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OBD2 high manifold pressure reading?
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=78676		 Page 1 of 1
Author:  arasb [ Sun Mar 23, 2014 11:53 pm ]
Post subject:  OBD2 high manifold pressure reading?
I recently got a cheap wifi OBD2 ELM327 device and connected to it with my iPhone using a couple different apps. The only one that gave me any readings was OBD Car Doctor and shows data from my sensors as just numbers.

I am puzzled by the reading from my MAP sensor - I'm reading 249 with the engine off, 249 at idle, and a max of 654 at WOT. The app says these are in kPa, but seems way high. I read somewhere that the MAP sensor reads low when failing.

Is there a conversion scale for the raw output of these sensors (i.e., converting from ohms to kPa)?
Author:  Auberon [ Mon Mar 24, 2014 4:46 am ]
Post subject:  Re: OBD2 high manifold pressure reading?
If you subtract the pressure (atmospheric) for whatever elevation you live at does it give you a better number.
It might be giving you an absolute pressure reading - so at sea level adding 1 bar, 14.7 psi or 100 kPa.(all approximate).
I can take a look in my notes (in Jeep) or a reading tomorrow - its dark out and there are blacksnakes playing in our yard at night so I'm not keen to stumble out there in t'dark but someone else may be able to give you an idea of the numbers of the top of their heads.
As I remember:
My boost gauge reads 0 psi at idle (whereas UltraGauge) reads absolute at idle of 14.7 psi
Boost gauge reads 20 psi @ WOT (close it could go a bit harder)
Whereas @ near WOT Ultragauge reads 20 psi (roughly) +14.7 psi (1 atm)= 34.7 psi = 239 kPa
It would probably go to 24 psi absolute which is 266 KPa absolute.
I know it isn't a direct answer to your question - I need time for that.
Hope this helps for now.
Mongo have too much to remember for present. :SOMBRERO:
Author:  arasb [ Mon Mar 24, 2014 9:40 am ]
Post subject:  Re: OBD2 high manifold pressure reading?
Thanks for the reply, no need to battle blacksnakes, lol.
Since I'm at around 5000ft, my correction value is about 84.3 kpa which still leaves me at:

165 kpa at Idle (24psi) and 561 kpa at WOT (81 psi)

Even if my idle was 0 psi, the difference at WOT would make it 57 psi.

Before spending $50-$100 on a new MAP sensor, I will test the sensor voltages per OLDNAVYs sensor description post.

Seems strange that I would have such readings, but not have a CEL.
Author:  Auberon [ Tue Mar 25, 2014 4:44 am ]
Post subject:  Re: OBD2 high manifold pressure reading?
I thought you would've had lots of replies - sorry I left it so long.
You may not need to replace. It (MAP) could just be feelthy dirty and not reading.
At the risk of putting out a number wrongly - but I've no doubt be picked up on it - it's a 5 mm (or 4) internal hex to undo - just watch you don't drop the set screw. I don't try different ones - I do it every other oil change and have a long reach hex driver dedicated to it in my toolbox.
Thanks for the reprieve on the snakes. I'm not joking - I nearly stepped on a juvenile about 2 feet long the other night with no protection on ankles or feet. Pretty little specimen but I'm in no mood to catch them at the moment and find a home for them. I don't think my reflexes would be up to it, frankly. Some would think I'm crazy but I once would've done this.
Author:  Auberon [ Wed Mar 26, 2014 10:44 pm ]
Post subject:  Re: OBD2 high manifold pressure reading?
Any luck? Surprised more didn't weigh in to help you.
Author:  arasb [ Thu Mar 27, 2014 1:22 am ]
Post subject:  Re: OBD2 high manifold pressure reading?
No luck, but I haven't had time to look into it other than trying to get another obd app working to make sure it's not just the Car Doctor app.
I've cleaned the map from time to time and never really found it gunked up other than the first time and checked it recently.
The engine seems to run fine.
Author:  CATCRD [ Thu Mar 27, 2014 11:17 pm ]
Post subject:  Re: OBD2 high manifold pressure reading?
When I rode in your jeep the other week it ran great. I think the fact that I couldn't see boost or rail pressure at all on my android linked to your obd says that your ECU just isn't talking right. I've used it on three other crds and saw boost OK. Maybe you have that flash that broke communication with scanners. I think F31 was supposed to fix that.
Author:  arasb [ Thu Apr 10, 2014 2:55 pm ]
Post subject:  Re: OBD2 high manifold pressure reading?
Yeah, I'm suspecting the F31 also. Since I can't spend the $300 on a GDE trans flash, is it possible to take my jeep to a dealership to have them just do the F31 without wiping out my GDE hot tune from my main computer? I'm sure there's always the risk, but maybe if I stand next to the guy doing the flash, give him a 6-pack, and ensure he doesn't overwrite my Hot tune. Is there another way of flashing the F31?
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