Yet another, BMW E-36 LSx conversion... by BRAAPZ

3 minute(s) of a 105 minute read

Quick update and info on the GM Fuel tank pressure sensor.

Wiring is approx 95% complete, only couple connections to make, prime the oil system with the remote oil primer then fire it up.


In short, you must use the GM fuel tank pressure sensor with GM PCM to control the BMW evap canister vent and either the BMW or GM canister purge valve and you can NOT use a GM MAP sensoras fuel tank pressure sensor.

Specifics for us geeks;
Using and old PC power supply with its 5 volt DC power source I was able to test a GM fuel tank pressure sensor and the BMW fuel tank pressure sensor to see if the BMW sensor could be retained. In short, NO, it cannot be used with GM PCM. The BMW pressure sensor and GM fuel tank pressure sensor are reversed in their Voltage-to-pressure readings so it will not work properly. In using the GMPCM to control the evap canister and fuel tank purge valves, you must also use the GM fuel tank pressure sensor.

Also, you can NOT use a GM MAP sensor for this purpose. I have seen information on the net saying the GM MAP and GM Fuel Pressure sensor are the same, that is incorrect! Those claims are guesses, NOT tested or measured. After testing them side by side, (applying 5 volt and ground, measuring the signal while applying vacuum and pressure), they are not even close! They may look sort of similar, even have the same sealing grommet, but they are NOT the same nor interchangeable.
GM MAP sensors;
Testing the GM MAP sensors, my findings matched exactly what is found online. Registers atmospheric pressure and below i.e. vacuum. At atmospheric pressure, (WOT or engine off), voltage is 5 volt. Pull 5" of vacuum on the MAP sensor and the voltage drops to 4v. Pull another 5", (10" of vacuum total), voltage drops to 3 v. Pull another 5", (15" total), voltage drops to 2v. Pull another 5" of vacuum, (20" inches of vacuum total) and the voltage drops to 1v.

GM Fuel tank pressure sensor;
The fuel tank pressure sensor visually looks sort of similar to the MAP, has the same silicone sealing grommet but the MAP connector will not fit. For what it's worth, the crank position and Cam position sensor connector does fit the fuel tank pressure sensor. This sensor also registers pressure above atmospheric, but only barely and is MUCH more sensitive registering minute pressures below andabove atmospheric i.e. vacuum and pressure. Of greater importance is its voltage to pressure values are reverse of the MAP sensor.
At atmospheric pressure, the fuel tank pressure sensor delivers 1.5 volts. As vacuum is applied, voltage rise is linear to 5 volts at approx. 1” or 2" of vacuum. My vacuum gauge is not accurate enough at that low of vacuum for an accurate measurement. Using your mouth you can easily max out the sensor with light suction or light pressure. Blow into it the voltage continues to drop below 1.5 down to minimum of .23v, I guess that to be pressure to be approx. 1-2 inches of pressure, not even half a PSI. Just blowing across the opening the voltage swings, this sensor is very sensitive.