OBDII Scanner Accuracy Question

[Realism]

This is for the Lemur Bluedriver. I have had this for a while and just realized that the intake manifold absolute pressure on the app has been giving me a low reading (avg 8.5 inHg). Not willing to take it at faith value, I hooked a vacuum gauge to the testing port and was given a normal reading of about 18 inHg. elevation of 2,500 ft. so this is about normal.

Does anyone else use the BlueDriver and had any issues with accuracy?

 

[AWD V8]

No experience with BlueDriver

However, any OBD displayed on any app is getting it's info from the PCM. Any inaccuracies through an OBD Bluetooth adapter will be in the form of a delay. So it is possible for example to be graphing a TPS and miss a drop out, or at the least see a delay in the display.

I have had this ELM327 adapter freeze momentarily on me, left me thinking that I had a bad sensor because of no data change.

Don't know if this is the adapter you are referring to.

EDIT: OK, I see it's not. I have no experience with that adapter.

I've used this one for years. Always connected, never froze.

 

[Realism]

It does use bluetooth, but its not freezing or giving delays; everything going and reporting as normal in this regard. I'm comparing previous logs from a year ago and they seem to all be telling me (at idle) about 26 kPa and 8 inHg.



Wish I had a old school OBD2 to hook up and check with that. If, by chance, it has nothing to do with the scanner and even an old wired OBD2 picks up the same readings, why would a PCM be giving incorrect data values?

 

[AWD V8]

So the PCM is taking a reading from a different location then your mechanical gauge. Also the PCM is relying on the MAP sensor readings.

So we have a few things to consider as to why the reading is grossly different.

Are you have running issues? Maybe the MAP sensor is giving a bad reading to the PCM, that is what you are seeing on your scanning app. A lower manifold vacuum reading will tell the PCM to increase the fuel injector time on making the mixture richer.

 

[Realism]

I suppose the MAP sensor could be giving a bad reading. I'll test it and check.
No real running issues other than excessive fuel consumption (fuel trims always show in the negs) and a high RPM idle (1800) at start that slowly drops down to normal idle range (600) 1-2 minutes later. Only does it if its been a few hours since I last started.
Driving - feels like it seriously lacks power going from full stop.

With the reading I was thinking vacuum leak somewhere, but I smoke tested last year and never found anything. Checked the valve cover bolts and they felt a bit loose. Dont have a inlb torque wrench so used a ftlb @ 8ftlb. I will admit 89inlb torque seems awfully low.

 

[Realism]

Hooked MAP sensor to vacuum gauge and probed the voltage while applying vacuum. At 0 inHg voltage reads 4.65V. 5 inHg = 3.65V, 10 inHg = 2.40V, 15 inHg = 1.3V. 20 inHg = 0.88V.

MAP sensor appears to be functioning correctly.

 

[budwich]

depending on what app you are using, the "formula" that has been "encoded" may not be correct... even though it seems like it is partially functioning.

 

[Tiggerr]

I tried the blue driver a long time ago...because it was the only iPhone compatible one available....wasn't impressed
Sent it back got a cheapo dongle and a cheapo android tablet....torque app

 

[AtlWrk]

This is for the Lemur Bluedriver. I have had this for a while and just realized that the intake manifold absolute pressure on the app has been giving me a low reading (avg 8.5 inHg). Not willing to take it at faith value, I hooked a vacuum gauge to the testing port and was given a normal reading of about 18 inHg. elevation of 2,500 ft. so this is about normal.

Does anyone else use the BlueDriver and had any issues with accuracy?

Budwich and you touched on it: it's just an interpretation thing:
The OBD standard is to report absolute manifold pressure--pressure above a perfect vacuum.
What your vacuum gauge is reading is pressure below ambient (i.e. "vacuum"). Ambient pressure at 2500ft is about 27inHg:

8.5inHg+18inHg=26.5inHg

Same(ish) reading, just different reference points. Ignition on, engine off: does your scanner report about 27inHg for absolute manifold pressure?

This method allows values above and below ambient pressure (boost and vacuum, respectively) to be stored without having to manage a minus sign in the data--more efficient. Like most OBD data, it's up to the software reporting it out to know what to do with it.

 

[budwich]

on that note, note that torque isn't an "official" (ie. paid gm for the obd interface structure which cost a lot of dollars) interface, it is based on some "I think it is like this figurin'" so as mentioned depending on what the "wiki equivalent deduction" was, it could be a simple formula. You can go into the "edit mode" for the particular "pid" and modify it to see if you can get it to agree with your independent reading.

