2001 V70 T5 Question on Boost Sensor (MAP)

[DamoV70T5]

So I don't have a specific issue or problem with the car. Just seems like it does not have quite the oomph that it used to when I got it first (at 120K mi, has 140K mi now). Could be that I am just used to the power now and simply want more.

So I decided I would tinker a bit and replace the TCV hoses with silicone and adjust the wastegate to factory specs (if needed). But rather than just do the work and then convince myself that "Yeah, that feels much better now!", I plan to gather some performance data pre and post change so that I have a somewhat objective measure. As part of this, I temporarily installed a vacuum/pressure gauge on the inlet manifold and this works fine (getting what seems to be reasonable max boost under load of 10-12 psi). Next, I decided that I would monitor the signal coming from the boost sensor (MAP) and ultimately correlate it to the readings on the "boost" gauge so that in the end, I could log the signal from the MAP and know what the boost is. When I checked the signal from the MAP at idle I was getting around 1.9 V. The "boost" gauge gives me somewhere close to 20 inHg vacuum at idle. When I rev her up a bit, the gauge moves towards 0 vacuum temporarily (which I think is normal) but the voltage from the MAP hardly changes. If I remove the MAP so that its reading just atmospheric pressure, I get 1.9V. If I subject it to slight vacuum or pressure the voltage responds appropriately (vacuum = voltage decrease, pressure = voltage increase).

I understand that the MAP is positioned upstream of the throttle and, therefore, would not experience the same vacuum as the gauge, which is attached downstream of the throttle, directly on the manifold. But does this mean that the boost sensor is really just that, a sensor that will only provide information about pressures above atmospheric, or should I be seeing some vacuum on the manifold to which the sensor is attached?

Thanks in advance

Damo.

[jimmy57]

Your boost sensor on that car is on the outlet of intercooler for the pipe to throttle. Your manifold pressure is immediately influenced by throttle opening but boost pressure is indirectly affected by thottle. Once you go past a certain load and throttle opening then the boost pressure will be very sensitive and more immediately impacted by throttle opening changes. Wwhen you close throttle from a high load and throttle operation you will see boost pressure spike as you suddenly closed the door for the pumped air to enter.
MAP sensors are plumbed to measure manifold pressure but the boost pressure on your car is located differently and its readings do not always mirror the pressure gauge on intake manifold you are watching.

[jimmy57]

As for power loss. Hot weather makes the turbo engines, especially high pressure turbo engines, much more octane sensitive. My 04 R is a totally different car on some 93 octane fuels I can find locally. Surprisingly it runs best on whatever Sam's Wholesale Club has in their fuel station in front of their store. This also is the lowest price premium in my area today. I can watch knock levels as interpereted by ECM through knock sensors and this fuel rarely gives any knock sensor activity. The timing stays at more advanced level with no knock retard and throttle response is much more crisp.

[DamoV70T5]

Thanks for the info Jimmy. Just to confirm then, there is no actual MAP on this car? Just the boost sensor on the intercooler manifold?

I have only ever run 87 octane (it's a big tank to fill!) so there will be a limit to what I can get out of it power wise. Cheapest regular around here is $3.85, $4.05 for premium.

Damo

[jimmy57]

The fuel system on that car is mass air flow sensing. Some systems use both MAF and MAP but Bosch on Volvos back into the 80's was MAF with no MAP.