes are not actually occuring at least at that high pressure. A clue is the multiple spikes happening at approximately the same time.

The turbo compressor develops pressure, from the high velocity air flow accelerated by the compressor impeller blades, then converting the kinetic energy of the high velocity air from the impeller, into pressure in the compressor housing volute, that slows high speed air flow from the impeller. High pressure discharge air does not rely on mass air flow, but the radial flow compressor needs air flow to operate.

[G-Ride]

Thanks for the info. I had a feeling it was wastegate related, I was hoping these symptoms might signal a failing n75. I recently actually tested that by removing the actuator line from the n75 and went for a brief drive, still wouldn't reach full boost, so I don't think that's the issue, but I'm hoping. As for the DV, I simply just reconnected stock hoses to the corresponding locations on the new DV (not sure what exactly the control hose is, sorry). The n249 should be active, I haven't started messing with it yet and I don't see any links in the control pressure hose. The DV is a forge 008 with the yellow spring installed, which probably isn't helping anything if I'm only pushing 12 peak psi (the forge DV is piston, I know).

Use the green spring - lightest in the forge 008 and orient it so that charge air pushes against the side of the piston and releases through the bottom. That will give you the lightest setup and still hold maximum boost. I used a yellow spring at first on a hybrid k04 and it gave me light compressor surge, so I swapped it out for the lightest setup and its great.

Does this ODB scanner remove the ambient pressure (1 atmos) during its calculation of output/actual boost? If it's reading raw ODB values, I'd expect to be seeing a reading that is 1 atmosphere too high.

[Dirty|Dubs]

That is an important question. In neutral gear the only load on the engine is the crankshaft power needed to balance the friction losses, pumping loss, and fluid dynamics related losses since there is not enough mass exhaust gas flow and the EGT is to low to drive the turbine with the energy needed to develop the shaft power necessary to achieve the high energy equivalent of a boost spike at ~30 psig. This suggests the apparent boost spikes are not actually occuring at least at that high pressure. A clue is the multiple spikes happening at approximately the same time.

The turbo compressor develops pressure, from the high velocity air flow accelerated by the compressor impeller blades, then converting the kinetic energy of the high velocity air from the impeller, into pressure in the compressor housing volute, that slows high speed air flow from the impeller. High pressure discharge air does not rely on mass air flow, but the radial flow compressor needs air flow to operate.

Exactly, unless a sensor is misreporting the readings. Would a dirty MAF cause anything like that?

[Dirty|Dubs]

Use the green spring - lightest in the forge 008 and orient it so that charge air pushes against the side of the piston and releases through the bottom. That will give you the lightest setup and still hold maximum boost. I used a yellow spring at first on a hybrid k04 and it gave me light compressor surge, so I swapped it out for the lightest setup and its great.

Does this ODB scanner remove the ambient pressure (1 atmos) during its calculation of output/actual boost? If it's reading raw ODB values, I'd expect to be seeing a reading that is 1 atmosphere too high.

I'll have to swap it out tomorrow, I originally used the yellow because it WAS running 20-21 psi and I thought that the OEM DV was the culprit of the underboost. Also, yes, this is the torque app and it does account for current elevation and standard atmospheric at said elevation.