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502 mg problem..

Old 12-12-2020, 08:04 PM
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Default 502 mg problem..

Self described shade tree mechanic and here is the problem. Went out today to do a dry run on a upcoming Christmas parade route that was 2.5 hours at idle. At about 2.75 hours at idle, the Stbd engine started to sound like the power steering pump was failing so I gave it a little throttle to investigate and the engine didnt respond. In fact, I goosed it and got nothing but idle thrust until it died a few seconds later. Would not restart. Limped home on the other engine. Gauge readings normal IE Oil Press, water temp.

It attempted a few restarts, and the engine fired up, hung at idle for a second or two then died. Tried this 2-3 times and now it wont even fire. I can hear the electric pump prime when I turn on the key. I opened the drain plug on the Racor water fuel separator to drain it, unscrewed the white oil filter style fuel filter and drained it to look for contaminates or water (None). So out of panic, I ordered a new Quicksilver fuel filter and fuel regulator. I read up on some old threads about vapor lock with VST system and dont know if this applies to me or not.

Anything i can try to get this thing up and running for the boat parade on Dec 19 would be appreciated. Im all ears for tips and tricks about what to try and test. Will the ol, pull the air cleaner and spray in carb cleaner trick work to confirm fuel delivery? Im thinking fuel but........?

1996 Mercury 502 Mag MPI stock but rebuilt 20 hours/10 months ago.
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Old 12-12-2020, 09:23 PM
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How old is the fuel line? Pump? Fuel filter? Ive seen where the inner lining on some fuel lines will collapse. Also I would check to see if the Pump is actually putting out. You dont want to do brain surgery to cure a cold. Start with the easy and work your way up.
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Old 12-12-2020, 10:33 PM
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The motors were rebuilt about 10 months ago with new bolt ons including new filters etc. Could they have reused fuel lines? Possible, but all the spendy bolt on components were new at time of rebuild so it wouldnt make sens to reuse cheap hoses.....but I could be wrong. I assume the fuel flows from the front of the engine to the electric pump towards the back of the engine? Ill pull the rear fuel line in the AM and have the wife crank it. Ive done some reading that this motor might have a mechanical fuel pump (lift pump) that delivers fuel to the electric pump. Correct?

Fuel is about 3 weeks old as I take her out a few times a week.
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Old 12-13-2020, 01:50 AM
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The mechanical pump would be on top of the sea pump. Just pick up a pressure gauge and put it on to check pressure. Sounds like the cape parade.

also maybe try swapping iac and tps with other motor. Could possibly those
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Old 12-13-2020, 07:46 AM
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What is the engine serial number?
Right now, don't know what configuration the motor is. I read VST, but being a 96 it would probably have cool fuel

When the canister filter was removed, was the filter full of gas or was it low?
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Old 12-13-2020, 01:36 PM
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Originally Posted by Crude Intentions View Post
The mechanical pump would be on top of the sea pump. Just pick up a pressure gauge and put it on to check pressure. Sounds like the cape parade.

also maybe try swapping iac and tps with other motor. Could possibly those
After going over some pics of the engine, I dont believe Ihave them. If a mechanical pump is connected to the raw water pump...then thats a big negative. TPS.. Thats an easy one to swap from the other engine to test, thank you. IAC another easy swap to confirm or rule out.

Originally Posted by AllDodge View Post
What is the engine serial number?
Right now, don't know what configuration the motor is. I read VST, but being a 96 it would probably have cool fuel

When the canister filter was removed, was the filter full of gas or was it low?
Canister was half full? didnt pay attention. L315584. And you are correct, after further digging, I have cool fuel. Ordered the following just to have them on site for tomorrow when I dig into this: Fuel filter, Fuel pump, fuel regulator. I figure I can clean out the screen under the regulator if needed. Ill know more when I can test pressure.

So plan of attack is try to start it when cold just to see if its a heat soak issue, cant hurt. Check fuel pressure with a gauge. Swap out the IAC and TPS since they are easy to get to and swap. If no bueno, pull the cool fuel system and install the new parts. Then cross fingers. If no go after that, Ill call a pro as I dont have the time to tinker with it. Wife is on my case to have it running for the Christmas parade as she has been looking forward to it for months (its our first at the new house on the water).

Last edited by lear60man; 12-13-2020 at 01:53 PM.
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Old 12-13-2020, 01:54 PM
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Good luck and keep us posted.
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Old 12-13-2020, 03:56 PM
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Check the fuel tank antisiphon valve. It can clog or stick. Its a cheap part but if it sticks the pump will have trouble pulling fuel. The rubber fuel line connects to it iin the tank



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Old 12-13-2020, 09:07 PM
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Originally Posted by AllDodge View Post
Check the fuel tank antisiphon valve. It can clog or stick. Its a cheap part but if it sticks the pump will have trouble pulling fuel. The rubber fuel line connects to it iin the tank


I have three valves coming off of the gas tank that feed Port eng, Stbd eng and Genset. Ill swap the fuel line from Port to Stbd. I assume that is what you are referring to? They all have quarter turn valves to shut them off. thanks for the diagram BTW....this guy needs picture books.
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Old 12-14-2020, 02:00 PM
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AllDodge, swapped the lines Port to Stbd and no fuel. That was a good guess. Right after that it started pouring rain so delay of game.

Solved! Bad fuel sender. It stuck on 1/4 gauge. Blind luck that we got home on 1 engine.

Last edited by lear60man; 12-14-2020 at 06:40 PM.
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The following 6 users liked this post by lear60man:
92nsx (12-23-2020), AllDodge (12-14-2020), CDShack (12-14-2020), Eddienel (12-14-2020), Gimme Fuel (12-15-2020), SB (12-14-2020)

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