A buddy and i just recently put a 6.0l chevy motor in his jeep. Running a computer and harness from street and performance. The problem is that his map sensor appears to be reading opposite of what it should. He bought a scan tool and while watching the readings for the map while driving they are opposite of what they should be. Take off from a light and the vac(inhg.) shoots up to 25ish. Let your foot off the gas while cruising and vac reading drops to about 8. He has driven around with a vac gauge and the scanner hooked up and watched as they gave out opposing readings. the vac gauge did exactly what you would expect. 20" at idle, drop hard on accelerating, and sky rocket while coasting. Could a map sensor be wired backward and produce this effect? or could faulty computer programing possibly do it? He has called S&P and goten no where with them. Any ideas?


EDIT:
So can anyone tell me what readings i should have on the scanner? Im finding conflicting information. At idle, i know how much vacuum the engine has is about 19-20 or so per the vac gauge. But per the scanner, it only has something like 8-10. is the scanner supposed to be giving those readings? With the key in run but the engine not actually running the map reading is like 29.5in.Hg. i would think it that it should have 0 in that situation?

 

His scanner is wrong end of story. Get a quality scanner and see what it reads.

 

i think whats confusing is your units. they are inhg. that being the case you might think "yeah like they allways are" but think of your zero point as your 29.5 inhg then subtract your 8-10 and you get your 20". most guys are thinking of map in terms of kpa. 105 or so being atmosphere and vac being 30-40 kpa boost being >105 so the trend is the same. i think its just a glitch in the scanner your using. i unit converted it in my hp and all of your numbers corrosponded to what my car does.

 

Your scantool is showing MAP rather than VAC...

VAC and MAP are related like this: VAC = BARO - MAP or VAC + MAP = BARO

where BARO = 29.5 inHg = 14.5 psi = 100 kPa if you're close to sealevel;

the math works out like robby showed above (using units inHg).

i.e. MAP is supposed to go in the opposite direction of VAC.