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vapor lock 502 mag

H

hlrich24

New member
does anybody have any information on the 502 mag vapor locking??? I put in a new fuel pump in mine last year and everything was fine.... but yesterday it stranded me for about 20 min and then it came to life after I babied it with the throttle :brickwall:. I have heard of the anti-vapor lock kits that Merc. has but I wanted to dig a little deeper into it.... the boat is a 2000 ZT280 502 MAG MPI with the cool fuel system. TIA!!
 
That is common with a return-less fuel system like Merc uses on that motor. #1 The easiest way would probably be the Merc kit that adds a mechanical lift fuel pump to your sea pump. #2 The not-so-easy way (but far superior) to fix the issue is to remove the cool fuel system, install a quality Aeromotive (or similar) pump, replumb the line to the fuel rail, add a fuel return line off the end of the fuel rail, and install your regulator on that line. Option #2 will possibly be more expensive depending on what the Merc gods charge for the retrofit kit but will be the proper way and allow for easily fueling bigger horsepower if you choose to boost/build things up.
 
Gimme Fuel said:
That is common with a return-less fuel system like Merc uses on that motor. #1 The easiest way would probably be the Merc kit that adds a mechanical lift fuel pump to your sea pump. #2 The not-so-easy way (but far superior) to fix the issue is to remove the cool fuel system, install a quality Aeromotive (or similar) pump, replumb the line to the fuel rail, add a fuel return line off the end of the fuel rail, and install your regulator on that line. Option #2 will possibly be more expensive depending on what the Merc gods charge for the retrofit kit but will be the proper way and allow for easily fueling bigger horsepower if you choose to boost/build things up.
Click to expand...

couldn't he use fuel rails off of a donor bbc to accommodate his fuel-return goals without being raped for "boat conversion kits"?

also, if the rails currently on his motor are billet aluminum. . .theres a good possibility that the shape would allow him to drill and tap an end for a regulator/return which would save him a lot of wrenchin, parts hunting, and money.

Ive never seen one, so hopefully my suggestions are not ridiculous, but that's what comes to mind.
 
The fuel rail that motor uses are aluminum but triangular in shape. One large rail feeds all 8 injectors. There should already be a tapped hole with either a plug or Schrader valve on the front engine end of rail. The single cool fuel line feeds from the rear but that is not a tapped hole. It uses a crappy clamp and o ring to seal the line. It can be tapped though. I did it with mine. I removed my schrader valve, built a block off plate for the opening, and screwed a fuel pressure sender in the tapped hole. It can be done, it just takes some machining/fab knowhow. Some of the bigger marine engine shops will do tjis all for a fee because it is a common modifacation for this fuel system to support more horsepower. There also were three different versions of this fuel rail used in the mpi engines. His 2000 should be the 3rd version that uses the short injectors and has the enlarged port feeding #7 injector. Previous versions of that fuel rail (like mine) had a smaller oriface deeding that injector that would cause lean conditions and damage if not enlarged on built motors.
 
hmm. ive never encountered a single rail feeding 8 injectors, must be a funky lookin piece. then again, ive never seen efi without a return bleedoff, also not the best idea for a boat (what were they thinking?). I was under the impression that, because there was no return, there would be no provision. makes sense logistically though to have the rail ready to accept a return, just plugged off.

if you modify the fuel rail for the solid feed connection, might also want to consider a fuel press provision as well (scared to run my gassers without FP/wideband gauges).

you obviously have this topic on lock down, if nothing else i'm buildin my post count :cheers:
 
Yea, the rail is tucked between the lower cross ram manifold and covered by the upper large plenum. When you shut motor off after a hard run that rail just bakes in the latent heat of the motor causing vapor lock. The scrader valve is supposed to vent the vapor up into the upper plenum but tends not to. When mine was stock i always let it idle for a few min before shutting it off. The cold lakewater cooled her down quick and cured most of my vapor lock issues.

Yes, fuel press gauge and afr meter are a must for in boat tuning. I run them both.
 
could it also be the fuel? I have also heard that this can be the case also. The boat did not act up all summer until just this very last time and then when it came to life it ran excellent. Plus that day was almost 100 and we had been running it a fair amount... again thanks for all the replies guys!.. I know that when something goes wrong I can get an answer here FAST and RELIABLE!:)
 
The ethanol in todays fuel has made it even worse. It vaporlocks much easier. Yea, your issues happened from a hot day and hard run engine.
 
two issues here, 1st the merc EFI fuel system is a joke, they tried a couple of bandaid fixes that worked in some cases, like gimme fuel said, can the merc, pump, cool fuel, lines, use a aeromotive A1000 pump and EFI regulator and run a return line and that will cure the issue..and yes the ethanol fuel is making it worse as well....
 
that returnless system is horrible for a boat (perhaps anything for that matter). diving the front end (obviously not typical deceleration) by killin the throttle after hard WOT fueling has to cause problems too. moving fuel that all the sudden has nowhere to go, could cause vapor lock also.
 
being that this was a one time deal (so far), do any of you guys use a ethanol diluter in your tanks when you fill up?
 
off topic, what bodies of water do you boat by casper? I just spent 2 weeks out there (my first time in Wyoming) and let me say, what an amazing place. . .beautiful lakes too.
 
by Casper there is Alcova, Pathfinder, Glendo (where I boat) and then there are some more further out from there. And yes it is beautiful country!
 
nothing will remove the ethanol, you can try opening the hatch when you stop, keep blower on all times while running if you do not, and even for a while after shutting down...have to get the heat out, and thats just another bandaid !
 
All the ethanol additives accomplish is to stabilize the fuel for extended storage periods. Any alcohol containing fuel is extremely hydrophilic meaning it loves to absorb water from the air. This is especially problematic in boats where moisture is a constant factor (alcohol is main ingredient to Drigas....the alcohol molecules attract to the water molecules allowing them to be burned off or filtered out). That is why with today's fuels cause constant issues with boats and fuel/water separators should be changed religiously. When an alcohol combines with water, it may burn, but not effectively or efficiently.
 
Replace the cool fuel filter and make sure the system is flowing both fuel and the cooling water. Same thing happened on my 2005 SeaRay Amberjack and another friends Express Cruiser fuel systems.
 
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