Can anyone diagnose a Mercury 30hp EFI accurately?

I have a 2006 Mercury 30HP EFI. It starts and runs perfect, until I shut it down when hot. Then it will not start and run in idle. The hotter it gets the worse it is, until it will not start.

I ran it in the yard and the same thing. It starts and runs perfect, then about 2 minutes later, dies. I can restart it, but it will not idle.

The Rinda Tech shows that the throttle is at 13% when I can see that it is closed/shut.

I removed the IAC tube to see if that made a difference, it does not.

After it cooled down today, It would not start at all. Nothing.

I checked the fuel pressure, zero.

I hot-wired the fuel pump to build pressure and it runs.

I narrowed down the issue to the ground wire. If I ground the pump negative lead (blue with black stripe), It will build pressure and start. And run and run.

If I shut it down, it will not start again unless I unground my temporary ground wire. Then I have to re-ground the wire after it is started to keep the fuel pressure up.

I put this ECU on a few months ago, and have run it 2 times since. Both times it would not idle after warmed up (the engine would die unless up to speed).

Not an expert on yours but why did you replace the ecu? Sounds like your problem started after that.

Is there an alarm going off? Or the red led light on ? Its obvious the ecm is shutting down the motor. The ecm grounds the circuits to run the circuit..

Figured out the issue. I hope this helps many others.

The computer/ECM is fragile, expensive and unrepairable. a huge number of otherwise serviceable engines with many years left, are sitting in parts yards due to the expense and difficulty troubleshooting this issue.

If you have a fuel delivery issue. It may be the hp fuel pump is an aftermarket model that draws slightly more amps than the factory pump spec. This is the high pressure pump. The factory replacement is extremely expensive and many knock offs exist. They can also damage the fuel
circuit in the ECM. But no worries, a relay to turn on the hp pump with the ignition switch fixed it, perfect.

Or, if the ECM is still sending 12v to actuate the Hp pump, but it is an aftermarket and draws too many amps, use the ecm circuit to actuate the relay. Which can now deliver adequate amps to the hp pump.

Actually the ecm only controls the grounds to the circuits. The positive feed is always active.

mechanicman said:

Actually the ecm only controls the grounds to the circuits. The positive feed is always active.

Click to expand...

Both pos and neg to the hp fuel pump come out of the ecm plug/harness.

If anyone wants help with this issue, feel free to send me a message direct, happy to walk you through all I learned.