DIY OBD2/CAN bus reading from MAP sensor not matching JB4

[Hyperjetta]

Short story is have a small arduino project I am working on that reads in PIDs from the OBD2 port under the dash via CAN bus and I don't think I have the correct formula for converting the value being returned when requesting the Manifold Absolute Pressure PID.

The PID that I am using specifically is 0x0B, which is supposed to return the absolute manifold pressure in kPa. I take this to mean that the barometric pressure needs to be subtracted from MAP sensor reading in order to get the gauge pressure. I get the barometric pressure from PID 0x33 and it is always returning ~100 kPa, which is normal atmospheric pressure at sea level, which is where I am.

So far so good, but when I subtract the barometric pressure reading from the MAP sensor reading I am getting values drastically different than what I get when I look at the boost coming from the JB4. For example, at idle, JB4 says 0 psi, which makes sense since there would be a vacuum detected on these cars since the MAP sensor is before the manifold. However, when I compare this to the boost being calculated through the OBD2 responses I am getting -7.86 psi at idle.

I haven't logged any actual drives yet. Just taking readings at idle to understand what I'm seeing.

Am I looking at this the wrong way? Wrong formula or PIDs? The internet has not helped much in troubleshooting this particular issue.

 

[ZyroXZ2]

While my particular experience with an Arduino was making an auto-leveling RC plane, I think the issue here might be that the data you're getting is not raw data. It may have already been converted or in some way adjusted before it even reaches the OBD2 port. In other words, the values you are getting don't correlate directly to manifold pressure because the values you're being given have already been adjusted for a scan tool (ergo, doing the very simple adjustment you're making to account for atmospheric pressure at different elevations). You can check this theory by tapping into a connector (boost sensor, MAP) instead of at the OBD2 port.

 

[Hyperjetta]

While my particular experience with an Arduino was making an auto-leveling RC plane, I think the issue here might be that the data you're getting is not raw data. It may have already been converted or in some way adjusted before it even reaches the OBD2 port. In other words, the values you are getting don't correlate directly to manifold pressure because the values you're being given have already been adjusted for a scan tool (ergo, doing the very simple adjustment you're making to account for atmospheric pressure at different elevations). You can check this theory by tapping into a connector (boost sensor, MAP) instead of at the OBD2 port.

Thanks for the response. I thought the OBD2 data would be more or less standardized over those PIDs so that universal scan tools and such would work, but I really don't know enough about the technology. Still learning!

I did some actual driving tests and got some more useful information. Over a ~30 minute trip with several WOT pulls I got OBD2 readings from the MAP sensor in the range of 3.05 to 27.56. Once adjusted for atmospheric pressure the range is -11.45 to 13.06, which actually looks spot on for stock boost if you ignore the fact that in some conditions there is a vacuum.

So this leaves me feeling like I am actually calculating manifold pressure correctly, but I would still like to confirm if I am supposed to be seeing a vacuum prior to the throttle body.

 

[Bigjohnb20]

You should be seeing vac at idle, low throttle and especially at engine braking. The -12 - 13 that you logged while driving around seems spot on for a stock vehicle on both ends of the spectrum IMO. Very surprised the JB4 doesn't log vacuum. It really bothers me that the OEM "boost gauge" a) goes up to 40psi when the stock max is ~13 and also, b) doesn't show vac readings. Why couldn't they at least center the gauge with 0-20psi to the right side and 0 to -20 on the left

 

[Hyperjetta]

You should be seeing vac at idle, low throttle and especially at engine braking. The -12 - 13 that you logged while driving around seems spot on for a stock vehicle on both ends of the spectrum IMO. Very surprised the JB4 doesn't log vacuum. It really bothers me that the OEM "boost gauge" a) goes up to 40psi when the stock max is ~13 and also, b) doesn't show vac readings. Why couldn't they at least center the gauge with 0-20psi to the right side and 0 to -20 on the left

Glad to know that I'm seeing good numbers. Is there any benefit to being able to see the amount of vacuum from the gauge?

 

[oddball]

Are you looking at the JB4 log or gauge? The JB4 gauge display is a boost-only display. Anything at or below atmo just displays as "0".
If you're pulling MAP, then that will show vacuum under most situations. MAP is behind the throttle body.
TMAP is the one in front of the throttle body, so it'll show either local atmosphere or boost.

Looks to me like your setup and math are all correct- you just have to remember that the JB4 display is purely a fancy boost-only gauge.

Also remember that JB4 fakes out the MAP and TMAP signals. So when the JB4 is commanding more boost than stock, reality won't match what you're reading off the OBDII port. Only the JB4 knows what's really going on.