Negative boost numbers

[wooner]

I very rarely view boost thru my scan gauge, but yesterday morning I started the truck and it was reading -3.x psi boost at idle. My Isspro boost gauge is reading correctly. Both go up and down with normal driving like usual, just the scan gauge reading starts out as a negative number. I am assuming the scan gauge gets its info from the map sensor. Is this an indication my map sensor is going bad? Never heard of these things going bad.

Thanks

 

[stroker2]

At idle your not building any boost so there would be a vacume like a gas/naturally aspirated engine. I dont know if it would be 3 pounds worth though....but not sure. I wouldn't be worried

 

[Fly By Night]

What kind of shape is your air filter in?

 

[youngstroker]

ya all engines suck air, a normal gas engine makes 18-20" of vacum, diesels don't have throttle plates, so there for they don't make a lot of vacum, witch is why we have vacum pumps.

 

[Arisley]

There should never be a vacuum.

 

[Jhaddox]

There should never be a vacuum.

:whs:

 

[SEABEE08FX4]

Up pipes leaking, boost leak?

 

[wooner]

I don't think I actually have negative boost because because my real boost gauge is reading correctly. But since my scan gauge is showing what the PCM is seeing from the map sensor, I would think a negative number may have some effect on how the truck runs.

 

[neverkickn]

I don't think I actually have negative boost because because my real boost gauge is reading correctly. But since my scan gauge is showing what the PCM is seeing from the map sensor, I would think a negative number may have some effect on how the truck runs.

Does your analog gauge have a stop or does it too read vaccum?

 

[Tom S]

I predict that if you read the data with a better quality scan tool you will see the MAP is just fine.

 

[JLDickmon]

I predict that if you read the data with a better quality scan tool you will see the MAP is just fine.

I predict if he read it with five different scanners he'd get five different readings

 

[cw96stroker]

My scan guage reads the same way except it doesnt ever go to -3, it stays around -.xx or -1.xx. Everything else is normal so im not really to worried about it.

 

[imelmo]

Does it read the same as your analog gauge when building boost?

.

 

[Tom S]

I predict if he read it with five different scanners he'd get five different readings

Interesting, do you really think there is that much differnce when you get to high end tool like the IDS?

 

[JLDickmon]

all the manufacturer tools have different software routines, as do the aftermarket ones like the Snap On and OTC
all they're doing is converting a voltage into a display value..

if the voltage and Hz values are within the correct range, disregard the actual display and drive the truck.

 

[washingtoncoastie]

My scan gauge does it too. But never more than .xx and just right at startup.

 

[youngstroker]

so when an engine is idleing, under no load, the turbo is not building boost, as the piston goes down, sucking in air from the intake, it wont creat a little vacum?

 

[Arisley]

Nope. Even at idle the turbo is building more boost than that.

 

[wooner]

Ok. Found out something interesting. When I first noticed the negative boost reading, I was in Helena Montana which is at about 4500 ft above sea level. I live in Portland Oregon which is about 250 ft above sea level. On my drive back to Portland today, I noticed the gap between the two gauge readings getting closer and closer as I got lower in elevation, and when I got back to Portland, both gauges were reading the same again, so apparently this negative reading from the map sensor had something to do with altitude. Not sure how, but everything is back to normal now. I would be interested to know if that has something to do tuning and what the individual tuners write in their programs for trucks at different altitudes.

 

[JLDickmon]

Ok. Found out something interesting. When I first noticed the negative boost reading, I was in Helena Montana which is at about 4500 ft above sea level. I live in Portland Oregon which is about 250 ft above sea level. On my drive back to Portland today, I noticed the gap between the two gauge readings getting closer and closer as I got lower in elevation, and when I got back to Portland, both gauges were reading the same again, so apparently this negative reading from the map sensor had something to do with altitude. Not sure how, but everything is back to normal now. I would be interested to know if that has something to do tuning and what the individual tuners write in their programs for trucks at different altitudes.

interesting observation..
I'd have to look, but I think MAP sensor diagnostic values differ for various altitudes.

I remember seeing it on the old BARO sensor EFI gassers..