Liberty RS suddently dumping manifold pressure

Hey guys,

Scratching our heads with this. 1992 Liberty RS, factory engine but now running Haltech Elite.

The issue occurred before the haltech, but at around 5500rpm it will go from 11-12 PSI to -6PSI in 500ms with throttle still wide open and no change in boost control. Even on open loop boost control.

We've checked the plumbing on the turbo, intercooler and bench tested the wastegase and BOV all are working as expected and show now evidence of boost leaks.

Car also idles stable and no signs in the logs of boost leaks other than and high rpms

If you use part throttle it will allow it to get up to nearly 6000rpm before dumping

It does appear to need more than 9-10 PSI of boost to do it.

The fact it can hold this boost pressure below 5500rpm is puzzling as it seems to be a combinations of airflow and pressure as pressure on its own isn't enough

I've uploaded some logs and the map for you to join in the head scratching

Soft turbo inlet hose collapse? Given it will make boost at part throttle I don't think it's as likely a hose or gasket opening up unless it's engine/gearbox twist in chassis related?

Could this be valve float? -- Are the valve springs stiff enough to close at the pressure and RPM conditions? When you have the throttle more closed, the actual pressure in the manifold is less (even if the boost pressure is the same) and thus the problem may not occur.

As it's going to below atmospheric pressure, I'd agree with Michael, there's most likely a restriction occuring up-stream of the turbo inlet.

Usually this is a collapsing hose, but may be something in the filter or even something loose around the filter intake that's being sucked into it when the air-flow is high enough, then dropping back when the airflow drops - this can cause a hose collapse, too, so check the whole thing.

It was the intake pipe collapsing.

Cheers for the help guys

Don't forget to check WHY the pressure is dropping so far in the hose - it's usually too small a filter, and/or something blocking it, and/or a restrictive section.

Some turbo inlets for these cars are known to collapse and simply need replaced with better units, but as Gord mentioned if you improve other restrictions you'll both reduce the likelihood of collapse and improve performance.