 

[Realism]

depending on what app you are using, the "formula" that has been "encoded" may not be correct... even though it seems like it is partially functioning.

Most likely the issue. When I get a chance I will stop by parts store and see if I can borrow a wired OBD2 to compare.
I did email Lemur about this and their reply was that they don't do any calculation or interpretation of sensor values, that the value in the app comes straight from the vehicle.
They offered for me to send in a debug email and they will double check that the raw data is being decoded properly. Might as well leave no stone un-turned.

Budwich and you touched on it: it's just an interpretation thing:
The OBD standard is to report absolute manifold pressure--pressure above a perfect vacuum.
What your vacuum gauge is reading is pressure below ambient (i.e. "vacuum"). Ambient pressure at 2500ft is about 27inHg:

8.5inHg+18inHg=26.5inHg

Same(ish) reading, just different reference points. Ignition on, engine off: does your scanner report about 27inHg for absolute manifold pressure?

Not sure I'm tracking you on the 26.5 inHg reference.
Ignition on, engine off = report of same 8.5ish inHg

I tried the blue driver a long time ago...because it was the only iPhone compatible one available....wasn't impressed
Sent it back got a cheapo dongle and a cheapo android tablet....torque app

I hear you, same reason I went with it as well, some days I regret the iPhone decision. I was disappointed when I discovered the handful of sensors and data that it is programmed to track, was much less than I expected and not near what it is for many other vehicles. Was told updates for it would come out when they got to it, but so far it's been a year and nil.

 

[budwich]

Its not the hardware blue tooth / hardwired interface that does the calculatiion, its the app that communicates with the interface that "interprets" the data stream coming in from the odb / pcm output. That stream is designed by gm. some streams / PID's are universal and as such anyone can "decode" those with little or no "guessing". Manufacturer specific PID's are ... well specfic. They are well known IF you pay the manufacturer for the "details" otherwise, one can potentially guess at what / how the data is encoded for a given pid. Torque basically uses the "collective intelligence of the internet" to figure out a number of these. As for a "generic reader", it may or may not have anything available for a given PID depending on how much they are willing to fork out to gm and how much they can recoup from selling their implementation thereafter.

As suggested, I suspect the reading is correct for the parameter they are displaying as per note by atlwrk.... although it is possible that the formula used by torque might not be "all knowing" in terms of "linearnous" and "correction" for various factors.

 

[Realism]

Ah, in either case, I did send off a debug report and am waiting to hear back from that. I'm thinking that I may be better off eating the cost of the BlueDriver and doing what was suggested and picking up a cheap Android tablet or phone and going the Torque route. At the very least I have two I can check against each other.

On a side note: is the torque for our valve covers correct at 89 inlb? It seems awfully low for what its for.

 

[budwich]

if i recall, that is correct but note that this thread has been moved from "tech" area to the scanner area.... your might want to post your "spec" question back in the other section.

 

[Realism]

Budwich and you touched on it: it's just an interpretation thing:
The OBD standard is to report absolute manifold pressure--pressure above a perfect vacuum.
What your vacuum gauge is reading is pressure below ambient (i.e. "vacuum"). Ambient pressure at 2500ft is about 27inHg:

8.5inHg+18inHg=26.5inHg

Same(ish) reading, just different reference points. Ignition on, engine off: does your scanner report about 27inHg for absolute manifold pressure?

This method allows values above and below ambient pressure (boost and vacuum, respectively) to be stored without having to manage a minus sign in the data--more efficient. Like most OBD data, it's up to the software reporting it out to know what to do with it.

Thank you AtlWrk, I finally had the bulb light up and did a face palm. Rookie mistake my not remembering and factoring in both.

For what it's worth, I will finish off this thread with posting what Lemur replied with, in case it helps anyone else with their aha! moment.

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Chris replied:


Hello!

Is your vacuum gauge an absolute measurement, or is it referenced to ambient pressure? I notice in the log that your truck is reporting 95 kPa when it isn’t running (28 inHg) and 8.8 inHg while idling (I double checked the decoding of the raw data and everything checks out) which would give a reading of roughly -19 inHG gauge.

Regards,

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*Note:
When Lemur adds "select 'more' then 'contact' then 'email', they are talking about in the app itself in order to send a bug report. Had to figure this one out for myself.