loading

MY 1991 GMC SUBURBAN HAS A ERRATIC IDLE. UP AND DOWN! WHEN I UNPLUG THE MAP SENSOR IT STOPS.

I REPLACD THE SENSOR BUT NO CHANGE. WHAT COULD BE THE PROBLEM?

First, check the EGR valve; it's the mushroom-shaped component near the intake manifold with at least one vacuum line attached.  If you feel the underside with your fingers, you will find a diaphragm.  Squeeze it inwards with your fingertips.  It should move smoothly and fairly easily.  If it doesn't, replace it.

The only other thing I would recommend if the idle "searches" up and down would be the throttle position sensor.  It's basically a potentiometer that, just like a dirty volume pot, will send crackly noise to the computer when it needs replaced.  However, this typically wouldn't be the problem if unplugging the MAP sensor yields a steady idle.

If you have a check engine light, take your vehicle to a mechanic to have them read the codes (or some auto parts stores like Advance will do this for free).  This will give you a better idea of what the computer "thinks" is wrong.  However, if this light is not on then the computer hasn't diagnosed a problem and either believes that all the sensor readings are correct or it can't make sense of the trouble.  Another idea might be to disconnect the negative battery terminal, attempt to start the vehicle to drain the computer, then reconnect the terminal.  While I doubt it will fix the problem, it may at least cause the computer to reevaluate the condition and return a trouble code.
Re-design7 years ago
With the MAP disconnected your mileage will suffer.

Some cars have the map and the EGR valve working together.  If the EGR valve is stuck or closed but the MAP thinks it's still working you could get a rough idle.

Then when you disconnect the map the it quits trying to mix the air with the egr valve.

Bet if you replace the egr valve that fixed it. 

Of course we're not really betting and if you replace it and it doesn't fix it the you won't really get anything from me.

Good luck either way.
vince 097 years ago
well i think the simple amswer is leave the MAP sensor unpluged