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502 Mag EFI Fuel Problems

Old 08-18-2005, 07:27 AM
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Angry 502 Mag EFI Fuel Problems

I have a mark V series 502 Mag EFI in my Baja Outlaw and am experiencing problems after a short run with it, and I slow to no wake or idle speed, It will stumble and die out and will not re-crank for a while. I have noticed that when it does this, the high pressure pump inside the vst tank doesn't sound like it is running to capacity, not nearly as strong as normal.
Could there be some sort of a voltage drop off here maybe? Please help I am getting tired of changing parts that don't need changing.
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Old 08-18-2005, 10:44 AM
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Default Re: 502 Mag EFI Fuel Problems

Have you checked fuel pressure when this occurs? The fuel pump will make more noise when its running dry or there is air in the system.

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Old 08-18-2005, 12:16 PM
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Default Re: 502 Mag EFI Fuel Problems

It's pretty easy to check fuel pressure on this motor. There's a valve right at the front of the engine to plug a pressure gauge in to. You should see 33-35 lbs at all times when running or ignition is on with pump running. It's fairly common to have a clogged VST filter on this model. They can be cleaned and re-used but take care of the O-rings for the reassembly and mark the fuel lines well when disassembling as they are easy to replace incorrectly. Some will crazy glue the O-rings down to re-use. Chances are, your VST screen may be clogged. The standard Merc water seperator/fuel filter is totally ineffective in protecting the VST screen from finite particles. The VST screen is way too fine a filter for the Merc to do anything. The RACOR is 10 micron and the Merc is 70 micron. Use a RACOR to protect the VST screen. The new VST screen is stupidly expensive @ $70.00 +. Try to re-use the old one.

Good luck.
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Old 08-18-2005, 02:17 PM
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Question Re: 502 Mag EFI Fuel Problems

I have recently replaced the entire high pressure pump in the vst tank itself, it's 498.00 and bran new only ran one time in Orange Beach at a Poker Run last Saturday. The old one was spotless, no particles in the screen even. The noise it makes when this occurs is faint, not as loud as when it seems to be operating normally. It's like it's not running as fast as it should be. What would cause this pump to stop running at it's capacity after running for a while? Could the map sensor have an affect? what about the throttle possition sensor or the fuel pressure regulator? When this situation occurs, you can really hear the hissing of air at the flame arrestor loudly. As it is hissing, the engine is sitting there stumbling and choking out gradually until it completely dies out. Then about 20-30 minutes later you can re-start it and it's as though nothing was wrong until you slow down again, same thing all over. When it's running, you can run half throttle, 3/4 and full all day. Never does it when you are up on plane, even a slow to moderate plane!
All help is greatly appreciated guy's really.

Last edited by Randy Nevers; 08-18-2005 at 06:20 PM.
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Old 08-18-2005, 06:40 PM
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Default Re: 502 Mag EFI Fuel Problems

Have you checked the mech pump? It draws from the tank and pumps to the vst and hi pressure pump. Do you have fuel pressure gauges?
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Old 08-18-2005, 07:50 PM
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Default Re: 502 Mag EFI Fuel Problems

I agree with cleaning the VST screen. If a clean screen doesn't fix the problem you may be getting vapor lock in the hot weather after a long run. The old 502 EFI motors with the VST were notorious for vapor locking. Mine would vapor lock when trying to idle after the motor had been run a while. The fuel pump in the VST may be vapor locked and that is why it sounds funny. The fix is to put a fuel cooler in the return line. When it occurs, open the throttle all the way which will cycle the injectors to 100% and the motor will usually start after a minute or two.

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Old 08-18-2005, 07:55 PM
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Default Re: 502 Mag EFI Fuel Problems

A couple of other comments, if the mechanical pump was bad the motor would not run at high speed. The VST would go empty under load. I also insulated the fuel lines with foam. The fuel lines run down the side of the intake so they get really hot. The hissing may be gas vapor coming out of the injectors in the intake. Also leave the blower on all the time to keep the engine compartment as cool as possible.
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Old 08-18-2005, 09:08 PM
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Default Re: 502 Mag EFI Fuel Problems

You can also get vapor lock in the mechanical pump. Try pouring ice water on the pump housing when the stalling occurs and see if it doesn't help.
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Old 08-18-2005, 10:44 PM
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Default Re: 502 Mag EFI Fuel Problems

I had the exact same symptom on my '95 VST 502 EFI. I'd decided the VST was running out of fuel at low revs and pulled the combo pump assembly off the engine and drained its oil resevoir. More metal drained out than oil.

There is a slip ring that fits over the cam lobe that drives the mech pump. It exploded into a tin foil jiblet like form, and as a result, the mech pump arm loses lots of its travel. Hence, low revs yield not enough fuel to the VST.

As a result of the destruction, a new drive shaft, slip ring, bearings,mechanical pump, and seals and now back to normal.

May not be your issue but you might take a look.

Last edited by mosport1; 08-19-2005 at 02:24 AM.
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Old 08-18-2005, 11:29 PM
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Default Re: 502 Mag EFI Fuel Problems

After checking further, I have noticed a slim trail of oil dripping down from what appears to be the front seal on the belt driven water pump and fuel pump resivoir. It seems to be loosing fluid around the back side of the pulley. I never saw this before. But would it still re-crank after sitting if the fuel pump drive rod was damaged? The VST pump just sounds different when it tries to re-pressurize after it stalls and dies, than it does when you first turn the key switch on. It's noticable it's working then. But when it dies and you try and re-crank the damn thing, it sounds totally different.
Thanks for everything guy's I do appreciate everyones help and input on this frusterating issue!!
Randy ( Bay St. Louis, Ms.)
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