PeachParts Mercedes-Benz Forum - View Single Post - 83 300TD 300d Air Bubbles in fuel lines hand pump? no start

JimSmith · #4 12-27-2003, 12:20 AM

bmitchell510

First lets try to clear up some of the symptoms. The prefilter has no way to be vented so it will typically be half or so full of air. This is normal. The filter is basically gravity fed from the tank, so the flow rate, pressure and basic geometry of the filter mean it will always have some air in it. And that is no big deal so don't fret over the air you see there.

The same is basically true of the clear line you see coming up and over to the main filter housing. It will often have a small bubble at the top because under normal operation the flow rate is not sufficient to drag the bubble with it, down into the filter housing. So the bubble stays trapped in the high point, which is often in the clear tubing.

So, to check if you have flow to the in-line pre-filter, unhook the fuel line going to the filter and you should get a steady stream of Diesel fuel. If not, you have a problem in the supply line from the fuel tank. If you have a stream, then the hand pump should be getting fuel, and unless it is squirting out the sides of the pump as you push the handle down and pull it back up, you have an air leak somewhere else. When I operate my hand pump, it makes a significantly different noise, like a faint hydraulic version of grinding, when the system begins to become "solid" and I do not really look at the bubbles and froth in the clear line too much. Just enough to check that there is flow there that corresponds to the action of the pump plunger.

If the stiffness of the system never changes (the pump becomes harder to push down on) then you must have a fuel flow probem or a massive air leak in the area of the pump, the main filter or the connection to the injection pump. There should be some evidence of air leaking, either audible as you operate the pump, or you should see some place where Diesel fuel is leaking out.

I hate to be a doubter, but it sounds to me like either the glow plugs are bad or the relay is bad, and you really don't have glow plugs. Leave the dome light on (either leave the door open or turn the light on) then insert and twist the key in the starting switch to glow position. You should see the dome light dim significantly. If you do not see it dim, you do not have power to the glow plugs.

If the car runs fine once started, and you have no visible signs or telltale odors of Diesel fuel in the engine compartment, you likely do not have a leak. Air or fuel.

I am curious about your characterization of the problem as a "ran out of gas (really Diesel fuel) symptom" when you did not indicate that once running you have any problems with the car running. It is only restarting it, especially after sitting for a while in cold weather.

I have a few other questions. When was the last time you changed the oil? What brand and viscosity rating was it? When was the last time you adjused the valves? How do you know the glow plugs are all good (how did you test them?) How old is the battery? When you take the fuel tank fill line cap off, does it make any hissing or sucking noises and is it hard to get the cap to break loose and twist off the filler neck fitting? How is the alternator belt? Tight, good condition?

Well, I hope this eventually helps, Jim