Hi, what would cause the TPS in a 502 mpi to receive 0.3v fully open. Tested the plug and it’s at 0.3. Should be at 5v. Is it from the ECM? Anyway to test it?

Unplug all of the remaining sensors that use a 5V reference. One of them is probably shorted and dropping your reference voltage.

do you know the remaining sensors that use the 5v?

I don't have a wiring diagram handy, but most likely ECT, IAC, MAP, IAT. Have you done any work on it lately like maybe pulling the intake plenum? It may have a wire pinched and grounded out.

I have seen a bad IAC cause weird voltage readings...is the boat idleing really bad?

it is yes, but can hear the iac kicking in. Soon as I hit the throttle the engine tends to die.

ok I’ll take a look. Is there a way to test the 5v reference wires to see if any are grounded?

You will need a wiring diagram and/or a pinout of the ECM. There is probably just one 5V reference wire from the ECM that will split off to all of the sensors. You will need to measure voltage at the ECM on that wire. If you have 5v I would then go to the splice and you will most likely find your problem there. If you don't have 5V out of the ECM, you will need to disconnect that wire from the ECM and measure again. Any short on that wire will drop the voltage out of the ECM. If still low voltage at ECM, you need a new one. If you now have your 5V at the ECM, you have a wire/sensor shorted. I would start by checking for 5V at the rest of the sensors. If you have voltage there, It could just be a bad connection at the splice going to the TPS. If no 5V with all of the sensors disconnected, probably bad ECM or wire shorted to ground or the splice has come apart.

what do you mean “receive .3 at WOT”? You should always have 5v reference voltage from ECM regardless of throttle position then output from sensor back to ECM is .5 - near 5v depending on throttle position.
Think the ECT sensor is a 3 wire sensor also.

Just tested the sensors, unplugged the MAP sensor, and that had 5v, so tried the TPS and IAT again and they now have 5v too, so does this mean it’s the MAP sensor or a short?