380 MAG Mpi fuel issues

[370Dancer]

Short story: run the engines for 30 minutes or more, cruise or idle, doesn't matter. Shut it down. Anywhere from 30 minutes to 4 hours later, the engine will not start, or starts and dies right away. This is an intermittent issue, and moves from port to starboard, and sometimes both (as in get towed in). Without fail, the engines can always be started the next morning as if nothing ever happened. Been going on now for 8 months.

Clue: spin off the fuel water separator, and the fuel level is down 1/2" or more. Now we know why they won't start.

It seems to be related to engines coming to temperature, or perhaps time. Last weekend we ran 120 miles without incident, but no temporary stopping and then going, except when we broke from the raft up, idled for 5 minutes, anchored, and then 4 hours later pulled the hook, and both fired right up. AUUUGGGHHHH.

Casual mechanic conversations think it might be vapor lock, but both engines, moving randomly from port to starboard? A Rinda showed no codes thrown on either engine.

I have watched both fuel vents when filling up for blockage. Last week, the port vent was clear, and the Starboard acted clogged. I cleared the Starboard vent thinking I had solved my problem. Nope, yesterday idled across the bay to take a small kid fishing, and cut both engines to drift. Port wouldn't start. Idled 1/2 mile to another spot, and cut the starboard engine (dumb). Now neither start. 2 hours later, the Starboard finally started. Port never did. Made it back, and this morning, both engines start like nothing ever happened. Double AAUUGGHHH.

The fuel is fresh, the tanks have been treated with StarTron (double dose) and I am at my wits end. So, what's causing my fuel level in the separators to be low? Why does it eventually get better over 8+ hours?

 

[northern]

Vapour lock, hot engine room, dirty filters, bad fuel pumps, no idea. Had this happen on an outboard once and fixed the problem by moving fuel line in engine to a cooler area.

 

[fc3]

Did it start with the last refueling? Has the problem changed since refueling? Have the temperatures changed when the problem started? Air? Water? Next step would be to check the fuel pressures on the rail when the problem occurs. That will prove or disprove fuel problems.

Best regards,
Frank

 

[keokie]

Do you have the VST's (Vapor Suppression Tank), or cool fuel system? I'm betting you have the VST's and you are experiencing vapor lock. There is a diaphram in the VST's that can fail, but over all, the VST's were not that good at preventing vapor lock. Merc knew this and changed to an electric fuel pump and their "cool fuel" system in later years of production of the 502 and 454 Mags. I have run both, and was much happier with the second system.

Incidentally, if you do have the VST's and it is vapor lock, you can often (but not always) overcome it by advancing to WOT while cranking the motor. Obviously, even though the motors have rev limiters, be ready to pull off the throttle immediately upon starting.

 

[370Dancer]

Did it start with the last refueling?
No, it was there
Has the problem changed since refueling?
No
Have the temperatures changed when the problem started?
Nope. Had it in March, have it now
Air? Water?
Next step would be to check the fuel pressures on the rail when the problem occurs. That will prove or disprove fuel problems.
Pretty sure I won't have fuel pressure as the fuel in the separator is about 1" low, at least on the Starboard side. The Port engine separator will be much more difficult to get to, especially with a hot engine. I'm starting to look backward in the fuel system to some sort of siphon going on, or electric fuel pumps that don't run when warm.

Best regards,
Frank

Thanks for your insight, and interest.

 

[370Dancer]

Do you have the VST's (Vapor Suppression Tank), or cool fuel system? I'm betting you have the VST's and you are experiencing vapor lock. There is a diaphram in the VST's that can fail, but over all, the VST's were not that good at preventing vapor lock. Merc knew this and changed to an electric fuel pump and their "cool fuel" system in later years of production of the 502 and 454 Mags. I have run both, and was much happier with the second system.

Incidentally, if you do have the VST's and it is vapor lock, you can often (but not always) overcome it by advancing to WOT while cranking the motor. Obviously, even though the motors have rev limiters, be ready to pull off the throttle immediately upon starting.

