MAP sensor scaling
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On The Tree
Joined: Feb 2020
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MAP sensor scaling 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.
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.
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).
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
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
Thread Starter
On The Tree
Joined: Feb 2020
Posts: 180
Likes: 14
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
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
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!
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!
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!
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).
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).
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Thread Starter
On The Tree
Joined: Feb 2020
Posts: 180
Likes: 14
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!
I’m sorry to bother you guys with this simple question. Thanks for the help and being patient with me!
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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!
I’m sorry to bother you guys with this simple question. Thanks for the help and being patient with me!
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!
I’m sorry to bother you guys with this simple question. Thanks for the help and being patient with me!
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.