1999 3.0 L Optimax in "no start" condition

[Anchors Aweigh Marine Service]

Engine will gain 80 PSI air pressure, but only 80 PSI fuel pressure. Previous repair facility replaced the high pressure fuel pump which I verified by removing and disassembling the VST. Fuel pump and associated grommet were new, and there was no change after reassembly. With the duel air/fuel gauges hooked up I blipped the air dump to see if fuel was present after the engine had built pressure, but saw no fuel. With the prevalent 10 PSI difference, I removed and replaced the tracker valve, and removed the fuel pressure regulator from the opposite rail and found no abrasions or tears in the diaphram there either. I crimped off the air compressor dump line to see if the air compressor was capable of producing more, and found that the air pressure AND the fuel pressure climbed over 100 lbs. when the line was uncrimped the fuel pressure fell to 90 PSI and held, and the air pressure went to 80 PSI and held. After cranking the engine, both pressures went to 80 PSI as before. I know that the 80/90 are the older pressure specs, but without the difference there this engine will never start. Somehow, and somewhere there is a compromised seal between the two chambers. Any help or direction with this headache will be greatly appreciated. David.

 

[schematic]

check what pressure you can get if you crimp off the fuel return. should get 115 or better. Also, with the engine off, apply shop air to the air check shrader valve and see if the fuel side increases.

 

[Anchors Aweigh Marine Service]

Fuel pressure rose when return line was crimped, but I've narrowed it down to a weak air compressor I believe. I applied regulated air pressure to the schrader valve and once stable @ 80 PSI I heard no hissing, or lost any pressure off of the compressor in the shop. Crimping off the air dump produced no major change in air pressure when cranking, and the fuel pressure rose a little as it should. I do know that having an air compressor that is capable of more than 100 PSI..more like 120 when crimped like right now, is critical and all pressures both fuel and air are based off of air pressure. I ordered the latest and greatest compressor p/n 8M0063279 because the compressor is suspect after eighteen years of service, and I need to move forward. Thanks for your reply, and I will keep you posted on my newest puzzle.

 

[schematic]

Doesn't sound like compressor issues. If you want to eliminate the compressor, simply apply regulated 90psi shop air to the air test port and see if it starts.....
What fuel pressure do you get when you "key on"? Does it hold this pressure for long when key off?

 

[Anchors Aweigh Marine Service]

Well you were right about it not being compressor issues. I did install the new compressor, and saw no change on my gauges when hooked up. The fuel and air pressure are @ 80 PSI. This time when the return fuel hose was pinched off, I did not see a rise in fuel pressure at all when cranking. I removed the hose and put a clear hose in it's place with a schrader valve t'd in, and hooked my fuel pressure gauge to the fitting. When cranking the engine it is my belief that I should see 6-9 PSI and no bubbles in the fuel which could be a sign of fuel aeration by the low pressure pump. It takes more than one key cycle to gain the 20-25 lbs. from the pump when I was hooked up at the rails. This indicates to me a fuel supply problem that mystifies me due to the fact that the previous service center told the customer that both pumps had been replaced. The fuel pressure does hold ~20 lbs with just key on, and I do not see any traces of fuel in the air supply as before. It is my thinking that if I had a fuel injector leaking off then I would see some traces of fuel in the air system, and there is none. I have checked this several times with the same result. This leads me to the low pressure fuel pump due to the fact that I have verified that the high pressure pump/ grommet are in good shape. Any thoughts?

 

[Anchors Aweigh Marine Service]

When the gauge was hooked in line on the fuel return hose I saw no measurable fuel pressure. I had left that part out. The clear hose showed no signs of air.