E34 525tds starting problem

Tue Dec 01, 2009 9:31 pm

Has anybody had trouble with these getting air in the fuel line?one im working on keeps getting air bubles in the pipe between the filter on the inner wing and the diesel pump on the engine.once switched off i can see diesel feeding back down this pipe.the car also strugles to start.

Tue Dec 01, 2009 10:06 pm

Wed Dec 02, 2009 7:54 am

There's a non-return valve available to go between the filter & pump. I'll try to remember to dig the part number out.

Wed Dec 02, 2009 12:18 pm

Chris is the expert on these

Wed Dec 02, 2009 1:17 pm

The fuel pick-up in the tank has a O ring that may allow air to leak in. Thisis more of a problem when the tank is low

Wed Dec 02, 2009 1:34 pm

I doubt it would be a tank related problem, the E34 has a low pressure submerged pump

Wed Dec 02, 2009 6:39 pm

Thanks guys.

Wed Dec 02, 2009 6:44 pm

Had a look for the part number earlier, but the invoice is in archive, sorry. Try asking BMW for it, if you come up against a brick wall let me know and I'll track it down.

Problem appears to be that the in tank pump only primes on ignition with plugs, ie. cold. The valve mostly sorts the problem.

There is a frig to be done on the plug wiring which kicks them in regardless of engine temp, but I've never bothered with it.

Wed Dec 02, 2009 6:57 pm

There is a hot start mod for the pump now you mention it Chris, break into the wire and run an extra feed to the pump so you can turn it on via a dash mounted switch before cranking, once started simply turn off the switch

Wed Dec 02, 2009 7:00 pm

Yup, the other being what I mentioned involving a resistor in the plug loom.

Valve sorts it though, and with no risk of electricity spilling onto the floor.

Wed Dec 02, 2009 7:02 pm

Do you have a part number?

Wed Dec 02, 2009 7:04 pm

Nope, as mentioned above the invoice is in archive. Try the parts desk and if still no luck I'll dig it out.

Wed Dec 02, 2009 7:07 pm

Chris-W wrote:no risk of electricity spilling onto the floor.

Have you put those safety plugs in all your sockets now?

Wed Dec 02, 2009 7:09 pm

Bob_S wrote:
Chris-W wrote:no risk of electricity spilling onto the floor.
Have you put those safety plugs in all your sockets now?

Little pyrex bowls under each socket like you told me. Still waiting on the method you promised for putting it back in.

Wed Dec 02, 2009 8:56 pm

Any old non return valve will do, mine's in the return line IIRC. The lift pump only works for one second when the ignition key gets to the glow-plug position, so if stuck, lots of key on and off normally gets fuel there in the end.

If you require the 'hotstart fix' box of tricks, I've got one and it works brilliantly, and have contact details if needed.

Thu Dec 03, 2009 12:24 am

dont bother with one way valves and all the hassle! i had a 325tds e36 with exactly the same problem. the air leaks in from either the outer seal on the fuel filter (take the filter off and clean the rubber seal and contact area) or one of the seals on the feed pipe to or from the filter. make sure you fit a genuine fuel filter or seal from bmw and you should be fine. try the seals on the pre and after filter pipes first and im sure you problem will be solved..

Thu Dec 03, 2009 11:00 am

On The M51, the lift pump operates all the time to feed the main engine pump. It's operated by the glow plug relay to prime the main pump before start up, and once the glow plugs turn off, the relay switches the lift pump over the power from the alternator. This is a safety feature - if the car crashes and the engine stops, the alternator stops powering the lift pump and it stops pumping fuel.
The seals on the filter as the main issue, and on the 525tds there is a small bluey green O ring where the pump assembly fits into the plastic pump mounting cage in the tank. This goes hard with age and air gets in when the fuel level gets low.
It can be also the plastic pipes where they fit onto the filter housing, and fitting a couple of good and tight jubilee clips helps.
I have seen a 325td with a dash switch (an old HRW switch I think) that powered the lift pump. Get in in the morning, press the switch, leave it a few seconds whilst it primes the filter and pump and off it went. This was because the entire car was basically shagged.
Be aware that if you need to replace the lift pump, a petrol one will not do as it's a much higher pressure unit and will flood the thing with diesel.

Thu Dec 03, 2009 3:53 pm

I have a E36 325Td and had the same problem you could see the bubbles in the fuel line on mine to.
It turned out to be the fuel return hose on no1 injector had a small hole in it that allowed air to be drawn in when the engine was turned off , therefore making it a pig to restart .
Replaced the hose linking all 6 injectors as a precation and problem was solved for about a fiver
Cheers
Andy