question for pnwboat or jerryjerry05

[rcb130]

This concerns a previous post by myself concerning a 1995 force 90 Hp S/N OE104323 I am hoping that you guys can explain to me why the engine straightened out and ran correctly when I did the tests specified in Mercury Service advisory 2001-17. This motor had a sneeze at idle and would also stall at idle for no apparent reason. The advisory said to hook up a short piece of clear tubing to the fuel pump input with a tee inserted into the the fuel line. The other leg of the tee went to the fuel supply while the tee branch went to a vacuum/pressure gauge. When starting the motor after hooking the above arrangement up, I noticed a few bubbles coming from the fuel pump which stopped after about five seconds and the motor ran perfectly. I have since replaced the fuel pump and the motor runs as it should. My question is this: What happened with those bubbles, where did they come from and what did hooking up the test apparatus do to make the engine run correctly?
The only thing I can figure is a check valve was sticking or had crud on it. Please provide your opinions.
Thanks
rcb130

 

[pnwboat]

Just to clarify.....was the TEE installed in the fuel line that runs between the fuel pump and carburetors? If so, then I would suspect that you may have had a small air leak in the fuel delivery system. Most likely a leak in the fuel pump diaphragm, maybe in the fuel line itself or a hose clamp on the fuel line. You shouldn't see any bubbles in the fuel line between the carburetor and fuel pump.

I've never watched for bubbles when first starting up, so I don't know if that's normal or not. I would think that if the fuel supply line going to the fuel pump was not completely full of fuel, you might see a few bubbles when first starting, however, if it's primed I would think that you should have cleared out any air in the line.

If the motor has started to run OK after you messed with the fuel lines, then I would be inclined to replace all the fuel lines/hose clamps just to be sure. It's not that expensive and is usually not that difficult, although the line between the tank and the fuel pump can be a pain depending on accessability. You also can't rule out a sticky check valve, bad fuel pump diaphragm (too stiff). Any of them could cause the symptoms that you found with your test.

 

[rcb130]

pnwboat,
The tee was installed in the supply line TO THE FUEL PUMP.

 

[Frank Acampora]

What was the vacuum reading on the inlet side of the pump? Assuming it was a Mercury style square pump, the valves are a rubber compound with plastic "keepers" to prevent them from opening too far. Since the bubbles were on the inlet side it is unlikely that the fuel pump was the problem.

Bubbles in the inlet side indicate either an air leak in the line or tank OR--and I am reaching here---Volatile fuel compounds which vaporize under normal fuel pump vacuum, especially if the vacuum is high due to a partial clog or sticking check valves in the primer bulb.. Remember: The fuel pump can generate enough vacuum to collapse an unvented tank so it is not improbable to vaporize light volatiles.

 

[pnwboat]

From your previous post regarding the vacuum test, you were only pulling 1 to 1.5 in. of vacuum when you should have been pulling 2.5, then it sounds like the fuel pump itself was not working properly. Replacing the fuel pump was the cure as you experienced. As for the bubbles, I suspect that what you were seeing may be normal as the fuel initially begins to flow when you first start the motor. Definitely not normal to see bubbles once the motor starts and fuel flow stabilizes.

I think that you had marginal fuel pump and messing with the lines and momentarily pinching off the fuel line to the pump, installing the "T" for testing was just coincidental which appeared to cure your problem.