Question on map sensor readings

[dusty76]

I borrowed my buddies Snap-On scanner trying to find any issues that would be causing my bad fuel economy. about the same time, i for some reason checked the Distributor cap and found all of the terminals were corroded bad. 1500 miles on complete rebuild and all new ignition componets. so i got the cap and rotors warrantied and it fixed the bad idle but not the fuel economy. so i notice that on the scanner, my vacuum and map values never add up to baro. And with key on engine off or under hard acceleration, the vacuum readin never goes less than about 6 inches hg. is this where these computers transition to the boost map? i know that vacuum is a calculated value, but also engine off key on, the map reading is short of being what baro reads. is this a problem? I did verify that it is a 2- bar sensor. And while cruising at like 70, it reads about 10-11 inches hg vacuum but the map still doesnt add up to 29, which is the usual baro. I think it must have a heavy load on it for the speed, because just one small increase in throttle, and the egr slams shut and boost is right there. and I on the right track?

 

[dusty76]

Re: Question on map sensor readings

Nothing? C'mon, you guys seem to know these trucks down to every nut bolt and washer. Has nobody ever noticed this oddity.

 

[Don W.]

Re: Question on map sensor readings

Well to start with actual baro is gonna depend on where you live. Book is at the shop but as I recall MAP sensor kicks out a voltage dependent on vac/boost. Vac is in inches hg (mercury), boost is in pounds per square inch. Two different things. As you add throttle vac goes to -0- and boost comes in and builds from there. There will be a point in the middle where they will both be at -0-. I would imagine this is close to but not exactly equiv to full throttle in a non turbo vech. and, if you could measure it, would be very close to baro or whatever your pressure is at your altitude. Notwithstanding pumping loss. (Air filter, plumbing, venturi, ect.) Don't forget what you're reading is actual manifold vac or pressure not what's happening before the the throttle.

I have never heard of vac and MAP values adding to baro. To me the MAP reading and the vac reading should be the same. Or, if under boost, those readings should be the same.

I'm no expert so if someone want; to shoot this down go ahead,,,:lol:

Don't know if that helps any but if someone doesn't beat me to it I'll be glad to get those MAP numbers tomorrow for ya.

 

[dgoodhue]

Re: Question on map sensor readings

What are you considering bad fuel economy? and What type of driving are you doing?

 

[dusty76]

Re: Question on map sensor readings

The best I have got is 14 to 14.5 mpg. I drive to school on the interstate and keep it around 65-70 trying to get good fuel mileage. The only non engine thing i can think of is it seems to need an alignment, it pulls to the right pretty bad and feels darty with small inputs. But, I can't see how an alignment would gain 4 or 5 mpg. It runs great, has only like 1-2 degrees of KR at wot, and just seems like a happy engine. But the fuel mileage sucks and it makes me ill every time i calculate the mileage.

 

[Don W.]

Re: Question on map sensor readings

Alignment will help a bit. Especially your safety. You don't want a "darty" truck at 70 mph. Try 55-60 and see what happens. These trucks were not built for mileage.