Please Help - Bad 502
 

My 1995 Baja 272 has a 502 Mag in it. It runs great for the first hour then it starts going down hill. At full throttle 4900 RPM's. It will miss like it looses fire or fuel for a split second. Then if the water is rough it will start doing it when you hit a decent wave. After a while it get so bad that the boat will hardly get on plane. If you are ideling in a six knot zone it will sound fine. If you put it in neutral and rev it up sometimes it will rev fine and sometime it will just die and not rev up at all and sometimes it will pop out the intake a little. The boat always makes it in under its own power and always starts fine. Im stumped. Obviously it is a fuel for ignition problem. I have inspected the Distributer, plugs, wires, all the grounds on the motor, all the plugs in the wiring harness for tightness corosion and dryness and everything checks a ok. Also I have replced the fuel water seperation filter twice along with dry gas in the tank. The boat ran fine the first few trips out this year so the problem has developed since the winter sit. My latest thought is that the vapor seperation tank has a problem in it. Maybe a faulty float or valve. If anyone has heard of something like this and can give a suggestion, I would be very happy. If I dont get it fixed soon, I will be looking at a pretty big bill from the local dealer when i take it in.

I am no expert but it sounds like the floats. If they are too high a good jarring will flood the motor. If may be time to rebuild the carb or a simple adjustment may do the trick. Good luck

Its EFI...

You need to check the fuel pressure. Low fuel pressure will cause the symptoms you describe.

Adjusting the floats on an EFI won't help.:D

Could be fuel pressure or clogged injectors.

Did you check the anti syphon valve? If you are getting crud in your tank this can become dirty and cause intermittant fuel loss problems.

The other thought I have was vapor lock. Make sure all your fuel lines are not in contact with something getting hot.

If you shut it down and let sit until cool, does it miracculously run well again?

BT :cool:

I just got back from the water with the boat. I had tried a few things to no availe. I dont think its vapor lock becasue It started doing it right away and its only 60 degrees here. I am going to order a code scanner from Rinda tech tomorrow. Hopefully that will tell me something.

Where should I check the fuel pressure at. The injector rails after the high pressure pump or inbetween the two pumps. What should they be??
John

Check for a good hot spark at the plug wires with a spark tester the pop in the intake sounds like it is too lean.

Probably a bad set of wires and a bad coil. plug wires only last about 5 years. also check for water under the dist cap.

502 problem
 

Merc used the VST tanks in 1995 on EFI's I believe. The problem you describe sounds exactly the same as I experienced myself with 1996 502 EFI's. My engine would run OK to a certain RPM and then sometimes shut off for an instant like total ignition shut off.

When the fuel flow is restricted in the VST screen it usually is not completely stopped, but rather just limited. Thus limited RPM operation.

First, there is a schrader valve in the front of the EFI system that is easily accesible to check electric fuel pump fuel pressure. This valve should have a cover on it and is easy to get to. Your test gauge should have a bleed hose and release to drain gas into a bottle instead of your bilge. You should get somewhere around 33-35 lbs fuel pressure at that check point with engine running. make sure to test at various higher RPM. Pressure should remain pretty stable through the RPM range..

The most common reason for low pressure at that point is a clogged VST screen. You can remove the screen and clean or install a new screen and keep the old one for a spare as I do. The removal of the VST tank is reasonably straight-forward. There are a couple of bolts at the rear of the engine that bolt the entire assembly on. When removing the two ( 2 ) fuel lines make sure to mark them for location as they look very similar and both will fit both locations. There is also electrical to disconnect.

When separating the VST tank take care. The assembly is sealed with O-rings. Sometimes a small amount of super glue will keep the used O rings in place for reassembly. This job is best done on a clean repair bench.

I solved the problem with clogged VST screens by installing RACOR 10 micron fuel filters before engine but after standard Merc fuel filters. I have not experienced any clogged VST screens in over 125 hrs running.

I hope this helps and is relevant to your application.

Good luck.

Its fixed
 

To those who replied I appreciate it. I wound up installing a gaffrig fuel pressure gauge perminattly in the dash. Right away I knew something was wrong because I only had at max 32 psi and the service manual says I should have 42. Upon trying it out in the water in noticed that Every time the engine would miss/die whatever you want to call it the fuel pressure was dropping to less tha 10 pounds. I though I had a bad fuel pump untill I blocked off the return line to the tank and the pressure pegged the 60 psi gauge. I replaced the Fuel pressure regulator with a new one and add a hardin adjustable cover. Problem solved!!! Good thing it wasnt the electric fuel pump it was $456. Compared to a total of about 120 for the regulator and the Hardin add on.

That's the quick and easy fix we like hearing about!!

jadento what part of maryland are you from?

Near fort Meade central between Baltimore and Washington