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1999 225/ 3.0L Optimax in "no start" condition

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BDboatguy

New member
Let me preface this thread by saying that I have a small marine service business, and am familiar with the Optimax engine. This particular boat was brought to me after the previous service facility threw in the towel, and told the customer that they could not fix it. They had replaced the low pressure fuel pump, the high pressure fuel pump, had the fuel and air injectors cleaned, and still were unsuccessful in getting the engine to start. First thing that I noticed was that this engine still had the old original grey fuel hose on it with the red writing from Mercury. I rebuilt the mechanical fuel pump with diaphram/check valve kit, replaced the fuel/water separating filter on the engine, and put the engine on a separate 6 gallon fuel supply from a remote tank. Cranking the engine would only build 80 psi of fuel and air pressure, and with the dual gauges hooked up I noticed a little fuel in the air side when pressing the relief valve on the gauge. I checked the tracker valve diaphram due to the fact that the injectors had been recently cleaned, and the 10 psi difference not there. I did find some creases in the diaphram and replaced it with new. I also removed the fuel pressure regulator housing and diaphram to inspect. It looked good, so I just reassembled the fuel pressure regulator and reinstalled. Cranking the engine would only produce 80 psi of air pressure, and 80 psi of fuel pressure. I installed a clear line on the fuel return hose with a schrader valve t'd in, and saw no air bubbles, but also saw no fuel pressure. My experience tells me that I should be 6-9 psi on the fuel return hose. I moved the hose to the output side of the low pressure pump, and saw very little fuel pressure key on. I replaced the low pressure fuel pump, and now key on the fuel pressure will rise to 20-30 psi, but still only 80 psi maximum. Knowing that this engine derives all pressures as related to air pressure, I crimped the air dump line going to the exhaust housing and so no pressure spike when cranking the engine. I replaced the air compressor with the latest and greatest from Mercury and ran through the oil pump prime to fill all feed lines with oil. I applied regulated 80 psi of shop air pressure to the air test valve with the spark plugs removed, and could not hear any air passing through like a direct injector not holding air pressure and bleeding off, or a compromised air pressure regulator diaphram. Pressure would hold @ 80 psi when the air line was clipped on to the test port and left for 5 minutes. The engine still will not start, and only gains 80 psi of fuel pressure, and 80 psi of air pressure. When the air dump line is crimped when cranking the engine, I do not see a pressure spike at all even with a brand new air compressor on it. When the fuel return line is crimped when cranking the engine, I do not see a pressure spike on the fuel side either. After cranking the engine I noticed that it immediately starts to lose air pressure on the gauge, and fuel pressure falls also, but always the 10 psi greater when falling off. Fuel pressure will then stabalize at 20 psi and hold. This engine has gotten in my crawl, and I'm ready to get it fixed. Any thoughts/help would be greatly appreciated.
 
I have attempted just what you suggested to no avail. as you know the 10 psi difference is critical to get this engine to start, and I am not getting 90 psi of fuel pressure even with the fuel return line crimped. I previously disassembled the VST to inspect the high pressure pump and associated grommet, and both looked new and in good shape so I just reassembled. As I stated the air compressor is new. The tracker valve is new, and I have inspected the fuel pressure regulator which looked good. I am not getting why when either the fuel or air dump line is crimped I do not see a spike in pressure on either side of the gauge. I have applied air to the test port with the spark plugs removed also, and heard no air escaping. I know it is critical to get the fuel 10 psi over air pressure (in this case) and I am unable to figure out why fuel pressure will not build when the return line is pinched off.
 
The air pressure has to come up to close the fuel pressure reg to bring up the fuel pressure to 90psi.

So the grommet on the high pressure pump is in the correct position....

I would take the VST apart again and rig a fuel pressure gauge right to the HP pump and make sure the pump is making pressure.
 
I rigged a fuel pressure test port t-d in just after the high pressure fuel pump. First I checked fuel pressure at the test port on the fuel rail, and only saw 80 psi again. I moved the gauge to the test port that I had rigged in just 3-4 inches from the high pressure fitting exiting the VST. Key on I saw 35-40 psi, and when I cranked the engine I only saw 80 psi. I feel that the high pressure pump, although new, is compromised somehow from the ability to make the appropriate fuel pressure. I feel that the upper and lower seals (they're the same p/n 8554331) need to be replaced as well as the pump that will come with a new sleeve. The gasket is now ~$160.00 for the top of the VST, so I will disassemble and determine if it absolutely needs to be replaced also. Any thoughts on these, and your good ideas are welcome. thanks David.
 
If you hook up shop air to the rail and regulate it at 70 psi the engine should start if the fuel pump is only making 80 psi. I have done this.
 
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Shop air pressure regulated at 70 psi with the compressor belt removed, the engine would not start. I installed a fuel pressure gauge at the test port on the fuel rail, and saw only 70 psi of fuel pressure. I relieved fuel pressure and checked to make sure shop air pressure was still at 70 psi, and after squeezing the primer bulb to again prime the system I saw only 70 psi of fuel pressure again. I dropped the air pressure regulated at 50 psi, and when cranking the engine I only saw 50 psi of fuel pressure. It seems like the two chambers are interconnected somewhere not allowing for the pressure difference to happen or for a spike in pressure either on the fuel side or the air side when either the fuel return line for fuel, or the air dump line for air are crimped off. I know that the high pressure fuel pump is capable of at least 80 psi from previous testing with the compressor belt on while cranking, but why the fuel pressure would mimic the air pressure is puzzling after all of the previous checks that I have made.
 
The fuel pressure will not come up until the air pressure comes up and closes the fuel pressure regulator. Pull the diaphragms again and look for small cracks and/or holes...

Hard to diagnose these engine through a keyboard.

Short work if it was in front of me.
 
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