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502s weird problem

Old 06-12-2017, 10:58 AM
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Default 502s weird problem

Ok so my friends boat just started with this weird issue yesterday. while the engines are above 3500 rpms the starboard side engine will drop out after about 10 min. it loses rpms and then just quits but restarts and drives normally immediately. It happens faster the higher the rpms you go over 3500 and doesnt do it at all under 3500. I thought initially it was a cap and rotor, so i replaced them both, they needed it anyway. Problem is still present so i think it might be the coil. I figure i should swap the coils from engine to engine to see if problem moves. beyond that not sure, plugs are fine and wires are good. Guess i have to take it one step at a time. I don't think its a fuel pump issue because he has mechanical fuel pumps and it runs for a good while before it presents itself. Anything else I should be looking at?

another thing the wires are not in the same position on the caps, but the firing order is identical on the spec plate attached to each engine. All the wires are labelled and the starboard wires are 1 position counter clockwise different then the port. I thought i was crazy or did something wrong when i put the new caps on but i verified all the wires each side cylinder to caps and they all are correct. I ran both engines and they both run fine and start normal. I even switched the wires on the starboard side to match the port side and it wouldn't run at all. Why would identical engines with same firing order have different wire locations on the caps, like i side its only one position off counter clock wise all the way around the cap.

example if cylinder 1 started at the 12 o'clock position on the port side then his cylinder 1 on starboard is at 11 o'clock and so on all the way around the cap, cylinder 2 at 1 o'clock port then cylinder 2 at 12 o'clock starboard

i cant figure that out at all.
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Old 06-12-2017, 11:09 AM
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the distributors are just clocked in different positions, no big deal.............502 what ? carb or efi.........sound like it may be a fuel issue

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Old 06-12-2017, 11:14 AM
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Originally Posted by F-2 Speedy View Post
the distributors are just clocked in different positions, no big deal.............502 what ? carb or efi.........sound like it may be a fuel issue
502 MAGS EFI.

I thought it could be fuel but once it starts happening even if you back off on the throttle it still drops rpms and stalls. so i thought it may just be the coil getting hot and on its way out.
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Old 06-12-2017, 11:19 AM
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have the engine scanned
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Old 06-12-2017, 11:39 AM
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Originally Posted by F-2 Speedy View Post
have the engine scanned
Does it store info of that nature, i have never had any experience with that.
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Old 06-12-2017, 11:51 AM
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The fact that your one engine quits at a certain rpm and quits sooner at higher rpm is indicative of a fuel supply issue. In other words the fuel pump(s), both mechanical and electric can only supply enough fuel up to XXXX rpm, in your case 3500.

Before BUP chimes in I'd recommend posting the serial number of the engine(s) in question so we know exactly what we're dealing with.

Has there been any work done on the motors recently?

Check fuel pressure. My guess is one of your pumps on the suspect motor is going south OR there is a restriction in the fuel supply lines/filters.

It would also be wise to have the injectors pulled, cleaned and flow tested. The filter baskets in the top of the injector can get clogged. This issue would not manifest itself with the systems you are currently experiencing but none the less you should address injector cleaning.
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Old 06-12-2017, 11:58 AM
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502mpi with VST. The VST is always suspect on these engine's.
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Old 06-12-2017, 01:34 PM
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I will have to get the serial numbers off the engines tonight or tomorrow. The boat is a 96 wellcraft 45 excalibur engines were replaced 5 years ago and this issue is new. I know that the engine in question had a fuel pump replaced 2 seasons ago, he has mechanical pumps on his engines. I guess I better do a fuel pressure test also. I not sure if he has the vst units never paid much attention to whats on it, but will check.

It just seems weird that it takes like 10 min or more for this issue to present itself, I would think if it was a fuel issue it would almost be instant. Like i said i will do some basic checks and see how it goes.
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Old 06-12-2017, 06:37 PM
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Unfortunately I am t boat now and can not read serial numbers it does have vst on it and two belts for the drive pulley system if that helps I brought a fuel pressure gauge but realized mine is made for a Schrader valve set up this does not have one I can see.
can vsts be removed completely or do they need replacing? I am still gonna do a coil swap
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Old 06-12-2017, 06:47 PM
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Schrader valve should be just behind t-stat housing on frt of fuel rail. I believe it has a brass cap on it.

Last edited by SB; 06-12-2017 at 06:50 PM.
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