These are Cool Fuel systems. Anyone know if there is an anti-siphon valve in the fuel system? The only one I saw in the service manual was for a carb engine, and located in the gas tank outlet fitting.
I'm thinking that with the electric fuel pumps, you don't need one.

 

[keokie]

When you turn the key on, do you hear the electric fuel pump cycle?

This is a tough one. I'm with Frank on testing the fuel pressure. Sure sounds like fuel, but Frank's right about being positive.

Some other thoughts... Have you tried advancing the throttle when they won't start? Might at least get you home.

Also, do you have a crossover fuel system, or anything that ties the fuel systems together? What about the shut off solenoids on the tanks?

 

[Larry]

I have the same motors and have had the same problem intermitently on both engines, I have thought it was vapor lock and have always (5 years) got the engines started by doing exactly what Keokie said, advancing the throttle foward as I crank the throttle and then catch it before it revs too high. I have had this happen only on a warm engine after it has sat for a few hours, but I have noticed that when the boat was anchored or on a mooring and the boat rocked allot it seemed to insure me having to start the engines with this WOT procedure. If I was going from my dock to say another dock were the water was calm where I shut down I don't seem to have any issues... It's on my list of things to look into but I havn't had any trouble getting the engines started using this trick and it is not all the time just under certain circmstances.

 

[fc3]

Pretty sure that the 1998 fuel system plumbing is the same as the 1999 system. Fuel from the tank is routed though a solenoid valve to the fuel filter/water separator. From the separator to the fuel pump and cooler, then though a regulator to the fuel rail. There is no return line from the fuel rail. The return from the regulator connects back to the fuel filter / water separator and begins another circuit. The GM fuel pressure regulator on the fuel rail is in-place, but capped.

The only thing that will keep fuel from getting to the separators is the solenoid valve. Is it set to the manual open position and allowing fuel to drain back? There is no anti-syphon valve. Sea Ray eliminated them since they are prone to vapor locking the fuel line. The solenoid valves take the place of anti-syphons.

I believe that MEFI shuts down fuel delivery during engine start at 1/2 throttle. But, it returns the fuel system to operation at WOT. So you can play with that during the start cycle. If the engine fires with the throttle at 1/2 or a bit more, then fuel is vaporizing and being "pumped" into the intake manifold past the injectors. Don't think this is likely. Or if it fires at WOT then the fuel system needs to be purged of vapor in the lines and fuel rail. That could be the case since there is no return line on the fuel rail of these engines to purge vapor back to the fuel cooler.

Interesting to see which helps.

Best regards,
Frank

 

[370Dancer]

(light bulb over head)
this may be an issue: "The only thing that will keep fuel from getting to the separators is the solenoid valve. Is it set to the manual open position and allowing fuel to drain back? There is no anti-syphon valve. Sea Ray eliminated them since they are prone to vapor locking the fuel line. The solenoid valves take the place of anti-syphons."

I have messed with these in the past, and don't remember what their original position was. However, open and closed are not the markings. It is OFF and ON, with ON being fully rotated clockwise, and OFF full counterclockwise (I assume). It is a full 360 degree rotation, not 90 degrees or 180. The solenoid is made by Parker. I'm going to do some research on it to see what I can find about "normal" operation settings.
I guessed that On was the correct setting, but since it is a cut off valve, perhaps that's backwards.

 

[fc3]

Off is the correct, default setting. When an electric current is applied, the valve will open. You do not want to leave the valve in the on position. That's only for emergencies if the electrics fail.

Best regards,
Frank

 

[370Dancer]

Makes sense.
Time for a boat ride to see if that cures it.
Thanks, you were faster than 2 Marine Max service managers, and one of them is my nephew!

 

[RubberDuckeeToo2]

Where are these solenoid valves? I'm having a similar problem. But my boat's fuel tank lines each run to a filter, then two manual switches for port or starboard tanks or "off" leading to a fuel line going to either the port or starboard engine. On each engine is another filter. I can hear the fuel pump pressurize prior to starting, but I haven't located them or this solenoid switch.

