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I would have to say you are getting air in the system, somewhere near the pump(s). It's a circular return system and is near impossible to actually "vapor lock". That pretty much went away with carbs.

Mine has never done what you are describing. I have ran the tanks dry before, but it was no problem to get it going again.

The fuel filter is the only type of "reservoir" that is in the system. Check to make sure it's not leaking. If it was, it would take a few key cycles to "recharge" the system and would also be want to displace the air that replaced the fuel. Nothing is hot enough under the hatch to cause vapor lock anyway. I would suspect a small leak around the filter area.
 

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It's what we do
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Actually Mercruiser has been fighting vapor lock problems with there EFI systems since the beginning. The metal fuel lines and fuel pumps get so hot after the engine shuts off it boils the fuel. They generally wont vapor lock while running, only after shut down and trying to re-start.
With that said, I don't think his problem is vapor lock, or he has another problem also. By morning everything would be cool enough that vapor lock would not be a problem.
 

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Can I assume these engines run an electric EFI pump? It would seem that if it were vapored fuel in the lines, simply running the fuel pump (in a return style fuel system) would purge the rails fairly quickly.

When you say you purge the fuel rail, is there a schrader valve in a test port on the rail somewhere? When you press this, you get vapor out for some period of time, then when you get fuel it will start right up?
 

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I think that the 496 ho has a cool fuel system that uses the freshwater to cool the fuel. in the morning it does not start? sounds like it is not vapor lock. Buddy of mine had similar problems and it turned out to be a tank venting issue. Could that fuel valve be sucking air?
 

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You need to hang a fuel pressure gauge on the purge valve. See what you are pushing, key on / engine off. Should be 43.

I wonder if the CTS is faulty? Maybe giving the ECM improper information? I would think it would throw a code though, and be beeping at you while underway.

Maybe not quite out of spec enough to trip Guardian mode? Just a thought.
 

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Maybe the fuel is draining back to the tank over night. Not so much a vapor lock, but a no fuel. I know when you do a fuel filter change with out a boost pump, you have to prime the system and bleed air out of the rails. Maybe it's just syphoning back to the tank?
 

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It is NOT vapor locking the morning after.

All the early model 496 engines serial number OM025000 thru OW059999 had a cool fuel system. After that, serial numbers OW060000 thru now have a fuel module system. BOTH fuel systems have a low and high pressure pump.

I will bet the low pressure (supply) pump is the culprit.

Keith, it does have a Schrader valve on the fuel rail.
 

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When it won't start, do you hear the fuel pumps turn on when you move the key switch to the first position? They should turn on for about 3 to 5 seconds and then shut off.
 

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I will bet the low pressure (supply) pump is the culprit.

Keith, it does have a Schrader valve on the fuel rail.
Thanks for the confirmation Brian. Your suggestion on the fuel pump is probably spot on, but if not, given his scenario, a simple nitrous fuel solenoid plumbed to the test port and activated before startup while the fuel pump is running would be a bandaid until the real problem is found. I'd run the outlet back to the return line or fuel fill hose. Simple, cheap and easy to use.
 
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