Difference in primer/bleeder pumps
 

I have a quick question. I recently retired an '84 300D that had the screw-type primer on the lift pump. The '82 240D that I replaced it with has the straight primer pump.

I really liked the screw-type style, which seemed to act also as a bleeder. It seems that this other style pushes air back to the fuel tank to bleed. Is this true?

NO, its not true

Any insights on how it does work? I always assumed that the screw-type released the air at the handle (based on the fact that it begins to spill diesel once the air has passed).

I am confused by where the air goes with this type primer. When I pump it I see air bubbling at the engine side of my in-line fuel filter. I'm trying to get the air out of the system or find the leak that is keeping my car from running.

[QUOTE=vjw;1827290] (based on the fact that it begins to spill diesel once the air has passed).

The problem with the older style is they do spill fuel (leak) when they need replacing, they shouldn't do that.

When you pump your primer pump the air exits a small hole on the primary (big) fuel filter housing, not sure where, doesn't matter, but you will hear a difference in the sound when your pumping. You can still get the old "screw down" type but I would stick with the new "black handle" one, more better:)

THanks Stevo

The black one is considered an upgrade to the older screw down type.

Danny

For some reason, I'm partial to the old style. Guess I'm just an old fogey who gets set in his ways... tell me I'll change my tune before I rip the old one off my previous block!

V

There is only going to be a "hole" in the filter housing if you make one by loosening a hose or fitting. The return line hollow bolt works well. The sound that you hear is most likely the relief valve on the injection pump. It just relieves excess pressure by diverting fuel (or air) into the return line.

As counterintuitive as it may seem, the large filter is the secondary filter. The inline filter upstream from the fuel pump is the primary.

Yes it is true. The air goes to the canister filter at the three-point connection, to the cigar hose and returns to the tank.

Don't belive it? Change your fuel filter without filling it with anything. Tighten it, then hit the primer 30 times. No cracking of anything required. Where did the air go? No big mystery.

It might be more obvious if you could see through your clear fuel lines and watch the bubbles.

If I did that, my car wouldn't start:D.


Still - both new/old pumps work the same....and a leaky old-style pumps means it needs to be replaced, not that it's bleeding air as designed

I did that earlier today. Don't tell me our cars are different?

Here's a question for you: Have you tried it?

Having battled a rusty fuel tank - yes - I've had to change filters literally 20-25 times in the past year. The first two times, I hadn't this fine forum as a friend and I changed the secondary filter w/o filling it up. I had to pump until my fingers bled and I killed my battery once too.

I'm not saying it COULDN'T work....I've just had much better luck loosening the bolt.

Hmm. Sounds like a problem with the condition of your car?
I have no problems whatsoever. Like I said, I did it earlier today.

But back on point, yes. The air returns to the tank.

Huh, I had always thought it was the air escaping the system that could be herd, learn something everyday:)