Identifying a Failing MAP

Customer's engine was dying while cruising down the road and you can see very little response in engine RPM to modulation of throttle.

The cause appeared to be a failing MAP sensor resulting in lean mixture (speed density). The smoking gun, IMO, was the sudden dip in reading while the throttle was closed. Without that, it might have been a little more difficult to identify the lower-than-normal readings with the throttle open, but even that is pretty clear if you're looking for it. You do not expect nearly 15 inHg of vacuum with 40-50% throttle…

NOTE: This seems to be a 5 or 6 year-old MAP sensor with unknown history to it. These Omni 4-bar sensor really don't fail very often.