Can't Prime! - PeachParts Mercedes-Benz Forum

tobybul · #1 06-06-2010, 02:02 PM

I finally got (50 psi rubber) fuel lines installed on my 85 300D. This is temporary to get the car running until I replace them with hard lines. Only problem is I can't get the system to prime. How many times do I have to pump the primer pump? I know the priming is working because some fuel is entering and bubbles too into the primary fuel filter. But I can't get it to fully prime. I open the nut on top of the secondary spin-on filter when I'm priming.

Does the amount of fuel in the tank have any bearing to this? Its low but not empty.

Anything I'm missing here?

bustedbenz · #2 06-06-2010, 02:05 PM

The whole point of priming is to fill the filter canister up and then the lines.

That nut on top of the secondary filter has to be down tight as you need to seal it COMPLETELY before priming has any effect. You're pumping your fuel/air right out through the leak you introduce by having the filter partially unfastened.

That's why you have to re-prime after changing that filter. The filter is part of what must BE primed under normal conditions. And to prime it, it has to be airtight.

RML · #3 06-06-2010, 02:12 PM

Yep. BustedBenz explained it well. You do not want to loosen the secondary fuel filter. When I changed my secondary filter and needed to prime it back up, I am guessing it was only 10 or 15 strokes. It then started up in short order with not problems.

Give it another try and I am guessing you will be firing up within a few minutes.

tobybul · #4 06-06-2010, 02:19 PM

Quote:

Originally Posted by bustedbenz

The whole point of priming is to fill the filter canister up and then the lines.

That nut on top of the secondary filter has to be down tight as you need to seal it COMPLETELY before priming has any effect. You're pumping your fuel/air right out through the leak you introduce by having the filter partially unfastened.

That's why you have to re-prime after changing that filter. The filter is part of what must BE primed under normal conditions. And to prime it, it has to be airtight.

I understand that part. I never took off the spin-on filter. SO there should be fuel in it (unless it drained out when I removed the old lines). All I did was replaced the supply and return fuel lines between the engine and the tank.

But I will try your suggestion - gain, since I did it that way the first time - all nuts around filter closed.

jt20 · #5 06-06-2010, 03:27 PM

when priming with the pump, you will hear and feel a vibration when the system is full - this is the pressure relief valve

then, crack the injector lines at the injectors, and crank at full throttle 'till fuel appears at the nuts.

**edit: I didn't really understand what was going on, ignore the post.

nhdoc · #6 06-06-2010, 03:36 PM

Are you 100% certain that you didn't switch the supply and return hoses? If you switched them then on a low tank the return line will suck air and not fuel. For the heck of it try swapping them and see. It is a common problem and I'd bet you a beer that's what happened.

Brian Carlton · #7 06-06-2010, 05:06 PM

The proper way to prime the system to the secondary filter is to crack the outlet fitting on the secondary (there's an arrow pointing out) and pump the primer pump until no air is observed.

If you simply pump the primer, as others have suggested, the air in the secondary has nowhere to go but onward to the IP.

nhdoc · #8 06-06-2010, 05:18 PM

Quote:

Originally Posted by Brian Carlton

The proper way to prime the system to the secondary filter is to crack the outlet fitting on the secondary (there's an arrow pointing out) and pump the primer pump until no air is observed.

If you simply pump the primer, as others have suggested, the air in the secondary has nowhere to go but onward to the IP.

My experience has been that the priming can be done without opening any lines. Eventually the IP and filters fill with fuel and the relief valve opens and returns whatever needs to go back (air bubbles or fuel) - that's when you get to hear the buzzing noise on every pump and the primer is hard to press down - that's how you know it is primed.

I also know some people say to open the fitting on the main (spin on filter) outlet which is what I think Brian is saying to do...that will work too but I have never done this and always just primed by pumping until I heard the buzzing of the relief valve and never had a problem.

jt20 · #9 06-06-2010, 05:27 PM

Quote:

Originally Posted by nhdoc

My experience has been that the priming can be done without opening any lines. ..

the original post is incomplete. He did not open the fuel system after the lift pump. I do it the same way you mention.

He only replaced the lines from the tank to the LP.

Since his tank is low... he needs the assistance of the LP to prime the line before the LP and in the clear filter.

It should have been stated.

Brian's suggestion sounds right.

jt20 · #10 06-06-2010, 05:35 PM

actually, thinking about it again.

if all that was disturbed was the line to the tank ,and nothing else, he should just start the car up.. .the LP will prime the line in short time.

tobybul · #11 06-06-2010, 08:56 PM

Guys, you are making this more complicated than it should be.

After Post # 4, I got it to start

I primed the system just like I always have. I can tell you that it is necessary to crack open the topmost bolt above the spin-on filter to let air out. Cracking open the 'exit' bolt does not always do it.

Otherwise, as Brian said, you will let air into the IP which the IP does not like.

In my case, I think it was taking longer to prime because the lines between the tank and the engine were empty. The 2nd filter was also 2/3rd full of which I filled manually thru the topmost bolt hole. This assited in filling the system faster and made the priming work better.

Just a few more pumps after that and I could see the system getting filled. Cracked that topmost bolt open to let residual air out and tightened it up.

Turned the ignition and halleluia!! Thanks for eveyone's suggestions.

......... and the winner is.... Mr Brian Carlton.

vstech · #12 06-06-2010, 09:48 PM

Quote:

Originally Posted by tobybul

......... and the winner is.... Mr Brian Carlton.

as is the case whenever he deems us with his knowledge. unless he's very sleepy, and misread something.

Renntag · #13 06-07-2010, 11:48 AM

Glad you got it going.

Just for the newbs searching the forums, which motor are you working with? Assuming its your 84 300D with a 617?

tobybul · #14 06-07-2010, 12:30 PM

Quote:

Originally Posted by Renntag

Glad you got it going.

Just for the newbs searching the forums, which motor are you working with? Assuming its your 84 300D with a 617?

Thanks. 85 300D 617. Also added to post #1.

Another reason I thought the priming was more difficult could have been that the car sat in the garage since last October tilted up on jack stands from the driver's side. I know that some fuel leaked out from wherever the break was.

The pickup is much better too compared to b4. I'm guessing it had been very slowly sucking air before until too much air started going in.