Engine/Transmission (1998.5 - 2002) Help me understand this graph

[SBall]

I recently purchased an Actron scan tool in order to clear some persistent codes and to be better prepared to fix this truck in the future. With the expectation that I'll have a MAP sensor go bad sooner or later I thought I'd use this new scan tool to capture a graph of the MAP pressure vs Load% vs engine RPM while it is running good with its new VP44. I figured this data would be considered a good "baseline". To perform this test I triggered the start of the data capture while running at a constant 2000 RPM in 5th gear. I started the trend logging just as I was starting up our local steep hill figuring I'd see the increase in manifold pressure and load %. At point A you can see the load % (red), and manifold pressure (blue) increasing as the gradient of the hill increases. I held the RPM as close to 2000 as I could (no cruise control). About half-way up the hill at point B, I goosed the throttle for a brief period of time which resulted in an obvious increase in load % and RPM. At point C I eased off the throttle which can be seen by the steep drop in load %. See graph below.

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Now for my questions to the experts out there ... .

1. Manifold Absolute Pressure (shown in blue) seems to peak and saturate much earlier than I would have thought, at 75 in-hg, (36. 8 psi). Does this look like a reading from a good MAP sensor?

2. Assuming the signal from the MAP sensor is what would be considered normal, is the flat top part of the graph due to the action of the waste gate easing the supply gas to the turbo?

3. With the truck not running at my altitude, an "absolute" pressure reading would be 14. 5 psia. My truck produces a quiescent reading of 29 psi. Does this sound right or am I missing something here?

4. Does the flat top portion of the load % graph at 100 % represent fuel limiting to the VP44 to limit torque output?

5. Wouldn't I expect to get a fault code representing "Turbo boost limit exceeded" with the signal from the MAP sensor saturated as shown in the graph above? No fault codes were produced as a result of the above test. (I did in fact have a turbo boost limit exceeded fault code previously and found a sticking waste gate linkage. )



The Actron scan tool expresses the MAP signal as in-hg. I have assumed that in-hg X . 4912 = PSI. Also note that I have graphed RPM/100 to allow the data to fit into the graph.



I kept the size of the graph small to comply with image restrictions, and hope it is readable. I welcome your response and hope to learn something about this Cummins.

 

[AKAMAC]

I would check the scaling of the MAP pressure. Zero should be ATM at sea level. You are also showing load % that doesn't match what the MAP should be doing. Stock PSI should max out at ~24 to 27 psi.

 

[WestTN]

29 psi absolute pressure would be about right with 14. 5 boost. But not at zero boost. You should probably change the MAP sensor, or try to clean it.

 

[NoSeeUm]

The ECM on my truck reports MAP in absolute pressure. At my altitude, with the truck off its about 27 or so InHG.

 

[SBall]

NoSeeUm,

That's very good information. 27 in-hg (13. 2 psia), absolute pressure sounds about right for your altitude (4000' if I were to guess). At my altitude (300') I figure I should be reading about 29 in-hg (14. 2 psia), not 59 in-hg that I'm measuring now with the engine off. I'm beginning to wonder if perhaps the MAP sensor calibration has shifted on my truck, possibly triggering a false "excessive boost fault". It doesn't seem to be hindering the engine performance though.

Steve