tonyorlo

I have a problem with the vacuum reading the PCM is seeing. Watching it live on a datastream with a scan tool It shows 11.5” of vacuum, I purchased a new sensor and it reads the same. When I put a vacuum gauge on the intake it’s showing 17.5”. I’m having a misfire problem and all the plugs verify it’s running rich.

I used PCM Hammer and Tuner Pro RT to make the changes to the PCM. I used a BIN file an OS from a 01 LQ4, mine is a 02, that shouldn’t matter. Could I have the wrong offset or scaler for the map on my tune?

I hope I don’t upset anyone having multiple threads, but I believe this is my problem.

QwkTrip

iTrader: (23)

Where are you getting these numbers from? A Manifold Absolute Pressure sensor doesn't report vacuum in inches mercury. It doesn't report vacuum at all. The ECM reports MAP value as absolute pressure in kPa (not relative to atmosphere).

tonyorlo

This has confused me. All the tables I have seen use kPa, but the 2 scan tools I have used both show it in inches of mercury.

My mechanical gauge is showing 17.5”.

QwkTrip

iTrader: (23)

It's probably just a user setting in the scan tool for units to display. Turn it back to metric or whatever works.

11.5" Hg vacuum is roughly MAP value of 60 kPa with atmosphere at 100 kPa.

1.0" Hg = 3.37 kPa
https://www.engineeringtoolbox.com/p...ter-d_825.html

tonyorlo

Thanks for the clarity on the tool.

So wouldn’t
11.5 x 3.37 = 38.75kPa
and
17.5 x 3.37 = 58.97kPa

Im not trying to be smart, I could be wrong. But either way, the gauges should be the same no matter the unit of measure?

Im just trying to sort out this misfire problem I have, so far everything mechanically checks out ok.

Thanks for the replies!

gametech

iTrader: (1)

You are looking at this as vacuum, but your MAP sensor reports absolute pressure. Total vacuum would be zero KP, and zero vacuum at sea level would be about 105 KP. If the unit conversion you were given above is accurate, then your 11.5" HG worth of vacuum would be 105KP - 38.75KP= 66.25KP. Your 17.5"HG vacuum would be 105KP-58.97KP=46.03KP. Your mechanical vacuum gauge will always have an offset from a MAP sensor, because the mechanical gauge shows a reading relative to ambient pressure, while a MAP sensor reading shows pressure above zero. It can seem confusing until you learn to think in absolute pressure. What you commonly think of as zero vacuum is actually almost 15psi, because that is the actual pressure of the earth's atmosphere at sea level. I hope I did not just make things more confusing for you.

QwkTrip

iTrader: (23)

It's the concept of "gauge pressure" relative to atmosphere vs. "absolute pressure" relative to true zero.

MAP stands for Manifold Absolute Pressure. It's the real, full scale pressure where 0 kPa really means zero, nadda, nothing, like the void of outer space. There is no such thing as negative pressure to a MAP sensor. The software in your engine ECM thinks in terms of absolute pressure with units of kPa (kilo Pascals).

0" Hg (mercury) gauge isn't real specific, it just means it's same as the atmosphere that day. What was atmosphere? I don't know, depends what is the elevation where you live and the weather that day.

-11.5" Hg gauge means the pressure is below atmosphere, almost 40 kPa below atmosphere if you do the conversion. If atmosphere was at 100 kPa that day then the MAP sensor would read something close to 60 kPA absolute (100 - 40 = 60).

tonyorlo

Nope you guys cleared it up for me perfectly. It’s been some years since college, but we went over this. PSIA vs PSIG. I guess I was stuck on a full atmosphere being removed which isn’t far from 14.7 being I’m 650’ above sea level. It’s been years since I’ve even had to think about that stuff.

I’m sorry to bother you guys with this simple question. Thanks for the help and being patient with me!

LilJayV10

iTrader: (39)

I'm glad you figured it out. I was getting ready to post that. It does get confusing.

gametech

iTrader: (1)

Your post made it obvious that you were putting in the effort, rather than just asking stuff google is for. I don't think anyone minds answering an intelligent question. I know I have found myself stuck on what seemed like something very simple once the answer clicked in my brain. Also, I have read things while doing research that made me think in an entirely different direction.

rsb213

can you not derive manifold vacuum using barometric pressure and subtracting manifold absolute pressure?

gametech

iTrader: (1)

There is no reason to do that, as Manifold Absolute Pressure is the only thing your engine cares about. Manifold vacuum is just the only thing we could really measure before MAP sensors were readily available. Nowadays, unless you are completely devoid of electronics, vacuum is meaningless.