The MAP mystery continues. Did some more troubleshooting and while it’s not complete, it helps clear up a few things, albeit also posing more questions.
Checked out the blowoff valve, which dumps air out of the intake tract when there’s vacuum downstream of the throttle plate. It works fine, at least when tested statically with a handheld vacuum gauge, opening at about -20 kpa*. I checked every fitting and hose for looseness and holes – nothing. What keeps bugging me is how MAP will stay at 200 kpa for a few seconds, then leaks down to 150 kpa. Why it’s not a steady leak at 150 kpa I don’t know.
I then tested the wastegates, feeding 20 psi into the control lines to check their operation and to make sure they aren’t leaking. Pressurizing the backside of the diaphragms (which pushes the valve open, helping the spring) showed there were no leaks in that circuit. Pressurizing the front side of the diaphragms and… hmm, they leak, though the valves still opened as they should. The leaking isn’t good, but I’m unsure if that’s the problem. That is, if some of the MAP pressure was leaking out, it would make the wastegate valve LESS likely to open, not more likely. That’s what’s so strange about the problem, that running only on 200 kpa springs works for several seconds, then MAP magically drops to 150 kpa. Where is that pressure going? Blowoff valve? Intake track hose? A wastegate that suddenly gets weak? Something internal to the engine? A cylinder that stops firing? (I don’t detect any roughness in the engine when it happens)
Though it’s a bit of effort, I’d like to pressurize the entire engine. That is, feed about 30 psi into the turbo inlet and plug up the exhaust pipe. That would certainly locate any obvious leaks. However, it won’t find internal leaks, such as a wastegate opening early and bleeding air around the turbo. Still, it would help compartmentalize where the problem is.
* For the units-challenged, 101 kpa is 14.7 psi. I use kpa because that’s what the ECU uses.