To both:
OK... I will admit, there are a few ways for the plates to be open, and the engine not reving out of control. It's more common with a DFI engine. But, with our early 240's... there's more wrong than just a suspected sensor.
On these merc engines... the ECU will ignore all sensors, and will run from an internal base map. So it will rev. Basically, the TPS, and temp sender will "Trim" the fuel. SO... if the ECU is getting bad info from them, it will add up to 40% more fuel (to be safe) at idle. This is when we get smoke, hard starting, and fouling plugs when they go bad. But, if you open the throttle... the MAP will see the vac drop, and let the RPM's come up.
So, let's look at the reasons an early 240 could stay at a low RPM, with the plates open.
1) Lack of fuel. You could have lost the signal to a few of the injectors. There is a single injector per cyl.
2) Lost spark: If something is cutting spark randomly to the cyl's... that could keep you from reving up.
Reflecting on those.... If you have a couple fouled plugs, or coils that went bad.... the engine will still run remarkably smooth. The issue will present itself when you open the throttle. Basically, the peak RPM will be low. I've seen these engines run well on 3 cyls. The only way to get a rough, low, limping RPM, is for the misses to be random.
3) Cut wires or damage to the ignition parts. It could happen, since they are close to the flywheel. AND, if they are chaffed, that could cause the random ignition, as they ground out.
4) Mechanical: This isn't the one we want to talk about. BUT... have we checked the compression?? Did the engine loose a piston or two?? Even if it did... it may run better than you would expect. Mine ran OK with a connecting rod hanging out the side of the engine, and piston that was blown to pieces.
When I try to help... all I can do, is diagnose with the available info. If it's an unusual situation, we have to fall back on standard procedures to dial things in. SO... when I give my input... I'm giving the most likely situation, and I avoid the least likely until I get more info. AND, in this situation... it's is VERY unlikely that a situation will arise that will allow the Merc 240 EFI with the Cummins ECU to "Limp" at a low RPM with the throttle plates physically open.
That's my story, and I'm sticking to it. (but, I'm not always right either.)
Incidentally, I was going to suggest a compression test is highly in order and add that since these motors are so expensive he should not have run it back to the marina if it wasn't running perfectly. This is a great way to destroy a 2-stroke and most don't make it back to the marina and don't leave the marina without disappointing news and/or a new power head.
