[dafucwuzat]

Just throwing it out there again. Anyone found a good additive to prevent it yet. Getting real sick of having to leave hatch open all the time

[GaryDoug]

I don't know about these boats, but a weak fuel pump will contribute greatly to vapor lock. The gas in the lines and/or carb gets hot from heat of the stopped engine and boils, making it difficult to flow. A strong fuel pump will compress it back to liquid and push it through. Maybe check your fuel pressure to see if it's up to spec.

[dafucwuzat]

Thanks for reply. Current pump is good but seems to loose prime too. Thinking about adding another pump close to tank. Biggest problem I see is that the regulator just dumps back to w/f separator to stay in system instead of back to tank.

[GaryDoug]

I am not familiar with this setup yet (new owner), but could be the regulator is leaking internally and dropping the pressure. I understand the concern about the return path not letting cool fuel back into the system.

[Motoxxx43]

This happened to me yesterday, again. It was 100° out and I had not run the boat more than 20 min well below 3,000rpm. After about 30min of drifting she was locked up. I hit the fuel rail valve and a small amount of fuel along with intermittent vapors came out of it. After cycling the ignition a few times the pump sounded like it was ready to fire up but no dice. I hit the fuel valve again and nothing but vapor came out- no fuel what so ever. I drained the water cooling line to the fuel cooler and yes- it was hot. Cranked on it a little more and the audible difference on the fuel pump is very noticeable once it's primed with fuel and not vapors. It started and that was about the extent of my boating.

I'm not going to put up with this nonsense. I'm hard after an affordable fix. I will let you know what I come up with. This is a very well known issue as you type in "Mercruiser 7.4 mpi vapor lock" and you have plenty of pages to read from.

[Motoxxx43]

Just to put this out there I have done most of the trouble shooting off this page in the past. Like running for a few min at idle, cracking the cover, running blower.
https://www.perfprotech.com/blog/art...ser-vapor-lock

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[Motoxxx43]

I have fixed the issue and can say with confidence I am 100% certain the issue I had was coming from the crappy ethanol fuel we are being fed at the pump. The fuel that was in my boat consisted of 35 gals of January E10 fuel I threw in the boat because I had $1.00 off a gallon and Safeway fuel. I ran that fuel out of my boat last Thursday and Friday I put in 20 gal of 91 ethanol free fuel. Since the running this fresh fuel on Friday, Sunday and Monday I have not had one issue even with the outside temp being close to 100°. I have not even been running the blower while running.

After some Google searching I kept finding the ethanol boils at 173° and the 7.4 MPI runs at 175°. The vapor lock issue is being caused by ethanol boiling in the fuel rail.

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