 

[370Dancer]

Where are these solenoid valves? I'm having a similar problem. But my boat's fuel tank lines each run to a filter, then two manual switches for port or starboard tanks or "off" leading to a fuel line going to either the port or starboard engine. On each engine is another filter. I can hear the fuel pump pressurize prior to starting, but I haven't located them or this solenoid switch.

You may be mistaking the fuel cutoff solenoid as the first filter you mention. They are silver metal with an inlet port, outlet port, electrical connectors on the top, and a knurled knob on the front. The inlet hose comes from the fuel tank. The outlet hose goes to the fuel cooler/fuel pump assembly.

 

[370Dancer]

OK, more testing since turning the fuel cutoff solenoids to the OFF position.
Idled out to an anchorage yesterday. Blowers on, genny running full load, 1100 rpm for about 40 minutes. Dropped anchor, shut off the engines, left the genset and the blowers running. Granted, it was 90+ degrees out. 6 hours later, no start on either engine. Had the little boat with us, so we bridled up (getting used to this now), and settled in for a 3.2 knot tow back home. 3 hours later, the port engine started. Starboard would not start. It IS the engine right next to the genset, which continued to run without issue.

The next morning, both started like nothing ever happened, except they took a few more seconds to fire than normal. If I were alone, I would have shut everything down, opened the hatch, and waited for an hour. It was just too hot, with too many guests to go through that.
This is really starting to smell like vapour lock, yes to both engines at the same time.
So, smart guys, where would you go from here? If they are vapor locked, will they throw codes? I have not had a scanner on the engines when they are acting up.

 

[keokie]

I know this isn't a cure, but did you try to start with the throttles wide open?

 

[fc3]

Test the fuel pressure at the rail during the no start situation. You'll know immediately if it is a fuel problem. One such gauge is here.

If it's vapor lock, you'll see it when you open the bleed-off. Since gasoline vapor may be present during this testing, you must be careful to ventilate and avoid ignition sources.

Best regards,
Frank

 

[370Dancer]

Test the fuel pressure at the rail during the no start situation. You'll know immediately if it is a fuel problem. One such gauge is here.

If it's vapor lock, you'll see it when you open the bleed-off. Since gasoline vapor may be present during this testing, you must be careful to ventilate and avoid ignition sources.

Best regards,
Frank

I have this pressure tester. I haven't done that yet for the very reason you state, and I've had a boatfull of people each time. But, I don't think I need that test, as I have spun off a fuel water separator, and the fuel level is down about an inch in the cannister. That tells me that I'm definiately starving the engines. What I cannot figure out is WHY the system is not pressurized, and how the fuel got siphoned back to the tank, or siphoned off (fuel pump failing when hot?). And, what condition is causing BOTH engines to act up? In cooler weather, it was one, but it was also random as to which one would not start. I have never had this issue until this past winter/spring. I wonder if I'm getting into an E-10 issue as alcohol has a lower boiling point than gas.

Remember, I had run for 6 straight hours a couple of weeks ago without incident, although not at cruising speed for more than 10 minutes at a time (damn Manatee zones). Running home, I was outside for about an hour, and 45 minutes into a 3400 RPM run, the port engine started to miss every 5 seconds or so. I backed down, and they ran fine the rest of the way home.

air leak in the fuel system(s)? Would I not have a fuel odor or a visible leak? Sniff test says no. Logic says not both engines.

To me it's a real head scratcher, until we find the one dumb thing wrong. The only common elements I can think of are the fuel source, and ambient heat.

 

[370Dancer]

I know this isn't a cure, but did you try to start with the throttles wide open?

The ECU won't let you do that, but half throttle will work. As it's trying to start, I have pumped the throttle, and can get a bit of backfire, and popping. Not enough flow to get em going, and I do not want to blow a head gasket or worse with the backfiring.

See my other post. I know they are starving for fuel. I'm trying to figure out why.

 

[douglee25]

Is your gas tank vent clear? Maybe you're producing a vacuum in the tank as you draw gas out of them. As the engine compartment cools, this worsens the issue and causes the fuel to slowly flow back to the tank?

Just a thought.

Doug