GM 4.3L M.A.P. Sensor readings Incorrect

[Jamie]

I am using the latest version of PCMSCAN 2.2 and I cannot seem to get accurate readings from the MAP sensor at idle.
I am reading ~ 10"hg When A mechanical gauge shows 20. I tried two vehicles a 96 Dodge Caravan and a 97 Chevy Blazer. Has anyone been able to get accurate readings with their system? Maybe the PCM needs to be flashed? Does the PCMSCAN software do PROM programming? 

[Jamie]

I just tested a 2000 Blazer with the same engine and it too gas readings of ~ 12.2"hg at idle. Has anyont been able to get accurate readings at idle from the MAP sensor? Also I noticed on this 2000 vehicle It does get the VIN after it connects, on my 97 Blazer it does not get the VIN it reports N/A.

[Jamie]

Just tested another 2001 S10 pickup and the MAP readings are also in the 11 12 range.

[BrianP]

As I said in your other thread, I looked at the techlog you sent, and the software is displaying the correct number. That is what the vehicle is reporting as the MAP value.

[DiscoStu]

Here's what I found on S10Planet.com.  I think the way it might work is that at idle, the MAF is used rather than the MAP, but at part throttle the MAP takes over.  See what you can gather from this explanation:

"The stock '96 up computers will control any High impendence injector. The stock computers sequentially control the injection system.

To understand how the computer will control the injectors, to compensate for boost it is necessary to understand how the '96 up engine computers determine fueling.

Here I will explain 2 thoughts on injection control.
One is called Speed Density. This relies on a MAP (Manifold Absolute Pressure) Sensor to figure out how much air is entering the engine. Vehicles that use this method are '95 down s10s, and more recent the new HEMI uses this method for fueling. The computer measures how much vacuum is in the intake manifold. Then it has a lookup table, with rpm, and manifold pressure. The computer will then lookup the value for the intersecting points, and this pre programmed value will be used to determine fuel needs of the engine. Many engines use a 1 bar map sensor. These sensors cannot read boost, so for a speed density setup on a blown engine it is important to convert to a 2 bar or more map sensor. A 2 bar map will read up to 14.7psi of boost.

The second is called Mass air flow. This uses a Maf (mass air flow) sensor. This is a sensor with several wires in it. As air flows over the wires the sensor can determine how much air is flowing through the sensor. The sensor then outputs a frequency to the computer. The computer has a lookup table to know how much air is going through the sensor at different frequencies. Fords are known for having purely Maf style fueling. My '05 explorer sport track doesn’t even have a map sensor.
A '96 Up 4.3 is lucky enough to use both styles of control. At part throttle the computer relies on both the maf, and the map. My opinion on why Gm used both is this; Maf’s are good for measuring constant airflow. When you change the amount of air going through the maf, then the sensor has a lag, due to the heated wires changing temperatures. With small amounts of air going through, this lag error could be large. The map reacts almost instantly to these changes mostly at part throttle, and fills in while the maf is catching up.

Here is the important part for a boosted application. The maf is solely used at wide open throttle. This means that the computer will be able to measure the added air coming in due to the blower. The computer will be able to compensate to an extent for the air flow potential of the supercharger.

Timing is mostly controlled with the map sensor. A boost timing retard may be required here. Or converting to a 2 bar map sensor.

The transmission line pressure is calculated off of the load on the engine. Converting to a 2 bar map may cause the computer to think that there is little load on the engine, and cause the Trans to slip, and cause failure.

So, my plan is to keep the stock map sensor, and rely on the maf initially. I may try a 2 bar map, and may go to a full speed density tune. A vacuum modulator kit would be put in the transmission, or the transmission tables would be skewed to allow for proper line pressure. I am not exactly certain of how the transmission will react yet. I may also keep the maf, but force the computer into speed density if I can figure out a way to do so. Load for the transmission would be calculated off the maf, but fueling would be calculated off of the map. Not exactly sure of the direction I will take here.

A problem with ECM tuning on most factory injection systems is that they use a narrow band o2 sensor for feedback, to see how close the tune is to how the engine is actually performing. The o2 sensor is not used for Wot fueling directly though. The stock o2 sensor is not capable of accurately reading air fuel ratios outside of 14.7:1 like at wot.

So, I have a Wide Band o2 sensor to check the actual air fuel ratio of the engine. This will be a valuable tool for tuning the engine, weather it is boosted or not. Boosted engines are even pickier than N/A motors though, and the chance of melt down is more severe under boost. This will help me keep the engine healthy while tuning it in.

Note: Much of the information of this post is my opinion. I tried to keep it as factual as possible. Always do your own reserch from different sources before practicaly applying this to your own vehicle. Remember that this is reserch for my project that I have done, i cannot coment on real world application of this information as i have not done it yet to see if it actualy works."

[rat]

Here's what I found on S10Planet.com.  I think the way it might work is that at idle, the MAF is used rather than the MAP, but at part throttle the MAP takes over.  See what you can gather from this explanation:

lol, that looks familiar

The map sensor will still read though.  It is just the degree to whitch it is used by the computer changes.  Wot is all maf.  Part throttle being a mix between the two.

Post a datalog in kpa.  Also make sure the units are being read right, are you sure it is in-hg, and not psi?  In-hg is aproximately psi*2 i think, the numbers start to make a little more sense.

I am not real positive why the numbers really matter though.  I guess it depends on what you are trying to do.  I do all my datalogs in kpa since that is what my tuning program uses.  And i know the kpa readings are correct.

[nexum1919]

Hi, I'm just an apprentice auto technician in training, and came upon this forum during my search for an affordable scan software...

MAP sensor reports the _absolute_ pressure in the manifold. And assuming we're on the sea level, the atmospheric pressure is 1 atmosphere by definition and this corresponds to 30 inHg (inches of mercury). So, a MAP sensor just sitting outside will read 30 inHg. And in an perfect vacuum, it will read 0 inHg.

The mechanical vacuum gauge that is connected to the manifold reads the _vacuum_ applied by the engine. So if you have 22 inHg of vacuum at idle, this means the absolute pressure inside the manifold is 30-22=8 inHg. Since atmospheric pressure is 30 inHg, and engine vacuums up 22 inHg of this pressure and what's left inside the manifold is 8 inHg. Since the engine cannot produce perfect vacuum, MAP will never read 0, and vacuum gauge will never read 30. If you push the gas pedal hard, MAP sensor will momentarily read close to 30, since by pushing the pedal, you open up a large hole in the manifold and thus the vacuum is lost. Then the engine revs up, assuming the same angle of throttle is maintained, the revving engine will create partial vacuum and MAP reading will go down, since the pressure inside the manifold is decreased by the re-established engine vacuum.

But if you add two numbers up, you'll always get 30 inHg (unless you have a vacuum leak between the MAP sensor and the mechanical gauge connection point in the manifold, and this is a really rare occurence)