Fuel guage fluctuation while engine running

[Sailonguy]

Have a 30 y/o 36hp MD17 Volvo Penta diesel, when the engine is running the fuel gauge fluctuates (dances up and down ... as in electrical / magnetic interference from other gauges??). When the motor is not running, the gauge reads fine. I have replaced the sending unit (float) and checked all the electrical contacts at both ends and they are solid. Short of replacing the gauge itself as the next step... any recommendation on eliminating this fluctuation?

Of course if I'm ever in doubt about my fuel level I can just stop the engine and know that the gauge is reading the correct level because I set the float myself

[mm2187]

how much is it changing. Have you tried putting an multimeter on the contacts to see what the voltages are. It sounds like a short somethere in the system.

[Sailonguy]

at 1/2 tank level it jumps + - 1/4 to 1/3 of the gauge range. I have not tried a multimeter on the contacts, I will try that. Thanks. If it reads constant then perhaps its just in the gauge itself. If its a short in the system ... ugh... could be anywhere.... not sure where to begin to look..

[hellosailor]

Vibration from the engine could be tickling a loose or corroded wire, check all the connections from the guage back to power and ground.

Could also be noise from an alternator problem, since the alternator is running when the engine is. But if that were the case, more than the fuel gauge should be fluctuating.

Interference from other gauges, etc? Unlikely, unless they came from Area51.

[merc2dogs]

Also, check to see where the return line is routed into the tank, it could be disturbing the float. Quick test would be to disconnect the return, and route it into a fuel can.

Generally engine caused vibrations are too 'close' to make the gauge fluctuate, typically if visible it will be a slight blurring of the needle from the vibration acting directly on it.

Check voltage, connect a voltmeter somewhere in line and see if the voltage fluctuates, fuel senders are resistors and fluctuating voltage will make the gauge read exactly as if the float was moving. An alternator/voltage regulator not functioning exactly right, or some electric fuel pumps can cause it.

[klem]

This might be a silly question but is it simply the fuel sloshing in the tank? If it only happens when the boat is rolling, this could well be what you are seeing.

[Sailonguy]

Thanks for the all the feedback.. Regarding the in-motion rolling of tank contents, I had to think about that one but no, the "dancing needle" starts as soon as the motor is running, and stops when the motor stops while still tied at the dock. The analog voltage meters at the engine instrument cluster (next to the fuel gauge) reads ~14v steady when motor is running... and equally the voltage at the panel (battery voltage) is the same, neither fluctuates. So.. doesn't that indicate that the voltage regulator & alternator is doing its job correctly? I will have to put that meter to work this weekend.

[neverknow]

Hate to point out the obvious here, but you shouldn't rely on a fuel gauge anyway. Those things are never accurate and even if it is eventually they will fail. Just hope that when it does you are not in a dangerous situation.

The best way is to have a flowscan meter that will tell you to the tenth of a gallon what you have used. That's what we do. We keep a log and record the meter readings and how many gallons we have put in. A flowscan meter will also allow you find the most efficient RPM's to run at. With fuel prices so high it wouldn't take long to get the cost of the meter back.

[hellosailor]

"So.. doesn't that indicate that the voltage regulator & alternator is doing its job correctly?"
Should be. Try this: Turn on the key, don't start the engine. Have someone go below and frantically "play the drums" on the fuel tank. ("Wipeout" should do.)
If the fuel guage starts dancing, you'll know it is vibration and a bad contact. If it doesn't start dancing, get a better drummer.