300SEL Fuel Pump Leak - Mercedes-Benz Forum

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post #1 of 2 (permalink) Old 07-09-2005, 07:41 PM Thread Starter
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kennodes's Avatar
Date registered: Dec 2004
Vehicle: 69 300SEL
Location: Brisbane QLD Australia
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(Thread Starter)
300SEL Fuel Pump Leak

Hey fellow Benz lovers,

I have a small problem which I’m hoping one of you 108/9 knowledge ponds may be able to assist in solving.

Yesterday, as I was about to head off on my weekend drive, I noticed a fairly steady leak of petrol from the rear fuel pump of the car (69 300SEL 3.5.) It is still running and isn’t noticeably underpowered; however I have no desire to drive her anywhere until this issue is resolved.

Upon closer inspection the leak is coming from where the power leads join to the pump (See attached image if it works). However no cracks are immediately visible; that’s not to say they are there though.

Can anyone tell me whether?
1. The small cap which the power leads attach to screws into the pump, and thus there may simply be a deteriorated seal at fault?
2. If I could use the same fuel pump from a 280SE 2.8 or 3.5 of similar vintage if the issue is terminal?
3. If using a simple sealant would in your opinion provide a decent temporary fix?

Being a poor uni student, I’d love to save the AU$500 it will cost to get a replacement pump!

Thanks in advance for your help.
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post #2 of 2 (permalink) Old 07-09-2005, 08:50 PM
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Date registered: Apr 2005
Vehicle: 1966 250SE Coupe
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RE: 300SEL Fuel Pump Leak

I'm not familiar with your specific pump, but I did just repair the one in my '66 250se Coupe 2 days ago and it was doing basically the same thing...although it has a "weep" hole (actually a little pipe) in the side and that's where the fuel was dribbling out.

My guess is, that like my pump, yours has an internal seal that's failed and you're now getting fuel all the way up into the electric motor portion of the pump. That's kind of exciting isn't it! Good thing that fuel and air have to mix at a certain ratio in order to combust with a spark present......KA-BOOM!

Anyway, I carefully took mine apart, replace one tiny little 6mm diameter metric o-ring and it's good as new. Total cost for that o-ring and the other one I replaced around the impeller base, $3.90US.

You should be able to FINISSE that pump apart, figure out where/what's leaking and then repair it. Mine had a neat little copper bellows style spring that was moulded into a bronze bushing at one end. The bushing ran on a carbon seal, just like you find in a water pump. Cool design. At the other end of the bellows spring was another bushing which sealed against the electric motor shaft with the little 6mm o-ring. This was all held in place with a tiny metal ring in a recessed groove on the motor shaft. It was a real job to get that ring off, but some tiny jeweler's screw drivers did the trick without breaking it or sending it flying....or scratching the shaft, etc.

Since yours is basically ruined, you have nothing to loose, so take your time and be creative and VERY careful!
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