'03 LeSabre MAF vs. MAP?

[Alex_V]

3800 V6, only DTC is P0172, bank 1 rich. Also a complaint of hard shifting. The "code helps" on my OBD-2 app show mostly MAF sensor fixes for this code, but the MAP value at idle is 8.6-9.2" vacuum when actual reading on a gauge is about 20" via a gauge. Removing the MAP sensor, plugging the hole, and pumping the sensor up with a Mity Vac does not reflect the same reading on the gauge as on the live data, either. Swapped in a new sensor and it acts the same.

Ground to the sensor has good continuity to engine block, supply is 4.9V, and signal to ECM is 1.3 at idle, 4.6V with key on/engine off.

The low MAP reading would explain the hard shifting and, if I'm not mistaken, the rich code because it's richening to compensate for the false load indicated by the lower manifold vacuum reading. But the electrical signals to the MAP check out. What next?

 

[Tman220]

Have you ever done the upper and lower intake gaskets? The MAP on on the 3800's get their reading through some of the same porting as the PCV valve and is susceptible to getting gummed up and plugged when you are making snot boogers in the upper and lower intake manifold because of the coolant leak. You might want to check that out. Right below the map sensor is the pcv valve. You might notice that the attachment that holds the pcv valve snaps into the upper intake, you could try removing it to clean it out.

 

[mk378]

When you're not getting a proper reading with a known vacuum applied to a new (presumably good) sensor, it would have to be something to do with the app or the PCM.

I think on these the MAP is only used for direct engine control as a backup should the MAF fail.

 

[clinebarger]

Originally Posted by Alex_V
3800 V6, only DTC is P0172, bank 1 rich. Also a complaint of hard shifting. The "code helps" on my OBD-2 app show mostly MAF sensor fixes for this code, but the MAP value at idle is 8.6-9.2" vacuum when actual reading on a gauge is about 20" via a gauge. Removing the MAP sensor, plugging the hole, and pumping the sensor up with a Mity Vac does not reflect the same reading on the gauge as on the live data, either. Swapped in a new sensor and it acts the same.

Ground to the sensor has good continuity to engine block, supply is 4.9V, and signal to ECM is 1.3 at idle, 4.6V with key on/engine off.

The low MAP reading would explain the hard shifting and, if I'm not mistaken, the rich code because it's richening to compensate for the false load indicated by the lower manifold vacuum reading. But the electrical signals to the MAP check out. What next?


8.6-9.2" Hg is the Absolute Pressure reading, While your Vacuum Gauge is displaying a Relative Pressure reading.

As you can see in this chart, 8.9" & 20.7" are the same reading........I'm sure there is a better way of explaining this, But this is how I learned the differences between Absolute & Gauge readings.

 

[clinebarger]

What is the Calculated Load in Percent? At idle with no loads?
What is the MAF Grams Per Second value at idle with no loads?

Another useful trick.....
*While watching the Fuel Trims.....Unplug the MAF sensor, Let P0101/P0102 set. (This will force Speed Density mode)
*If the Fuel Trims correct.....A faulty MAF is a good possibility.

 

[Alex_V]

Excellent info, Clinebarger. Many thanks!

 

[Alex_V]

Based on Clinebarger's info I replaced the MAF sensor and the fuel trims got much better, so time will tell if it keeps the DTC for rich bank 1 from coming back but it appears to be fixed.

 

[clinebarger]

Reset the Fuel Trims, Sometimes.....The oil will need to be changed to completely correct the trims. Depends on how long & how rich it was running.