Merc 502 stalling/dying on acceleration after idling for awhile

[thunder550]

While we were at Havasu, I had an issue a couple times where after idling for awhile, I would go to take off, just about get on plane and the engine would cut out like I pulled the throttle back to neutral. Then it would stumble around a bit and finally die. If I let it sit for 2-3 minutes, it would fire right back up like nothing happened and run like a top the rest of the time, until I let it idle for a long time again, then rinse and repeat.

I had the same problem when I first bought the boat. On a whim, I changed the water separating fuel filter and it went away. When I changed it, I dumped the filter contents into one of my wife's clear Pyrex bowls (shh, don't tell on me) and let it sit overnight. Next day I did not see any debris or water/fuel separation at all. However, 3 months later and the problem is back again. Changed the filter while at Havasu and seems like the problem stopped once again, but for how long? I should not have to change the filter every 3 months.

I put a fuel pressure gauge on the rail (after changing the filter) and got 34 psi KOEO, 37 psi idling with the FPR vac line removed, and about 31 idling with it hooked up, which is all in spec according to the service manual. My setup has the electric pump in the VST along with the mechanical pump/raw water pump combo unit.

Any of you guys experience these symptoms before? I think it has to be a fuel delivery problem, I can't really see air or spark being intermittent issues.

I got a pic of the anti-siphon valve thinking they were cheap so I'd start by replacing it, but I can't find any that look like they match. All the ones I see have hose barb rather than a threaded fitting on the outlet side.

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[Big Block Power]

So hard to say. Why does it stop after changing the fuel separator filter? Is it dirty in the tank. You have to remove your fuel sender anyways. Also I know I had issues with a throttle switch at the throttle body. My buddy thinks mine is bad as mine fits coming to a idle. They are designed to stall the engine when you shift it into gear. Maybe it's bad and causing to to stall all together. Just a guess.

[Bfun220]

So hard to say. Why does it stop after changing the fuel separator filter? Is it dirty in the tank. You have to remove your fuel sender anyways. Also I know I had issues with a throttle switch at the throttle body. My buddy thinks mine is bad as mine fits coming to a idle. They are designed to stall the engine when you shift it into gear. Maybe it's bad and causing to to stall all together. Just a guess.

I thought if the throttle switch killed the motor it was good. I thought they failed the other way. As in no stalling ? Should be easy to check out the switch though.

[Big Block Power]

So hard to say. Why does it stop after changing the fuel separator filter? Is it dirty in the tank. You have to remove your fuel sender anyways. Also I know I had issues with a throttle switch at the throttle body. My buddy thinks mine is bad as mine fits coming to a idle. They are designed to stall the engine when you shift it into gear. Maybe it's bad and causing to to stall all together. Just a guess.

I thought if the throttle switch killed the motor it was good. I thought they failed the other way. As in no stalling ? Should be easy to check out the switch though.

I have no idea just throwing it out there like maybe it wasn't letting the engine restart.

[cmattj]

I'm guessing water in fuel.. it can create a surge starve type feeling and the mystery off idle lean bog when warm.

The fuel filter being the magic effect tells me .. full of water..which you have that symptom...but I would find a product called "Heat"

It a glycol solution that bonds with water and not only helps dissipate it into small particles but turns water into a combustible liquid.

Not sure how much you would need per gallon but as aposed to pulling and cleaning your tank.. it couldn't hurt!

Heat.. fuel filter.. done!!

[thunder550]

This weekend I'm going to pull the fuel sender to check level manually. Will try to get a sample of fuel from the bottom of the tank and see what it looks like. Thanks for the tip on the Heet cmatt, will check it out.

I hate tracking down intermittent issues, especially now that I've changed the filter and it seems to have stopped. Might have to wait for it to start acting up again before I can track it down. Since fuel pressure at the rail was good, I need to check pressure after the mechanical pump and see if it's in spec. I've read about bad anti-siphon valves, bad mechanical fuel pump arms, worn raw water pump shaft, vapor locked VST, etc.

One thing I forgot to mention, when I pulled the old fuel filter off it was only about 2/3 full. Is that normal, or do they fill all the way like oil filters do?

[cmattj]

One thing I forgot to mention, when I pulled the old fuel filter off it was only about 2/3 full. Is that normal, or do they fill all the way like oil filters do?

In my 22 Donzi with 502 it was electric lift pump so I never noticed that..? Key on I could here it prime up..

I had the original fuel tank and yes .. two fuel filters until I threw in 3 or 4 bottles of Heet.

I'll admit I was worried about how it would react to the main fuel pump but it took care of what I felt was a water issue.
I would imagine could be some draw down or syphon back when let set .. you have good fuel pressure so no worries there!

Be nice to get it to fail with a pressure gauge on it when it happens so it would tell you if was dropping during the occurrence.

Fuel filters are cheap right?? Haha.. it doesn't sound to broken.

[Big Block Power]

If you put heat in it get the one with isopropal in it. Also remember its alcohol so it's a cleaning agent don't over do it.

[thunder550]

I pulled the oil fill plug from my mechanical fuel pump today and got.about two tablespoons of thin fuel/oil mix come out. That means that fuel is leaking from the diaphragm in the pump. I had put fresh gear lube in there when I got the boat, so that's quite a bit that has leaked in since then. Going to replace the pump and just wait and see if the problem comes back.

I'm going to run a couple bottles of Iso-Heet through too just in case.

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[AsLan7]

I pulled the oil fill plug from my mechanical fuel pump today and got.about two tablespoons of thin fuel/oil mix come out. That means that fuel is leaking from the diaphragm in the pump. I had put fresh gear lube in there when I got the boat, so that's quite a bit that has leaked in since then. Going to replace the pump and just wait and see if the problem comes back.

I'm going to run a couple bottles of Iso-Heet through too just in case.

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Nice find, t550. I had the same issue once with a classic sports car. Rough running at times until I replaced the rubber diaphragm. Never had an issue since.

[thunder550]

I pulled the VST apart today since several threads I've read have referenced a dirty pump screen being part of the problem. Mine looked ok. It's gotta be the low pressure pump. Went to Oreilly a little bit ago to get some fittings to allow me to test pressure off the low pressure pump, but get home and none of them will seal correctly. Ugh. Gotta figure out plan B now. I found the Merc retrofit kit that adds a low pressure electric pump. Seems like it was designed for the cool fuel systems, but I'm getting mixed information on whether or not it will work for my VST system. My preference would be to ditch the mechanical lift pump and switch over to the electric lift pump, but I don't want to spend $400 on the kit if it's not going to work.

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[Big Block Power]

I have never seen that system at all. I'm not much help. Seeing that we like sending other people's money just do it.lol

[AsLan7]

Wish I could provide more help, T550. However, that conversion is out of my realm of expertise.

I’ll let the other experts who have done this one before chime in.

Cool pics though!

[Big Block Power]

Prob more of those engines on OSO or the Hull Truth forums. I know formula used that engine.

[Ytmsn]

Thunder I wonder what occupied your time before you bought a boat? Jeesh..