AUTO Veteran

Ramphele DL : At sea level, you’re looking at roughly 100 to 102 kPa, and even at the bottom of the Dead Sea (the lowest point on Earth), it doesn’t exceed 108 kPa. So, 199 kPa suggests either a serious sensor input error or a corruption in the ECU’s interpretation of the signal. On Jags of this era, the ECU performs a barometric pressure snap shot during key on before engine start, using the MAP sensor as the measuring device since there’s no separate BARO sensor. If the intake manifold or MAP port is contaminated, restricted, or sees abnormal pressure (like a stuck open EGR or boost control issue), it can skew this reading.

Ramphele DL : But more commonly, it’s a circuit issue check the 5V reference line to the MAP sensor for voltage spikes or resistance. I’ve seen cases where a failing alternator or poor grounding introduced enough electrical noise to make the ECU think it was under 2x atmospheric pressure. Verify the actual supply voltage at the sensor with the connector plugged in (back-probing), and ensure the ground is solid less than 0.2 ohms to battery negative.

Alvin Knex : If it’s the O2 sensor, why would it only happen when the engine is cold? Wouldn’t it affect performance all the time?

Jimna Mustafa : Good observation! Many O2 sensors only become active once the engine reaches operating temperature. When cold, the engine runs in 'open, loop' mode and ignores the O2 sensor, relying on preset fuel maps. If the sensor is failing, it might only cause issues once it starts influencing the fuel mixture, or if it sends incorrect signals during warm-up.

Alvin Knex : Turns out the upstream O2 sensor was failing. Swapped it out, and the violent shaking is gone. Thanks for pointing me in the right direction!

Jimna Mustafa : Updates?