I have a low of 1.29 volts and a max of 1.31 volts at idle!

I scanned it for about 14 seconds and it never went higher thant 1.31 volts. According to my information that translates to about 9 inches Hg or just about 31Kpa.


Can anyone tell me what voltage am I supposed to get at idle with 20 inches of vacum? Could my MAP sensor be all screwed up?

Test done with engine at 650 rpms, 205.7*F coolant temp, Intake Air temp 126*F, Barometric press 4.93 Volts, Right Block learn at 119, Left Block learn at 118, Left and right integrator at 123 and 124, Injector pulse for left and right Injector banks dead even at 2.7Ms each.

Anyone have any idea whats up with the MAP sensor? all the remaining readings are withing specs.


Marvin

 

That is not that far off. generaly depending on sea level about 1.5 volts is normal..

 

http://members.aol.com/InjuneerZZ/ScanMast.htm

 

If you get an SES reading MAP low voltage it generally means the MAP sensor is going out. Theyre $35 at O'reilly's and easy to replace.

 

I am trying to figure this out, If I only get a max of 9 inches Hg inside the Intake manifold isnt that like 21 inches of vacum when measured from outside the manifold?

I am comfused because from Freds (injuneers) Conversion table for MAP units it seems as if I should be pulling at least 3.3 volts to achieve 20.7 inches HG, but since the manifold is in a MINUS barometric pressured when measured from outside the manifold then my logic kicks in and tells me ok Max HG (BAR) is like 30HG minus intake manifold at 9HG = 21HG of Vacum? Is this correct?

To make things worse my scanner gives me the BAR in volts too.

I guess all I need is a Volts to BAR conversion table.

 

Oh I forgot. I got an SES but is for EGR Failure. I forgot to re-connect the EGR vacum line back. I plugged it with a screw while I was checking for a slight miss at idle a few days back and the puter caught it.

Marvin

 

With a voltmeter the MAP sensor signal voltage should read at 1.5 to 2 volts at idle and 4.5 to 5 volts at WOT or with the key ignition turned but the engine off.

With a scan tool the MAP sensor should read between 1.2 and 1.6 at idle.

 

Looking at your first post about both banks running lean, it sounds exactly like what happened with my car. It had right and left banks lean codes and later MAP low voltage. I replaced the MAP and O2 sensors right around the same time. The MAP sensor solved the problem.

 

MeenGreen Thanks for you Post!

Also......
I am suspecting the MAP sensor for two reasons... while testing the fuel presure on park (no engine load) I did notice that when I rev up the engine and stabilizes the fuel pressure is constant but every time it came down to Idle speed the fuel pressure went up a notch or two. So I concluded my Vacum may go down a bit when the revs come down below 100rpms. I did this test slowly so not to upset vacum and trigger a the fuel pressure regulator.. so again if I rev it say slowly from 900rpm to 5500 rpms with no load the fuel pressure is steady and if i let it RPMs drop slowly it will be consistent till almost 800RPMs and then fuel pressure begins to climb. So if the map sensor is not detecting any drop in the vacum yet the fuel pressure is going up at that moment I am almost dead sure vacum is not being reported right.


Now Correct me if I am wrong but my both banks are showing

(Long term Fuel Adjustment)
Right Block learn = 119
Left Block Learn = 118

(Short term Fuel Adjustment)
Right Integrator = 124
Left Integrator = 123

and the ideal measurement for both should be 128
doesnt that mean the PCM is Substracting Fuel?

My Scanner book reads "If the Exhaust is rich the integrator substracts fuel and the reading decreases, If the exhaust is lean the integrator adds fuel and the reading increases." The same goes for the Block Learn exept on a much slower pace.

Acording to the book my car is running rich and both the Block Learn and Integrator are substracting fuel therefore the low reading???

I just got my scanner yesterday and I began to learn about fuel trim last nite. So if I am wrong dont Fuggin Shoot me alright??





Marvin

Note*Merry Fogging Xmas!!!!!

 

You seem to be confused between absolute pressure and vacuum.

A stock cam LT1 should pull about 20-21"Hg vacuum at idle. MAP = BAR - vacuum, so if BAR is 30.0"Hg, 30.0 - 20.0 = 10.0"HG MAP. My conversion chart shows that 1.1V = 8.9"Hg MAP (21.1"Hg vacuum), and 1.7V = 11.9"hg MAP (18.1"Hg vacuum). Since you are getting about 1.3V MAP, if you interpolate between my chart points, you will see your MAP is about 9.9"Hg (20.1"Hg vacuum). That is completely normal for an LT1 with a stock cam.

Again, you are confusing MAP with vacuum. 3.3V = 20.7"Hg MAP = 9.3"Hg vacuum (assuming your barometer is about 30"Hg.

You don't need any other conversion chart for BAR. A MAP sensor reading of 5.0V = 30"Hg barometric reading. If you live in Denver at almost 6,000-ft elevation, your BAR will be closer to 25"Hg, and BAR will be about 4V.... taken from the same table as MAP.

 

Thank You Fred I finally got it.

I scanned her again at Sea Level today. (well give or take 2 or 3 feet above Sea Level, maybe less.) I Scanned her with the key On and the engine off. My Barometric pressure was at 4.93 but my MAP only read 4.91. Stuck?

Well then i decided to move to a higher location and the BARO now Read 4.91 But the MAP was still at 4.91. Hmm fishy I though.


Went back next to the ocean water again and scanned her one more time. The Baro was back at 4.93 However the MAP only reads 4.91. (is this normal at all?)

Could my MAP sensor be experiencing the slow bleeding death here? They are cheap to replace but why spend cash if I dont have to. Has anyone seen this before?


If I have the chance Ill drive up the mountain and check it out again. This is with the engine off to see if the MAP readings follow the Barometric pressure. oh yeah I did open the trottle plates and there was not change off-course.


Thanks for the info Fred I got it now.


Marvin

 

It's entirely possible for barometric pressure to change. The MAP sensor is used to measure BAR at startup, before there is any air flow. Then that number is saved, and not updated. Losing or gaining 2/100ths of a volt is not significant. I really think your MAP sensor is fine. I have not seen any go bad. What I do see on the older cars, like my 94, is the harness connector dries out and crumbles and you get a bad connection.