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Mechanical fuel pump issues/questions

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So I did some research on the forum about fuel pumps and have most of my questions answered. But there is something that I am trying to figure out.

The car has been running fine without issues, then today, after I stopped at a friends. (1 hr later) the car would not start. This happened once before to my son (same circumstance) and a quick diagnosis, it looked to be the fuel pump. I had it towed to a shop and they looked at it the next day... but it started just fine.

But today, I looked into the problem a littler deeper. I disconnect the fuel line at the carburetor and it looked dry. The line was not under pressure, nor did any fuel run out. So I cranked the engine over a few time and nothing. No fuel came out. So I disconnected the fuel line more up stream post fuel pump and I saw a few squirts of gas come out.

I would have expected fuel to come gushing out at these two point but it did not. The question I have... lets say the line leading to the fuel pump is just fine... not sucking air and not blocked in any way. Should I expect to see fuel gushing out at the two points where I disconnected the fuel hose? And do pumps like this exhibit intermittent problems like I have seen?

Any thoughts would help.



1985 318i (My First); 1996 535i (Loved it/sold it); 1999 323is (My kid likes'em too); 2003 325i (BMW Buy Back)

2005 530i (Best Ride Ever); 2007 x5 (Loved the look, but not the maintenance); 1976 2002 (Bitch'in ride)

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Remove the right side rear board covering the fuel tank and check the fuel line(s) at the tank. I believe there is a spacer sleeve that fits over the tube and inside the hose. If that sleeve is cracked or missing, then you will pull air and not fuel. This seems to be a common problem but difficult to diagnose. This happened to Marc Caden on his trip to Vintage NC in 2011.

Luckily there were some good helping hands available.


Jim Gerock

Ruby Red 73tii built 5/30/73 "Celeste"

Riviera 69 2002 built 5/30/69 "Oscar"

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the pump is very simple:

rubber diaphram stroking back and forth.

the diaphram is either good and soft

or it's brittle, old and torn?

now, you could have a supply problem

originating at the tank:

blocked tank pick-up?

dirt/silt covering the bottom of the tank and strainer?

other rubber hoses from the tank that are sucking air

because of the age of the hose?

a partially crushed metal fuel pipe some place under the car ?

does this happen only when the tank is 1/4 or less empty?

do you know how old the fuel pump is ?

if it is an 'older' pump with screws holding a cover

on it - remove the cover and look inside - report your findings

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

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Thanks for the feedback. I will look at these points and report back. Stay tuned.


1985 318i (My First); 1996 535i (Loved it/sold it); 1999 323is (My kid likes'em too); 2003 325i (BMW Buy Back)

2005 530i (Best Ride Ever); 2007 x5 (Loved the look, but not the maintenance); 1976 2002 (Bitch'in ride)

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c.d. is on to something with the diaphragm in the fuel pump. My son's '74 had this issue- after the car was fully warm, it would re-start but only run until the fuel from the pump to the carb was used up (usually about a block down the road) and wouldn't re-start until the engine cooled down (my son got into the habit of lifting the hood every time he parked; and in a pinch, he poured cold water on the fuel pump before re-starting).

I would like to re-build that fuel pump (anyone know where to find a good re-build kit?), but in the meantime we bought a new pump from Bavarian Auto for $85.95 and the car runs like a top now.

www.bavauto.com part # 13 31 1 265 192

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So this morning I went out to check on the issues and without hesitation, it started just fine. I removed the fuel line leading to the carburetor and sure enough, fuel gushes our when I crank her over.

I dont think that I have a sucking air problem, but more of a fuel pump issue when the pump gets warm. But it baffles me that I don't have the issue while driving as the car gets warm, it is only when I attempt to restart.

I am leaving on vacation today, so I will not be able to drill down on this further. But when I do, I will respond with my findings.


1985 318i (My First); 1996 535i (Loved it/sold it); 1999 323is (My kid likes'em too); 2003 325i (BMW Buy Back)

2005 530i (Best Ride Ever); 2007 x5 (Loved the look, but not the maintenance); 1976 2002 (Bitch'in ride)

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yeah... same deal- our '74 would run all day beautifully, but if you turned it off and tried to re-start it while it was warm.... a few seconds of motoring and then no go.

we had initially pursued fuel line and carb issues, but a simple pump swap did the trick.

I believe the rubber diaphragm was worn out and would stick when it got hot, but would run fine when cold fuel was running through the pump.

I'd bet a pint of stout on it-

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One thing not mentioned yet is junk in the tank, and no prefilter or screen.

It'll lodge in the valves in the pump, letting fuel drain back to the tank,

and then you'll have 'dead fuel pump' syndrome.

Perversely, putting the filter in front of the pump, which fixes THAT problem,

can cause other starting/running problems, as the air in the filter

can act as an air spring and reduce the efficiency of the pump, especially

when there's a little bit of an air leak somewhere in the line.



"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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  • 3 weeks later...

can you access the rubber diaphragm on the old Pierburg pumps? Mine does not have any screws or any visible means for access to the inside

1970 Agave work in progress

Born on May 14 1970 and delivered May 19th 1970 to NYC to Hoffman Motor Corp. Agave code 071

new guy

help appreciated!

other cars: 1991 318is / 1999 540iT

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  • 7 years later...

Hi Dave!  I'd have no clue as to how to open up that style of pump.  I replaced the one that came on my car with the screw-top variety, liking the idea of it being serviceable, but I goofed and bought the short style pump that sits down hard on the coolant hose running to the manifold.  I ran that pump for a few years, before putting my original crimped pump back in a couple years ago and It has been working just fine ever since. 


The inlet/outlet nipples come off of mine at different angles than IanIreland's though.  Is that a VW pump?  If you do wind up replacing that one, I'll suggest avoiding that short style pump that rides on top of that hot hose.  The taller ones cost a little more, but it is money well spent.  This is the one I'd avoid.



@IanIreland , how have you tested your failed pump?  Does it have a ruptured diaphragm?  Can you blow through it in both directions?  Just curious.  sometimes they have trouble supplying fuel due to old fuel hoses in the trunk, or a cracked plastic sleeve on the tank's sender nipple.


If you do find that the plastic sleeve is cracked, you can snip off a piece of the plastic fuel line that runs through the cabin and push that onto the nipple.  I learned that from Halboyles.  I applied the same trick to my fuel pump's outlet nipple too.








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