The dash oil temperature now reads right, but MAP continues to read low (only) at idle, with it reading 15-20 kpa, while the ECU, which is reading the very same sensor, sees 22-24 kpa. That’s nice.
Anyhow, Midlana went back onto the dyno to see whether the cause of the MAP drop could be found. At first, the tuner felt that the clutch might be slipping, because if it slips even a little, MAP will immediately drop. He said it doesn’t take much slip to cause MAP to drop. Later, he got suspicious of the closed-loop boost control, so it was disabled and the boost run at a fixed duty cycle. Like magic, all MAP variations disappeared, indicating that the PID control loop needs more work. With that “solved”, boost was increased and then the MAP-drop started happening, so he finally saw what I was experiencing.
At the end of the day we had no smoking gun, but do have some things to try. First is to swap in a 16-lb spring and disable boost control completely, running a hose directly from the intake tract to the wastegate. It’ll be run that way for a month or so with logs always running, to see if the MAP-drop still happens or disappears, then go from there. Depending how that turns out, the next thing to try is going back to a 3-port controller. He felt that the combination of a weak (5-psi) spring, 4-port controller, and poorly-tuned PID may have been causing the problem (falling out of boost and being unable to recover).
The tuner told me about one customer’s car with a twin-scroll turbo setup that was impossible to tune because that boost was all over the place, very sporadic and unstable. He said that it wasn’t until the owner switched to a regular T4 turbine housing did the problem go away. When I told him that I’m running a twin-scroll setup as well, he just said, “Hmmm.” I’d love to try some different turbine housings, a 1.0 A/R housing and a non-twin scroll unit, but at $550 each, it’s just too expensive for something that would potentially be used only once.