You just might be correct. The coil might be the issue, it reads 1.8 ohms primary 9800 secondary. Factory spec is 0.6 to 0.8 primary 9400-11700 secondary.

New plug wires, new cap and rotor. A bad coil can give the same symptoms as a fuel pressure issue. 👍🏻

Update: I ordered a new coil and fuel pump, I’m tired of messing around. I’m still trying to find a fuel pressure sensor online. I’ve tested it it to check for a short to ground and it checks out ok, bur I don’t have the resistance values to check the ohms on each terminal. I’m hoping it’s ok so I will put it back in. I’m going to replace the coil first before the fuel pump and do a lake test. If that doesn’t work I will then replace the fuel pump at the lake and give it a test. Will post the results. Fingers crossed.

You are still messing around.

You need to diagnose the problem. The best way to understand what is going on in an engine is to look at the spark plugs.

If replacing the coil and fuel pump don't solve the problem, do as follows:

1. Get a set of brand new spark plugs
2. Drive the boat to where you can test it
3. Replace spark plugs
4. Drive until the problem shows up
5. Cut ignition when the engine isn't running right. If you cannot do that, slow down first.
6. Look at the spark plugs. A wet plug indicates lack of ignition. A dry plug indicates lack of fuel. See what the problem is and which cylinders are affected.
7. Put in the old plugs again and drive back to the ramp so that you can keep the new plugs for the next test

Thanks for the advice, I will do that. The issue though is the problem is always there and a few times it went away after maximum throttle for at least 1 -2 minutes. And then the rpm “JUMPED” up 300-4000 rpm. And for example it’s not getting full power out of the hole either so under load and even not under load, like in cruise at 3000 rpm it feels like it is struggling and then WOT it is not there achieving only 3900-4100 rpm. Then after a couple of minutes it literally goes up the 300-400 rpm. Maybe the coil? I’ll find out after installing the new one.

On another area, yesterday I removed the fuel pressure sensor from the back of the TBI and there was a small bit of black paint in the sensor, yes the dreaded Mercruiser black paint. That may be causing an issue also?

THANKS for the help!

Well it’s been all winter and I went to the lake yesterday for a test. This winter I replaced the coil and spark plug wires and if I didn’t mention before I replaced the fuel pressure regulator which is located on top of the electric fuel pump.
WELL, bad news as the boat wouldn’t get beyond 2800 rpm and very bad vibration from the engine. I figured we’d just go and pull up on the beach, my wife could have a few drinks in the sun with her chair and feet in the water while I put the new fuel pump in. I unscrewed the bench seat, moved the seat out of the way, got down and removed the fuel pump from under the front of the engine ( I want to punch the engineer who designed this stupid system). But in my stupidity and after searching high and low for the new pump, my dumb azz left it at home. WTF. Anyhow I changed the pump in my garage in about 20 minutes. I wonder if this will be the issue. If after the next lake test. Is a failure then I will pull the plugs and inspect them.

Well it was the fuel pump (mostly)! I changed the fuel pump on the Mercruiser 5.7 EFI last week and took it out to the lake on Sunday. I also changed the spark plugs just to be sure. Instant power, no bogging under load and crisp throttle response without any hesitation along with a super smooth idle and no shuddering. My 2001 Stingray 220 LX with 475 hours, 23 inch 3 blade prop, 3 people, ice chest and a heavy toolbox, 107 degrees at Lake Mead did 61 mph GPS. Not bad.

But I think I can get more because of the rpm. Right now I max out at 4285 RPM from a digital timing light hooked up and read by my wife which was the same as the tach on the dash. Next weekend I’ll switch back to the 21 inch prop and see. I think the rev limiter on the MEFI 3 is 5200 for this engine but the manual says it’s 4800 rpm.

Some additional mods were a TBI spacer, 10mm spark plug wires and an aftermarket coil with 45000 volts and higher secondary ohms. I also installed another blower motor which sucks air from the outside to the back of the flame arrestor using the same circuit and on and off switch as the regular blower motor. It gets pretty hot in the engine compartment with very little airflow . This I believe helps. Thanks for all your replies and assistance.

The biggest lesson I learned from this crappy design on the cool fuel mercruiser system with ONLY 1 fuel pump ( no low fuel pressure pump) on this first gen cool fuel) is checking fuel pressure in the shop and running the engine without a load on the prop will not give you a correct readout of the fuel psi. It must be checked under load.

I had a 502 built by member here Eddie Young (Young Performance) and dyno at 530HP. It used only one fuel pump and it was the same as yours. The difference being, mine was MPI

They still make the single cool fuel pump for SBC using Alpha drives, but then they change to the box 2 pump systems (fuel pumps and filter) for Bravos, what's up with that???

If I ever go back to I/O's and get one of those box pump system, I'm ripping it out and putting the single pump in after warranty expires

Yesterday at Lake Mead, temperature 110 ugh, I ran a 21” Mercury Mirage SS prop, 60 MPH GPS and 4750 RPM, same load in the boat but hotter temps. Engine ran smooth and flawless. I think I have solved the problem and that’s probably the maximum output from this TBI vortec 350. Any thoughts on improving this?

3/4 race cam
nitrous
procharger
have a friend w/ a duramax tow you to 80 and throw water at you.lol

only way to get more out of the boat,,big block,mild 45-500hp and you'll be flying.