1999 502 Mag Stumble and Back Fire After Long Run Time
06-21-2013, 09:48 AM
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1999 502 Mag Stumble and Back Fire After Long Run Time
I need some help with my problem child...
I have a 502 Mag that will stumble and backfire ONLY when ran for long periods above 2300 RPMS. It usually takes 30-45 mins of run time (At any RPMS) to get the problem to show itself. Sometimes I can pull the throttle back and punch it to correct the problem and it will run out for a few minutes before coming back.
Again, it will run flawlessly for the first 30 minutes at any RPM. After it stumbles I can only run it at 2200 RPMs.
I have good fuel pressure, the anti flow back is good, fresh/clean fuel, new plugs.
I am trying to keep from buying the Diacom software.
Any ideas?
I have a 502 Mag that will stumble and backfire ONLY when ran for long periods above 2300 RPMS. It usually takes 30-45 mins of run time (At any RPMS) to get the problem to show itself. Sometimes I can pull the throttle back and punch it to correct the problem and it will run out for a few minutes before coming back.
Again, it will run flawlessly for the first 30 minutes at any RPM. After it stumbles I can only run it at 2200 RPMs.
I have good fuel pressure, the anti flow back is good, fresh/clean fuel, new plugs.
I am trying to keep from buying the Diacom software.
Any ideas?
06-25-2013, 05:09 PM
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Where are u located? I ask because if close, you can borrow mine.
What do you consider "good fuel pressure"? How/where did you get the psi info.
Do u have cool fuel?
If so.....then the two-cooler lines Are clogged. Gas is boiling in the cool fuel module. Prove it to yourself by pouring cold water on it and see the result.....which will only cure it for a few minutes.....then return.
If this is it, I can guide u through the problem...
What do you consider "good fuel pressure"? How/where did you get the psi info.
Do u have cool fuel?
If so.....then the two-cooler lines Are clogged. Gas is boiling in the cool fuel module. Prove it to yourself by pouring cold water on it and see the result.....which will only cure it for a few minutes.....then return.
If this is it, I can guide u through the problem...
06-25-2013, 05:50 PM
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Going to take a stab at this. I had sort of the same issues with mine. My ignition control module on my distributor went bad and I could not go over 3200 rpms or half throttle without it stumbling and backfiring. Drove me crazy. It could also be your coil though. I ended up doing the cap, rotor, coil and finally the ICM trying to figure this one out. The problem with this is that even if you hook up a computer it will not tell you anything. Ask me how I know 
06-25-2013, 08:01 PM
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You do not need a tool to check for codes see here: http://www.offshoreonly.com/forums/d...%241-00-a.html
I would be checking for foaming in the fuel rail when this occurs, especially if you don't have cool fuel. I have also seen an ICM failure like The Ace, but normally you will also get tach fluctuation when this occurs, does the tach stay steady or not?
I would be checking for foaming in the fuel rail when this occurs, especially if you don't have cool fuel. I have also seen an ICM failure like The Ace, but normally you will also get tach fluctuation when this occurs, does the tach stay steady or not?
06-26-2013, 11:49 AM
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Where are u located? I ask because if close, you can borrow mine.
What do you consider "good fuel pressure"? How/where did you get the psi info.
Do u have cool fuel?
If so.....then the two-cooler lines Are clogged. Gas is boiling in the cool fuel module. Prove it to yourself by pouring cold water on it and see the result.....which will only cure it for a few minutes.....then return.
If this is it, I can guide u through the problem...
What do you consider "good fuel pressure"? How/where did you get the psi info.
Do u have cool fuel?
If so.....then the two-cooler lines Are clogged. Gas is boiling in the cool fuel module. Prove it to yourself by pouring cold water on it and see the result.....which will only cure it for a few minutes.....then return.
If this is it, I can guide u through the problem...
Has cool fuel, just replaced regulator, and the screen under the regulator. All fuel pressure was taken at the rail. 30-33 PSI at idle, 38-43 WFO throttle.
Good idea on checking the water lines to the cool fuel. The engine has a ton of corrosion in the lines that I need to clean up.
You do not need a tool to check for codes see here: http://www.offshoreonly.com/forums/d...%241-00-a.html
I would be checking for foaming in the fuel rail when this occurs, especially if you don't have cool fuel. I have also seen an ICM failure like The Ace, but normally you will also get tach fluctuation when this occurs, does the tach stay steady or not?
I would be checking for foaming in the fuel rail when this occurs, especially if you don't have cool fuel. I have also seen an ICM failure like The Ace, but normally you will also get tach fluctuation when this occurs, does the tach stay steady or not?
I pulled the codes with my 2 cent, beer inspired code reader. Flashed 12 multiple times, no codes...
I am ready to have a parts swapping orgy between the two engines until I get it to move from the starboard to port, or until I get it to go away.
Last edited by justinp20012500; 06-26-2013 at 11:52 AM.
06-26-2013, 11:56 AM
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Going to take a stab at this. I had sort of the same issues with mine. My ignition control module on my distributor went bad and I could not go over 3200 rpms or half throttle without it stumbling and backfiring. Drove me crazy. It could also be your coil though. I ended up doing the cap, rotor, coil and finally the ICM trying to figure this one out. The problem with this is that even if you hook up a computer it will not tell you anything. Ask me how I know
07-01-2013, 05:21 PM
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I am certain that I have figured out my problem. Vapor locking SOB!
After a hard run and shutdown the Starboard engine will start then die. No fuel pressure at the rail. I can release the schrader valve and cycle the key a bunch of times (like 30) and slowly get the fuel pump to build pressure. Once pressure is built it is fine.
What's the common (most reliable) fix? Add an additional pusher pump? Insulate the fuel line? Cap the fuel return line to the filter and run the return it to the tank?
After a hard run and shutdown the Starboard engine will start then die. No fuel pressure at the rail. I can release the schrader valve and cycle the key a bunch of times (like 30) and slowly get the fuel pump to build pressure. Once pressure is built it is fine.
What's the common (most reliable) fix? Add an additional pusher pump? Insulate the fuel line? Cap the fuel return line to the filter and run the return it to the tank?
Last edited by justinp20012500; 07-01-2013 at 05:29 PM.
07-02-2013, 11:19 AM
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The most effective way is to run the return back to the fuel tank. It might be a little more work, but seems to be the most full proof fix. I would not add another pump because to me it's just another thing to fail.
