HOW ABOUT AV-GAS?

QUESTION: I have a question about fuel that is currently available where I boat. We are having fuel system problems caused by the ethanol-blended gas sold here. Is it OK to use 100-octane aviation gasoline?

My boat has an 800-horsepower EFI engine that is set up to run on 91-octane gasoline. What are your thoughts?

ANSWER: In some boating venues, where there is a lot of performance-boating activities, 100-octane gasoline is readily available. 100 LL (low lead) AV-GAS is one of the easier to find fuels. You can run it in your engine.

I highly recommend that you add about a pint of outboard motor oil or Marvel Mystery Oil to every 50 gallons of fuel, or so. The aviation fuels are a little “dry” and need a little more oil in them to prevent pistons from scuffing.

Now that ethanol-blended gasoline is becoming more the rule, rather than the exception, there are many companies developing additives to help protect your high-performance engine from negative affects.

It is still likely that many boats will have to have their fuel system upgraded using hoses and components that can tolerate the ethanol. I know of a few additives on the market now and others being made. I intend to do some studies before the next boating season starts to see which of these products really do work.

AERATED FUEL?

QUESTION: I recently converted two 454 MerCruiser engines from carbureted to MPI with Edelbrock Pro Flo systems. I currently have both engines dialed in at a 12:1 air/fuel ratio. Maximum spark advance is 33 degrees at 3,000 rpm. Once the engines reach 3,000 rpm and higher, I sometimes experience an intermittent backfire through the throttle body, which would indicate a lean condition. According to my air/fuel meter that is not the case.

The system as set up now relies on a single high-pressure pump to supply fuel. Pressure is also good and stays steady all the way up the rpm range. I do not have a fuel cooler. I am thinking that I am getting air bubbles in the fuel either from boiling or from the tank itself. Some days, the engines run flawlessly, the next day the popping returns.

My proposed solution at this point is this: Reinstall my OEM mechanical fuel pump that will fill a 1-gallon slosh tank. The high-pressure pump will draw from that and my return line will pass through that small tank and then back to the main fuel tanks. I will also add fuel coolers to the system. If I can find some, I am hoping this small tank will eliminate any air bubbles from the tanks and the coolers to get rid of boiling, if any.

Engine room temps are 110 to 120 degrees. I'm also running a hotter water temperature to satisfy the ECM at 190 degrees. Any ideas?

ANSWER: If you are returning all fuel to the tank, it is unlikely that your problem is associated with boiling or aerated fuel, assuming that your fuel system is correctly installed and there are no leaks on the suction side. Starting at the fuel tanks, make sure that the pickup tubes are sealed to the fittings in the top of the tank. Some fuel tanks have a plastic tube pressed onto a nipple that is connected to the pickup-fitting assembly. Over time, the plastic tube can stretch and lose its positive seal, which can allow air to enter the fuel stream from inside the tank.

Next, make sure that all of your fuel-line fittings and hose connections are tight and sealed. The high-pressure fuel pump must be mounted low in the boat. It is best if the fuel pump can be mounted below the level of the fuel tank. If the fuel pump is mounted too high, it can cavitate and cause the fuel to aerate. You should have a water/fuel separator filter between the tank and the fuel pump. And, you should have an inline micron filter between the fuel pump and the injectors.

The water/fuel separator protects the fuel pump and system, and the micron filter protects the injectors from minute particles or debris that cause pump failure. If your problem is more prevalent when the fuel level is low in your tanks, I would be suspicious of a problem with the inlet part of the fuel system.

Mercury Marine tried eliminating the manual diaphragm-style supply pumps on some of the fuel-injected motors a few years ago. One of these was the HP500EFI engine.

The engine seemed to be fine during the validation process. But with all the variables in actual use, there were many cases of engines with vapor-lock conditions. A kit was created to retrofit these motors that included the installation of the original-type supply pump system.

The difference with these motors compared to yours is that they do not return fuel to the tank. Instead, the Mercury Racing HP500EFI and HP525EFI return fuel to the fuel filter assembly. Because you are returning fuel to the tank, I do not believe that you need a supply pump to feed the high-pressure fuel pump.

A fuel cooler is always nice to have and would be mandatory if you were not returning all fuel to the main tank. It would be good to have a fuel cooler in the return side of the system, but I don't think it will solve your problem.

Don't overlook the basics. Sometimes we get so focused on what we think is causing the problem and overlook some of the basic principles that could cause the same problem. Make sure the ignition system is operating correctly. Check for problems with spark-plug wires, distributor caps, magnetic pickups and so on. I have had a few cases lately where a reluctor in the distributor was loose and caused erratic timing. If your engine is misfiring, it can actually cause a richer reading on your air/fuel ratio meter.

If your fuel system is sound, and the electronics are in good working order, I think that your problem is most likely associated with that 190-degree engine temperature. That temperature is a little too warm for a performance marine engine, especially a non-closed-cooling engine.

At 190 degrees, with your raw-water-cooled engine, it is very likely that you are getting steam pockets in your cooling passages especially adjacent to the exhaust valve seats in the cylinder heads. Do you have any idea what your oil temperature is? I can imagine that it is high as well. It is possible that your problem is associated with detonation or preignition.

I advise that you lower the water temperature by changing the thermostat and then having the ECU programmed to allow the motor to run with normal parameters at the lower temperature. Remember: Heat is death.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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