Sorry for the long post, but I want to provide as much detail as possible because this issue has me, and even my mechanic, scratching our heads.

I have a 1997 MerCruiser 454 MAG MPI with a VST system. I purchased the boat a little less than a year ago from someone who inherited it from his late father. He told me the boat didn't run, but when I got it home it actually started right up and seemed to run fine.

Since there were no maintenance records and the boat is nearly 30 years old, I decided to go through everything as preventative maintenance before really using it.

So far I have replaced:

• Spark plugs
• Plug wires
• Distributor cap and rotor
• Engine oil and filter
• All fuel filters
• Fuel pump
• Thermostat
• Fuel injectors
• Ignition coil
• MAP sensor
• Oil pressure sensor
• Fuel pump relay

I also performed a complete Bravo outdrive service:

• Bellows
• Gimbal bearing
• Fluids
• General inspection/service

Last season I finally got the boat in the water and it ran great for about 2 hours with no issues.

During the offseason I completely gutted and restored the interior:

• New carpet
• New headliner
• New stereo
• New gauges
• Interior refurbishment

No mechanical work was done to the engine during that time.

Fast forward to this season.

The first test run after the interior restoration went great. The boat ran perfectly for about an hour.

The following weekend, however, about 10 minutes into a full lake day, the raw water impeller hose blew off and flooded the bilge. The engine overheated enough to melt the exhaust couplers, but fortunately I was close to the marina and got back quickly.

I took it to my mechanic immediately.

Thankfully:

• Compression checked out
• Engine appeared undamaged
• Exhaust couplers were replaced
• Oil was changed again as a precaution
• Boat ran on muffs for about an hour with no issues

The next weekend I took it back out.

Again, the boat ran flawlessly.

At WOT it would run around 70 MPH and 4900-5000 RPM right before the rev limiter.

Then suddenly I felt it start losing power and it completely died.

It would crank but would not restart.

I opened the hatch, left the blower running, and let it sit for roughly 45 minutes.

After cooling off, it fired right back up and I drove it back to the ramp.

At that point my mechanic scanned it and found a MAP sensor code. He also suspected the ignition control module could be acting up when hot.

So I replaced:

• MAP sensor
• Ignition Control Module (ICM)

Next trip out...

Same issue.

This time I had a fuel pressure gauge connected to the Schrader valve.

Fuel pressure while running:

39-40 PSI

When it died:

Approximately 35 PSI

While cranking:

Pressure gradually dropped

What was different this time was that it didn't die at high speed.

I had already been running hard and it died when I slowed down entering a no-wake zone.

At that point I started wondering about vapor lock.

I raised the hatch.

Blower stayed on.

I even put ice directly on top of the VST.

After about 45 minutes it restarted immediately and ran perfectly again.

I made a high-speed run across the lake without issue.

Then I slowed down entering the marina.

It died again.

This time I had to walk it back to the trailer.

Still chasing the problem, I replaced:

• Ignition coil
• Oil pressure sensor
• Fuel pump relay

Then came this past weekend.

As soon as I left the no-wake zone and tried getting on plane, it briefly acted like it wanted to die.

I backed off the throttle.

Tried again.

It got on plane and then ran absolutely perfect.

I then ran the boat HARD.

• 6 adults onboard
• About 30 minutes nearly WOT
• Oil pressure perfect
• Temperature perfect
• No hesitation
• No loss of power

We beached for most of the day.

Later, I decided to really test it.

I spent another 1.5 to 2 hours running the boat hard.

Again:

• Perfect oil pressure
• Perfect temperature
• No power loss
• No misfires
• No hesitation

It ran flawlessly.

Then the kids wanted to jump in the water.

I slowed to idle.

The engine died.

And that was the end of the day.

This time it would NOT restart.

We floated for nearly 3 hours.

Engine hatch open.

Everything cooled down.

Still would not restart.

Eventually I had to get towed back to the marina.

A few observations:

• When the engine dies, I still appear to have spark.
• During the last failure I do not believe I had fuel pressure.
• I did not have the fuel pressure gauge connected on that trip.
• When turning the key, I could not hear the normal fuel system priming.
• Pressing the Schrader valve produced little to no fuel.

This is what makes me wonder if the problem is fuel-related rather than ignition-related.

I've researched:

• Vapor lock
• VST issues
• Cool Fuel retrofit kits
• Fuel return line modifications

But something about this doesn't completely fit vapor lock.

The engine can run at WOT for HOURS without issue.

The failure seems to happen after slowing down, idling, or coming off plane.

At this point I'm wondering:

• Bad ECU?
• Failing VST pump?
• VST needs rebuilding?
• Wiring issue?
• Ground issue?
• Heat-related ECU failure?
• Fuel pump losing power when hot?
• Something related to the oil pressure safety circuit?

Has anyone experienced something similar on a 454 MAG MPI with a VST system?

Even my mechanic, who has owned his own shop for over 30 years, is running out of ideas.

Any suggestions, tests, or areas I should focus on would be greatly appreciated.

Thanks for reading through all of this. I know it's a novel, but I wanted to provide enough detail to avoid the typical "check fuel pressure" responses and hopefully narrow this thing down once and for all.

I do not see checking for spark when it won’t start. A spark tester is under $20.

There is spark when it won't start. At least from the Ignition coil to the cap.

So, engine currently still will not start ? This is a good opportunity to see what’s up. Have you re-checked fuel psi and spark since now not starting ?

I have not checked anything yet, I dropped the boat off at the shop once I got off the lake. I will ask the mechanic to check before trying to start the motor.

Kill switch???? The clip gets old and stretched out. Most of the time it is fine but you can hit a wake or slow down and it cuts out. Had it happen and because of the random nature it drove me crazy.

Padraig

I had vapor lock issues w 454 Mag MPI but Never when running. Always after running then sitting for a period of time letting it heat up.

Next time it does it crack the fuel line by the vst to let pressure out and see if it starts up and runs

I had a 502mpi that did the same I ended up running a return line to the fuel tank and the issue went away

With the fuel flowing and circulating from the large heat sink source of the large volume of fuel, that would make sense that it would be difficult to get a vapor lock going. I suspect the vapor lock is in the draw pump? Unless it happens right at the 2nd pump's impellor.

Running WOT for 30 min...wonder what your oil temp is like.

If you're cranking and not firing...and have spark...you should be able to smell raw fuel a bit out the exh ports...if it's getting fueled but not firing.

Maybe you just have a bad fuel pump?