What does the MAP sensor do?

[jaxflg8r]

I have a 1995 VP 5.0 Fi. Yes, the dreaded Ford 5.0 liter.

Have been chasing an issue with the motor staying running. Will start right up, idle and dies after 15-20 seconds.

Hooked up the Ford EEC-IV tester. Showed code 22 which was MAP sensor. Of course, the Ford MAP sensor is "obsolete" with Ford. They had a replacement part number. Purchased one for >$100.00.

When installed, it runs, but the idle speed surges between 800 and 1200 RPM, with no throttle activity on my part. Just for kicks, I took off the new aftermarket MAP sensor and installed the old Ford one. It srarts, does not surge, but once again shuts off after 15-20 seconds. Will still start right back up.

Now, with the old MAP sensor installed, I get a code 11 (pass) however I get a 22 (MAP sensor) wwith the new aftermarket one installed.......

My question is what does the MAP sensor sense, and has anyone else had an issue with the aftermarket sensors? Any suggestions?

 

[BlueLightSpecial]

Re: What does the MAP sensor do?

MAP stands for Manifold Absolute Pressure. Pretty much measures negative pressure(vacuum) and positive pressure in the intake plenum/manifold. Surging idle is leading me to believe there may be a vacuum leak, perhaps in the sensor itself. You can use carb cleaner to verify a leak. The sensor may also not meet the parameters of the ECM, causing the idle hunt and codes. Hope this helps, and hopefully someone can shed more light than I did!

 

[jaxflg8r]

Re: What does the MAP sensor do?

Thanks for the input. I do not think there is a vacuum leak, but i will check it out. I am going to try another MAP sensor, if there is no vacuum leak. The "sensor may not meet the parameters of the ECM" comment is very concerning, especially since I have no idea what you are talking about. I know the sensors "talk" to the ECM, and give information, which the computer interprets. I get the feeling that you are saying the sensor is "talking" in a language that the ECM does not understand....Is that a oversimplified translation of what you were saying?

 

[BlueLightSpecial]

Re: What does the MAP sensor do?

Ok, this is a generalized explanation, your application may be different: the ECM sends out a 5v signal to the map, called a reference. Knowing that it sends out a 5v reference, it should see a return signal between 0 and 5v. Let's say, at idle the sensor is returning a 1.3v signal. The ECM knows the engine is running at idle, because of the data from the throttle position sensor, cam and crank sensors, etc. Let's say that the ECM is programmed to the effect, that it should be seeing .5v from the MAP sensor at idle, but instead, is seeing 1.3v. The ECM knows it should be reading around half a volt, but is reading 1.3. The ECM then sets a code for a MAP sensor. The sensor may only be capable of reaching a low voltage of 1.3. That is what I meant by being out of parameter. I hope I explained this well, I'm not the best with putting thoughts into words, lol