Air in hard plastic line from lift pump??
 

I am seeing a white surge of what i suspect maybe air in the hard plastic line coming from lift pump to spin on filter housing on my 1983 w123 300D us spec, im having battery drain every morning since about a week ago and im ruling out air in the system as the cause.
I see no bubbles but rather a whitish surge of what could be normal fuel ???
THANKS!

Can you elaborate on the battery drain issue? If your battery is healthy, you should be able to crank for a LONG time before it runs down.

I had to read what you said a couple of times to get it. So you mean that you have to crank it so much, the battery drains before you can get it going? Unless you've added something to the fuel, it sounds like the frothing you seem to be describing is air coming in.

When the diaphragm fails in the primer pump, air can be sucked in.

Try pumping the primer pump. You should feel resistance after a certain amount of pumping, accompanied by a 'squeaking' or 'farting' sound which is the relief valve. If you feel no backpressure when you pump and/or it never builds pressure to where the relief valve kicks in, then you need to replace the pump.

However, I should also mention that reluctance to start as the mornings start to get colder around this time of year is also a typical symptom of valves needing adjustment.

Valves recently adjusted, i do have an upgraded Bosh primer pump, however i do not get a strong surgig woosh sound at all, it is very faint pumping action, i double checked today all hoses coming and going to lift pump and they are nice and tight.

If you do need to replace any of those clear nylon fuel hoses, Goodyear Barricade rubber hose is good since rated for bio-diesel. NAPA carries it. Since opaque, you won't be troubled by small fuel bubbles anymore. Usually, such bubbles are entrained upstream of the lift pump where there is a slight vacuum. If the lift pump rod O-ring leaks, it usually leaks fuel into the engine oil rather than sucking in air.

No diaphragm in his W123 1985 300TD naturally aspirated Fuel Supply/Lift Pump. It as a spring loaded piston and 2 plastic sort of T shaped cylindrical valves.
the 204Ds have a similar Fuel Supply/Lift Pump but have 2 disc valves.


When the new type of Hand Primer is installed you need to be sure to remover the old copper crush washer and install a new one.

White surge or foamy fuel are air bubbles that has been whipped into smaller air bubbles by the lift pump. If your primer pump is new and not leaking, then the air is coming from upstream of the lift pump, i.e. the metal fuel supply line, which is known to develop pin hole rust under the fuel line clamps. Or could be a clogged in-tank fuel strainer. When the clog/ restriction is bad enough, makes the lift pump work extra hard and outputs foamy fuel- that process is called cavitation.

Inspect the Plastic Fuel Filter for hair line cracks.
When I got Air in mine it was that the Fuel Inlet Hoses that attached to the Plastic Filter were so hard and inelastic that they were not sealing and allowing are to pass but in my case only when under the hood temp was hot.

Yes, old, dry, hard rubber will not seal well against the smooth barb-less plastic prefilter, especially the ones without a brass insert/ support in the barbs. I have seen prefilters made by welding 2 halves together resulting in rough seams on the barbs. Plain garbage! I had to scrape the seams smooth to stop the air leak. Once I bought proper pre-filters with brass insert supports, threw the garbage out.

Not really. Cavitation would be fuel vapor in low pressure regions of the flow, usually behind a moving blade or sharp corner. It is temporary and the vapor soon collapses back to liquid (violently, can even pit metal). Bubbles that stay around are surely air.

Doesn't cavitation occur with any kind of pumps and not just blade or impeller pumps?

This is from wikipedia:

"Cavitation is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. "

Diesel, hydraulic fluid etc has a small percentage of air dissolved in it. When the restriction is great enough, e.g. a completely clogged fuel strainer or a clogged tank vent, the pump will suck this dissolved air out of the fuel/ hydraulic fluid and form air bubbles.

I am beginning to think that there is another possible. Perhaps the one way valve where the Fuel exits the Fuel Supply/Lfit Pump is not sealing and the Fuel is getting pushed out and then sucked back in and the bubbles are actually vacuum bubbles or the Fuel is frothing.

I had exactly that happen on the 3rd IP I put in the SDL. Lift pump valve wasn't sealing and it was just making a foam froth instead of pumping fuel. Took a bit to figure out what was going on, couldn't get the IP primed (603's have no manual primer). Took the pump out and tested it, and found it was pumping, but not sealing, any back pressure on the discharge side and it would lose pressure as soon as the pushrod was released. Installed the known-good pump off the original IP and it's been happy since.