[David and Teresa]

Are there any other explanations for tiny bubbles in the fuel filter besides the o-ring not seated correctly?

[Richard]

Hmmm,

While the engine is running? What happens to the bubbles after the engine is stopped?

Have you added anything to the tank such as fuel additives or stabilizers?

[David and Teresa]

While running. I had an issue last year where the fuel system was loosing pressure and lost prime and wouldn't crank. Detroit Diesel came out and found an air leak in a fuel line and replaced it. I noticed tiny bubbles while running it recently. I do put a stabilizer in.

[ccjohnson]

Sounds like you may have another air leak developing in the fuel system somewhere before it enters the intake side of the filter.

[Richard]

As Clint says, probably sucking air.

But, and forgive me, for I am not an additive fan. There can be a situation where additives can cause what appears to be bubbles in the fuel. Some additives contain methanol, which is a solvent which will dissolve in water and in diesel fuel. However, only to a small percentage before it saturates, and starts to coalescence into tiny droplets suspended in the fuel. This is probably not your case, but why I asked what happens when the fuel flow stops. Do the bubbles rise to the top, as air should? or do they stay suspended in the fuel as methanol would?

[77newell]

Additional thought; if the bubbles are present when the engine is running but not when it's stopped AND there is no loss of prime when stopped then the leak is in the filter housing. Besides the gasket, if you have the water collector and drain, the drain could leak under the suction of the fuel pump operating. In the scenario I've noted you would be looking for leaks anywhere downstream of the check valve in the fuel supply line to the filter and upstream from where you are observing them.

[David and Teresa]

Thanks, I'll look closely at all this. The bubbles do rise to the top.