Since the problem does not go away with less load on the engine with a smaller prop, is is most likely spark related and not fuel related.

Yes the symptoms are the same, regardless of prop size until wot for at least 2 mins, on plane, trimmed out correctly, then it jumps up 300 rpm like something just clicked or connected or dislodged. The previous post said it’s the fuel pressure regulator under load. You are saying spark related, can you elaborate a little more on the spark related theory? I’ve ordered a fuel pressure regulator and an anti siphon valve. Thanks for your advice 👍🏻

Lake Test Finally!

Ok, this is what I did:

New fuel pressure regulator
New anti-siphon valve
New spark plug wires

Boat ran the same as before, like a turd. So it’s not the fuel pressure regulator. The next step has to be changing out the high pressure fuel pump, even though it shop tested at 38 psi but not under load. Damn, this sucks.
I also mapped the timing in the driveway hooked up to the hose. It advances like it should, but again no load.

As far as spark, any ideas? Coil perhaps?

You still haven’t checked psi at full rom and load yet ? I don’t get it - why the reluctance to do this ? Just do it and scratch it off the diagnostic list. It’s the #1 or #2 thing to do when diagnosing. Replacing parts is not telling you facts.

You are correct, I should have hooked up the fuel pressure gauge today but I didn’t. First of all, whoever the engineers were who designed this Mercruiser fuel cool system should be fired. How inconvenient and inaccessible this is on the bottom of the engine in the frkn bilge. Volvo Penta put it right out in front with easy access both fuel pumps and a Schrader valve to hook up a fuel pressure gauge! And Mercruiser did not . How stupid. So it’s not easy changing the fuel pump and the fuel pressure regulator. The entire back seat has to come out in order to access it.

Anyhow, it ran worse today than it did before the new fuel pressure regulator, new plug wires and new anti siphon valve. I wish the shop put the prop dyno on when they determined fuel pressure was 38 psi throughout the rpm range but no load on it.

Is this your set up - has a fuel psi sensor (#16) in the fitting (#3) to the right of the fuel line inlet (#1)?

YES, This is my set up.I do have the #16 fuel psi sensor. You are the first person to talk about this, thank you for the insight. Could this be the culprit causing the issue and how does this work? Does it provide info to the ECM?

Ok, after reading a million blogs I decided to check my ignition coil. Just to recall: 2001 5.7 EFI (TBI) with the older oil filled type coil.Here are the factory specs ( I think) and what my coil is reading. Factory: Primary 0.6-0.8 ohms Secondary 9400 - 11700 ohms
My Coil: Primary 1.8 ohms. Secondary 9800 ohms

The added resistance on the primary will affect the output of the coil right? Maybe this is causing my lack of rpm under load because a bad coil will lead to symptoms mimicking a fuel pressure problem. Any thoughts?

Also still waiting to hear from y’all about the fuel pressure sensor # 16 on the back of the TBI, thanks for your help!

Thinking you maybe can put in a psi gauge (most are 1/8” npt) into the adapter the sender goes into.

I think it’s a 16 metric thread into the back of the TBI.

And after wasting $360 at a local shop, (they adjusted the timing and cleared a code and checked the plug wires were correct and also checked the fuel pressure which I already told them it was 38 psi) I decided to replace parts instead. Yes I know it isn’t the correct way but I’m down to 3 possible culprits: the coil, the fuel pump or the fuel pressure sensor on the back of the TBI. All 3 parts together are still cheaper than 1 hour at the boat repair shop. Yup, I’m frustrated but thanks for your help!