Can someone tell me what the hard black vacuum line, "J shaped" on the top of the throttle body and running to a switch, or sensor mounted on the firewall, passenger side, is? I've been checking for vacuum leaks, haven't found any, so far. Just replaced IAC, EGR, plugs, fuel filter,cap, rotor, and wires. The truck won't idle and surges at stop lights,accelerates hard like a plugged converter.Converter replaced also.The reason I ask about that particular vacuum line is, when I unplug it from the sensor on the firewall the truck idles almost normal,but blows black smoke from exhaust pipe. When I connect it again the truck stalls and runs like"you know what".. Ive spent so much money trying to find and fix this problem, I'm almost ready to give up and sell the beast, as is. Any help is appreciated.

 

The sensor on the firewall is the MAP sensor. It is the main sensor used by the computer to determine engine load. That could be your culprit, but I would test the sensor before buying a new one; they are rather expensive.

 

is the map sensor computer controled or vacuum activated and can it be checked off of the truck?

 

Well, MAP stands for manifold absolute pressure, and is a sensor for the computer. It tells the computer the manifold vacuum (pressure). It needs to be checked on the truck so that you can apply vacuum and watch the sensor output to see if it responds correctly to the applied vacuum.

 

Many Thanx to you and this "great site". I took a sensor off an old barnyard truck I sometimes use for parts and its working great.

 

Glad to hear you got it fixed!

 

How do you test the map sensor?

 

You don't test the MAP sensor. You read the codes from doing your KOEO scan. If the MAP sensor is bad the codes will tell you.

 

You can certainly test the MAP sensor. It outputs a frequency signal to the computer, which is easily read by a tachometer on the six cylinder scale or what I have done, is read it on a duty % scale on my digital multimeter. A haynes manual will give you the step by step procedure.