Normal intake psi?

[redfishsc]

Just asking for giggles here. Truck is a 99 Suburban with 5.7 Vortec

The Torque Pro app for Android has an Intake psi PID so I put a gauge on the the screen for it. Seems to hover around 5.2 psi on warm idle in Park.

Im assuming this reading is from the MAP sensor.

Pardon my ignorance but is this actually pressure, or vacuum?

I've tried to find info on this and havent found much.

Truck not misbehaving. I am just interested in what this PID is actually reading and what's normal at warm idle..

 

[evilunclegrimace]

Well, being that atmospheric pressure is around 14.7 PSI a reading of 5.2 psi would indicate a partial vacuum or a lower than normal Atmospheric pressure. So to answer your question it is both.

 

[redfishsc]

I've never seen a pressure gauge read anything but 0 at atmospheric STP, so that's interesting.

 

[evilunclegrimace]

That is because you measure atmospheric pressure with a barometer and a pressure gauge is used to measure a mechanically created pressure

 

[redfishsc]

Actually wasnt referring to a barometer but rather the pressure gauges seen on compressed air systems and tanks, they all read 0 at STP but obviously thats still a differential pressure like you said.

 

[someotherguy]

Someone that is regularly involved in tuning can probably give you a better answer, and honestly I dunno what measure that Torque is giving you that reading it. BUT:

Depending on altitude, you can expect roughly around 18~20" Hg of vacuum (at idle) out of a healthy stock small block from this generation of trucks. This is easy to check using a regular (mechanical) vacuum gauge.

Where it can get tricky is interpreting readings from the MAP sensor, even with a scanner. Behavior also changes depending on year model/ECM/PCM. I discovered this a while back while troubleshooting some TBI's that ran poorly.

Memory is fuzzy so beware - but it seemed the much earlier TBI's like an '88 I was working on, the ECM behaved this way: when you first turn the key on, but have not started the engine yet, the ECM would read the MAP at store the value as BARO (current barometric pressure) - then once you start the engine, the new reading while running would have BARO subtracted from it resulting in MAP. So, on the OTC Genesys I was getting this super low MAP reading making me think I had a big vacuum leak, until I did some reading and figured out the scheme. I do believe the later TBI PCM's did not do this but again memory is fuzzy.

Long story short? That 5.2 you're seeing is likely in/Hg (inches of mercury) of vacuum, and is the sum of MAP minus BARO as I described in the last paragraph. So if you know your barometric pressure, you can add that reading to it, and get what your actual vacuum reading is.

Richard