Recently, I picked up a E36 M3box A4 that I've been piecing back together. It probably has a blown headgasket, it's minor, but it also had/has a host of other issues.
One of those, was a consistent MIL for a P0234 overboost condition. Apparently, this is relatively common on the 1.8t engines, and is typically associated with the N75 valve (wastegate solenoid). The VAG hive mind says to test this by unplugging the valve, and letting it run on wastegate spring pressure only. Sure, makes sense, some data logging later, I'm still getting spikes to 15/16psi.
Testing the wastegate actuator itself with a pump reveals a working system, so what gives? I'm giving the boost pressure a straightline shot into the wastegate actuator via a wide open system.
As a last resort, I pulled apart the entire intake system upstream of the turbocharger. Checking all of the connections and lines. Turbocharger looks pretty new, so that's cool, but wait, what's that in the pressure feed banjo fitting? Is that grease? Maybe that's the issue? Clogged with goo?
Remove the banjo fitting, clean it out, and then I realize, the damn thing doesn't have the thru hole drilled through the snout into the cavity of the fitting where the banjo bolt passes the pressure. There is no actual way for pressure to travel from the compressor housing, into the banjo bolt, out through the fitting.
Entirely a manufacturing defect on what seems to be a replacement turbocharger, was causing a constant overboost condition as the only way the engine had any ability to regulate the pressure was a fuel cut. No wonder it has a blown headgasket....
Drill the appropriate thru hole, reinstall, and now there is a happy, regulated, 11-12psi chugging through the system.
It's amazing what can slip through QC...
