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.

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.

Dave

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.

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.

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?

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. :mad:

Tom

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.

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.

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.

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.)

Re: 502 Mag EFI Fuel Problems
 

In my case the metal fragments damaged the rear bearing supporting the drive shaft so much that the shaft was misaligned by belt tension enough that the H2O pump wear plate o-ring had a steady drip. The water leak was my hint that something was amiss with that assembly.

Your oil leak might mean something...

... or just need to be repaired.

My symptoms were: pull throttle to idle, a couple minutes of idle speed then stall, crank, stumble, stall, crack throttle(the hissing is the idle air control valve trying to make a really big air leak, this helps make a bigger one and gets the tps to increase injector duration to get some fuel for it to catch-that the vst pump is finding in short supply cause the vst tank is virtually empty), crank, fire, run in neutral approx 2500 revs, idle in gear for thirty or so seconds, stall, repeat crank procedure, put in gear and go home(about ten miles) at 4000 rpm. Ran fine while flushing on lift at 1200-1400 rpm, then stalled at idle speed after a couple of minutes. Then repair.

Re: 502 Mag EFI Fuel Problems
 

Appreciate everything, will tear into it today and see what comes out of it. Also if anyone else has any clues to this mystery, please let me know!!
My motor alway's ran perfect, I take very good care of my boat and it's contents. My engine never vapor locked before, I live where it's hot, the Miss. Gulf Coast, and lived in St. Petersburg Florida also where we bought the boat, ran in hot weather all of it's life and never once did anything like this till just recently. The boat is a 1992 Baja Outlaw in near perfect condition. The previous owner installed a new 502 Mag EFI crate engine into it in 95. I just want it back to it's old self again, I am sure yall understand!! Thanks alot,
Randy

Re: 502 Mag EFI Fuel Problems
 

If you can get a hold of a scanner that would help. You will see if it has thrown any codes. you can also leave it on there and run it, then be able to tell what the MAP sensor and TPS are reading when the problem occurs.

Re: 502 Mag EFI Fuel Problems
 

Last time I saw one of the combo raw water/ mechanical fuel pump leaking at the front seal like that, it was full of gas due to a failed diaphram. You might be on to something.

Dave

Re: 502 Mag EFI Fuel Problems
 

Hello Dave...If the diaphram was or say, is bad, would it still run part of the time like it does now? I mean it will crank right up right now and run perfectly and you can take off and get on plane and run. I have only noticed this problem when I come down off plane and slow down. Or sometimes it doesn't do it at all and when we get ready to leave later in the day after rafting off to other boats or spending the day at the islands, it'll start and when I get ready to take off and get up on plane, she stumbles right down and starts that spitting and sputering crap, then eventually dies. I need a code tester badly, I may end up having to bring it somewhere to have it repaired there. I am about tired of messing around with it. Keep the info comming guy's I check it regularly.
Randy

Re: 502 Mag EFI Fuel Problems
 

If you see oil leaking out at the mechanical fuel pump then the diaphragm has failed in the fuel pump and diluting the gear oil. I had this happen to me.

I also had the other problem that you have with a 502 EFI going dead after running a while and the motors sounded like they were starving for air. Made it real tough coming in a marina after running for a while. Anyway, I talked to an engine builder at the Sarasota offshore a few years ago. He told me that I more than likely had a carbon buildup on the pistons and it acts like charcoal and sucks up the fuel and creates a lean condition to the point of stalling. He told me to use Marvel mystery oil in my fuel and sure enough it corrected the problem. I tripled the dose in the beginning and then just continued to use it.

Re: 502 Mag EFI Fuel Problems
 

I am not sure if the diaphram failure would cause symptoms like yours or not. It's possible that due to lack of lubrication (gear lube diluted/washed away) the pump is overheating. Maybe check pump temp when symptoms occur.

Also, if you smear a little of the leakage from the pump on your finger and smell it, a strong smell of gas will confirm pump diaphram failure. Or just pull the fill plug on the pump (be careful it may be under pressure).

Dave

Re: 502 Mag EFI Fuel Problems
 

This type of problem may not generate a trouble code, but if you want to check for codes use the LED and paperclip trick.

Dave

Having same problem.
 

im having this same exact issue with my 502 mag efi in my 1993 formula to the T. Did you ever find out what your problem was?

Doing same thing
 

my 502 efi mag is doing the same exact thing right now. Did you ever find the problem? I put a cooler thermostat in mine so it runs in between 140-160 now so i wouldn’t think it’s a temp problem

This thread is 20 years old. Most of the guys who were posting back then aren't even on this site anymore. Some might be dead.