Merc 502 stalling/dying on acceleration after idling for awhile

[Mr Hahn]

In case you haven't seen it, this site helps you fine ethanol free stations, and it shows 2 in Scottsdale:
https://www.pure-gas.org/index.jsp?stateprov=AZ

Might be interesting and cheaper to see if the ethanol, which you probably know boils much lower than gas, is letting vapor bubbles form when the boat is at idle and the cooling is reduced. I say this because you cycling the key off and on several times to cycle the fuel pump is the same procedure I have to do if I let my diesel run out of fuel to push all of the air out of the system before I can start it again, and because you say this happens when you idle after getting it hot.

[Big Block Power]

I would try efree gas. Also do you run a e gas treatment? Something to try.

[Mr Hahn]

Found this advice on another forum:

What you are describing sounds like vapor lock that is VERY COMMON on 454 & 502 mag EFI's. With out redoing your fuel system, a couple things you can try. Leave blower running to help cool the engine. After running at speed/power let the engine idle 3-5 mins allowing it to cool using sea/raw water. Wile idling open the engine hatch to allow heat to escape.

Ultimate fix is you want to route the return line form the fuel rail back to the tank rather the VST.

VST= Vapor separator tank.

To explain whats happening its something like this. The gas in the VST starts to boil and make "vapor" when the engine is HOT, not running or cooling. This vapor pressure gets to high and the mechanical fuel pump can not pump gas into the black tank. Then to make matters worse what ever "hot" fuel you had in the black tank never goes back to the main tank for cooling. It just goes round and round form VST to fuel rail. Once its all gone its gone untill the pressure is low enought in the VST for the mechanical fuel pump to pump gas into the VST.

And here’s Mercury Service Bulletin 99-07 which warns about these engines vapor locking due to poor cooling in engine compartments that were over insulated against noise:
http://www.boatfix.com/merc/bullet/99/99_07.PDF

[thunder550]

Thanks for the links Mr Hahn, I was using something similar but it didn't have the Scottsdale locations listed, probably because they are higher octane race fuel. I'm not sure how much 95-114 octane fuel is but I'm pretty confident it's too rich for my blood

I don't run any type of treatment. I read a lot of different reviews and analysis about it and came to the personal conclusion that no treatment will "solve" the ethanol issue. I'm open to being convinced otherwise. I did find some info on Arizona fuel formulation and if I'm reading correctly, it specifies a max RVP value of 9 with minimum 10% ethanol in the winter and max RVP 7 with no ethanol minimum requirement in the summer. Both are in the "less likely to vapor lock" section of the boatfix link.
https://www.afpm.org/content.aspx?id=1336

I need to get a pressure gauge on the pump and check it when the problem occurs. I've taken fuel pressure with the engine running normally and it's within spec. Also sounds like the easiest potential fix will be to have the fuel return to the tank instead of the VST.

Is it as simple as moving the return line from the fuel rail from the VST tank to the fuel tank and capping the unused VST port?

[thunder550]

Here's a decent link I found that discusses vapor lock solutions:
http://www.speedwake.com/forums/9-techn ... -lock.html

[Mr Hahn]

Yeah, I just looked at the comments on the greater Phoenix area locations, they don't actually have E-free, but I guess somebody added them to the list because they sell race fuel. I'm lucky enough to have a Maverick and a Sinclair within an 8 mile circle from my house that sell 88 E-free, and the price is usually the same as the diesel fuel for my truck.

I've also checked all over the place for some of the classic cars I used to play with and your right, there's no such thing as an additive fix for the ethanol content in gasoline. Most additives, and especially cleaners, also have a bit of alcohol in them which just makes things worse instead of better. Another problem is that ethanol is usually added at the pump, it's not closely monitored, and there's basically some wiggle room with how much might actually be in your fuel.

The VST fix sounds simple enough that I know I'd probably try it next. There's also some fixes that are supposed to cool the fuel system somehow, but I'm guessing those cost more money and are going to be more complicated. Either way, I know I'll be watching out for this when it gets hot on Lake Powell and Flaming Gorge this summer on my boat.

[thunder550]

Return line sounds like the winner for now. It should be pretty cheap and also easily reversible. Here's what I'm thinking. To get the fuel back into the fuel tank, I will try to find a T fitting to put inline with the fill hose rather than drilling holes in the tank.

return line.jpg

[thunder550]

Putting this link here for my future usage. Need to figure out what size the current fill hose is.
https://www.fillernecksupply.com/auxili ... ation-kit/

[Big Block Power]

Good info here guys. Add the return line big thunder

[cmattj]

Let me add just a bit of a diag idea to what we believe to be heated fuel disapation or as known ol vapor lock.

I looked at the 1995-97 252 hatch set up. It seems limited on vents and air intake ducts as compared to newer models.mmmmmmmmm

I would say let make a cruise with either the hatch slightly ajar ( if it's hydraulic ram. ) and or leave the blower on for the engine compartment. Just to see if we are leaning towards what we believe. Once at idle after hard runs is where all heat soaking takes its toll.

Then we'll pick out some scoops or tough Donzi naca ducts for that newly upolhstered hatch

[Big Block Power]

Let me add just a bit of a diag idea to what we believe to be heated fuel disapation or as known ol vapor lock.

I looked at the 1995-97 252 hatch set up. It seems limited on vents and air intake ducts as compared to newer models.mmmmmmmmm

I would say let make a cruise with either the hatch slightly ajar ( if it's hydraulic ram. ) and or leave the blower on for the engine compartment. Just to see if we are leaning towards what we believe. Once at idle after hard runs is where all heat soaking takes its toll.

Then we'll pick out some scoops or tough Donzi naca ducts for that newly upolhstered hatch

I see A Bitchin Ride coming here

[thunder550]

Let me add just a bit of a diag idea to what we believe to be heated fuel disapation or as known ol vapor lock.

I looked at the 1995-97 252 hatch set up. It seems limited on vents and air intake ducts as compared to newer models.mmmmmmmmm

I would say let make a cruise with either the hatch slightly ajar ( if it's hydraulic ram. ) and or leave the blower on for the engine compartment. Just to see if we are leaning towards what we believe. Once at idle after hard runs is where all heat soaking takes its toll.

Then we'll pick out some scoops or tough Donzi naca ducts for that newly upolhstered hatch

Good ideas here, next time we go out I'll open up the hatch while idling and see what happens. There's a couple of vents near the front seats in the cockpit and then the vents on the transom and I think that's it, so not much. I'm sure there's no airflow at all through the engine compartment when idling, and probably minimal when cruising.

[NautiGirl]

Ok so we all know I'm not the mechanical one here but I've had a few vapor lock experiences with the 190...all would be on super hot humid weekends and if I shut it down right after a long or WOT run and sit for a bit it would start back up then die as she was trying to get up on plane. Different than what you are describing but still thought I would weigh in....

Letting her sit would sometimes work and sometimes not... Like we're talking it still wouldn't start the next day... But then when my mechanic would look at it she fired right back up.

I just learned to let her idle for a bit before shutting down on those hot as hell days.

Good luck tracking this down... Hope it's an easy fix.

[Big Block Power]

Turn the blower on as well.

[Driller]

When I did my engine compartment temperature test the blower made no difference. On / Off temp inside the engine compartment remained the same.

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