Wankel7

Nope, looks you have it.

However, its much easier to find a vacuum leak with the engine off and pressurizing the intake system with compressed air like I had mentioned earlier.

James

takahashiRyosukeFC3S

understood wankel7, thank you again.

Ryosuke

NZConvertible

Actually manifold pressure is in vacuum all the time on an NA engine and most of the time on a Turbo engine. Like I said, just watch the gauge...

tarmac_terror

I've got the exact same problem with the boost gauge not working as it should on my S5TII.

The way I undesrtand it (and please correct me if I'm wrong) is that the ECU sends the voltage to the boost gauge. The ECU detects the intake manifold pressure from the Pressure Sensor. If it's the sensor that is faulty and sending low readings to the ECU, yes installing an aftermarket boost gauge will tell you the correct manifold pressure... but does having a faulty sensor cause any other problems (wrong air/fuel mixtures etc)?

I guess the reason why I'm asking is because i've noticed that the gauge is barely working now and (although I might be imagining it) the car feels a bit more sluggish under normal driving...

- K

NZConvertible

Half right. The MAP sensor sends a signal to both the ECU and the boost gauge. They're essentially wired in parallel with the sensor.

The MAP sensor doesn't have much effect on fuelling but it does have an effect on ignition timing, with the timing being retarded as the manifold pressure increases. If the ECU sees a signal that's lower than it should be, the ignition won't be retarded as much. This increases the chance of detonation.

From what I've heard the stock ignition maps are pretty conservative, but I'd still rather have it working properly just to be sure. Check the MAP sensor signal voltage at the ECU. It should be 2.2-2.3V with the ignition on and the engine off, and drop to 0.8-1.1V with the engine idling.