I apologize in advance for bringing up the topic of vapor lock again but I haven't found anything that sounds just like what I'm dealing with.


Symptom: Engines fire immediate when cool. Run great, no issues until we go to our local watering hole & sit. I leave the engines idle for ~5-10 minutes while we thrown the anchor & get situated. Then a few hours later we get ready to head home. Engines start up & run great through all RPM's for ~30-60 seconds & then they die. No sputter, just quit. I then cycle the key & listen to the fuel pump which sounds like it has no load on it (trying to pump air). I can purge the schrader valve at the end of the fuel rail & I get a mixture of fuel & air. This leads me to believe I have an issue with vapor lock although I am baffled as to why it would start & run so well for the first 30-60 seconds. Getting the engines to re-start after that is a bear & often takes 20+ minutes of cycling the key & bleeding the fuel rail to get all the air out. Eventually the sound of the fuel pump changes to a significantly deeper tone & from that point, the engines will fire & run without any issue.

The other odd thing is that I never once had the issue last season (first season with the boat). I ran 93 octane fuel all last summer. This year I put fuel in around mid/late April (likely contained 10% ethanol). I thought that might be the issue so I burned through that tank & recently refilled with 90 octane rev fuel (ethanol free). Issue persists.


Details: It's a '99 290 Powerquest with a pair of Mercury 7.4MPI's with the CoolFuel II system. Both engines have dedicated 3/8" fuel lines running from the tank to the fuel/water separator. Stock fuel line from fuel/water separator to input side of cool fuel II fuel pump. I've recently replumbed the return fuel line from the regulator so it now feeds back to dedicated lines into the fuel tank (no t's back to the tank).

Note, I also added recently an in-line check valve between the tank & the fuel/water separator. This didn't change anything.


The part about all of this that confuses me is the 30-60 seconds that they each run great before they starve for fuel. If the fuel in the fuel rail is vaporizing, then I would expect it to sputter immediately.


The only remedy I've heard the OffshoreOnly community talk about that I haven't tried is installing the low-pressure fuel pump between the tank & the fuel/water separator.


Any suggestions from the community would be greatly appreciated & again, my apologies if this is a duplicate post from the past.

Thanks!

You added an additional check valve? Should have one on each tank line feeding the rubber fuel line (barbed fitting). Called an antisiphon valve


Have you check the tank fuel vent line?
When motors stop, open gas cap and listen for air being sucked in. If that happens its the vent line

Yes, I added a check-valve between the tank & the fuel/water separator. I'm still too new to OffshoreOnly to post a URL but If you search "EVIL ENERGY 3/8'' 10mm One Way Fuel Line Check Valve Non Return Diesel Aluminium Black Pack of 2" on Amazon, you'll see what I purchased.

I never thought about it already having these anti-siphon valves but it looks possible. I'll pop one off tonight & see if it's just an adaptor or the anti-siphon valve. If it already has the anti-siphon valve the the extra valve I added certainly isn't doing anything but causing a restriction & I'll look at removing that. I have a photo from the top of my tank which looks like it may have either an adaptor or this valve. Unfortunately, OffshoreOnly won't let me post that photo yet either. I can try to PM it if anyone is interested.

Can you explain a little more about the fuel vent line theory? I will certainly check this but my initial reaction is, if the vent line is plugged, wouldn't I eventually get a negative pressure in the tank that would case issues while running? Not necessarily hours after running.

You have to post 10 times before you can upload pics

The fuel pump is sucking fuel from the tank. If the vent line is clogged then air is unable to enter the tank and a vacuum starts being drawn inside the tank. If it runs long enough it will completely stop the flow of fuel. The vent line is a 3/8 to 1/2 fuel line that is next to the fuel fill most of the time

Not saying it's the case in your situation, but I thought I had vapor lock on my 525efi but it turned out to be water reversion. Makes me wonder how often people chase this problem thinking it's fuel related when it's water from reversion sitting in the headers. Mine was sloshing water into the cylinders with open valves when the boat rocked in waves.

Intrusion.

Baby J & SB - Can you elaborate on this some more? I'm not seeing how the symptoms I'm experiencing relate to reversion or intrusion.
I apologize for my ignorance here & thanks for your support.

i’m sorry. I was just trying to clarify terms. Reversion is from exhaust tuning snd cam event issues.

I'm not saying it's reversion but I had all kinds of weird symptoms with mine like you are talking about. My engine would seem to run fine but not restart. Then it would start idle for a while and then stall and not restart. I thought it was vapor lock until I got enough water in the oil to make it obvious what the issue was. I switched to dry exhaust all the way to right before the turn downs and problem solved.

This does not sound like a traditional vapor lock issue. Running a return line directly back to the tank should have eliminated any vapor lock as well.
Have you tried running with various levels of fuel in the tank?? Tried running it with the gas cap off?? or take the gas cap off when it happens to see if the is a vacuum??

It baffles me that it happens at 30-60 seconds of running at rpm and that it happens to both engines near simultaneously.