The MAP sensor is a triage tool

The MAP sensor is a triage tool. Most techs don't know how to use it in that particular manner. Before you pull the scope, pressure transducer, or cylinder leak tester, your scan tool MAP sensor data can already tell you where the problem actually lies. Here's how: Three readings, that's all you need. Key on, engine off. MAP sensor should read approximately 28 inches of mercury. That's barometric pressure depending on your location. Start the engine, warm the idle. The MAP sensor should drop approximately 20 inches of mercury down to about 8 to 9 inches. That's engine vacuum doing its thing. Now cranking, MAP test, pull the fuel pump fuse, crank the engine with the throttle closed. A healthy engine pulls 3 to 5 inches of mercury. A low pull could be a mechanical issue. Start looking at compression. Snap the throttle at idle. Vacuum should spike up when the throttle closes. No spike, restricted exhaust, EGR problem, or maybe even a valve timing issue. Jim Wilson put it the best: Use the MAP sensor as a triage tool before bringing out the big guns or the more costly tools in your arsenal. I'll see you on the next one.