Like All Forum HOT TO:'s this is a preliminary document. It is still under research and reworking.
Performance Tip:
You've checked everything else but the pedal responce off the line still an engine stumble? Check the legnth and size of the MAP vacuum line. If the line it too long or too fat the increased volume in the line can slow the ECM "vacuum advance" calculations. This will cause the engine to stumble when you punch it.
HOT TO:
Many GM cars with electronic fuel injection use a Manifold Absolute Pressure, or MAP, sensor. This sensor is used in a couple ways.
First, when the car is being started, the MAP sensor can be used to grab current barometric pressure.
Second it is used to measure engine manifold vacuum and/or boost pressure.
There are no serviceable portions of the MAP sensor itself. It either works or doesn't. There are 3 wires and one vacuum line that can cause problems.
The wires provide 5VDC, Ground, and Signal Out to the ECM. The vacuum line is connected to manifold vacuum, usually through a port on the throttle body. Obviously if the wires, terminals, or vacuum line are messed up then the MAP won't work right but some problems with the MAP can be very subtle and these subtle problems usually don't set codes.
Note that this sensor uses 5VDC and not a feed from the battery. In fact most sensors in GM cars use 5VDC. The reason for this is repeatability. The readings of the sensors must be consistent. Battery voltage can change by several volts, but the Sensor Supply Voltage must be constant or you'll get all kind of bizarre results when the ECM tries to read them.
So at rest with the key on, the MAP will show 5VDC on one lead, 0VDC on another, and something between 0V and 5V on the output lead. Usually the output near Sea Level is about 4.8VDC with the key on and the engine stopped. (This is for a 1ATM MAP. The 2ATM MAP used in boosted applications will read lower.)
If you apply vacuum, preferably with a hand pump but sucking on a length of clean line works for a quick test, then the output should change. (On the 2 ATM MAP you would also want to pressurize to a few PSI to make sure the reading moves both ways.)
You also need to inspect the vacuum line to the MAP very carefully. The accuracy of the MAP sensor readings is only as good as the vacuum line. If there is any leak or blockage at all the MAP sensor will be off or slow in responding. Check and if need be clean the manifold port that the MAP is connected to.
Another thing to keep mind of is the length of the vacuum line. You want it as short as you can get it without pinching or stretching. Most GM sensors come from the factory installed using hard vacuum lines with rubber couplers. If you repair the line with soft line then you MUST keep it as short as posible.
The reason that this is important is that the line contains a certain amount of volume. The bigger this volume is the longer it will take the MAP sensor to register changes in manifold pressure. Slow response of the MAP sensor often will not set an ECM code but will effect performance of the engine.
For example. You're sitting waiting to get into traffic, see a hole and punch the gas. The MAP however is responding slow so the ECM doesn't quite get the math right for injection and ignition timing. The engine stumbles a second and maybe even stalls.
Notes: this article is the result of a observation I made awhile back but just really thought about. When I replaced the OEM line it seemed rather long. I'd notice on a trip to the scrap yard that othe DIS motors had the MAP closer to the TBI port they plug into. Today I decided to cut the line down as short as posible. I am still testing but results look good.
