Hi, I hope some of you technical experts can help with a problem I am experiencing with #016. After a frustrating few months of inactivity caused by a faulty genuine BMW battery (less than 18 months old) I finally got the BMW dealer to replace it FOC after much persuasion. However on trying to restart the car after a few weeks inactivity it took most of a weekend to get it to run for more than a few seconds. It would turnover and when it fired would run OK for a minute or two and then just stop. It would then take many attempts to get it to fire again. Eventually I got it started and running OK.
Over a period of a couple of weeks it ran faultlessly but this weekend, after a couple of weeks inactivity, the problem has returned although a bit different. This time it started fine and ran for a couple of minutes before dying on me and leaving me stranded. I had to leave it for a few hours at the roadside and when I came to collect it, after a bit of coaxing it started and again has run fine since.
I am tempted to blame the fuel supply as the symptoms are just like running out of fuel but the problem only happens if the car has not been used for a while. I did replace the tank and the transfer pump assy earlier this year but it had covered several hundred miles since then before the battery problems resulted in lack of use for a period. Could a new transfer pump have put any extra load/pressure in the fuel system which has shown up a probelm somewhere else? It is almost as though there is an airlock getting into the fuel supply when the car is left for a period. When the car is running properly, though, there is no sign of any lack of fuel pressure caused by a failing pump.
Are there any obvious things I am missing, I was hoping that getting the battery sorted would give me some reliable motoring!
I have not experienced this problem but I think I am right in saying that the only thing left on the fuel side of things might be a dodgy fuel pressure regulator, unless of course there is a dodgy electrical connection to your fuel pump causing it to be intermittent...... Have you tested the rate of fuel supply at the fuel rail? The fact that the problem is intermittent suggests some form of electrical gremlin to me.
Thanks for your response, Piers M5. I am undecided as to whether it is an electrical or fuel supply issue but I don't have the means to test for either in any depth. All I know is that when it failed to start after I fitted the new battery all of my fiddling that finally got it going was related to fuel supply. I will try the car again this weekend , after standing for a week, and see if it re-occurs.
Update on yesterday's performance. I used the M5 for a 20 mile round trip. It started perfectly and I let it warm up for a couple of minutes before driving off (just to give me some confidence), the outbound trip was perfect with no signs of the previous cutting out problems. I left the car for a couple of hours and it started perfectly for the return trip. The initial drive away was fine but after a few hundred yards it lost all power and cut out, all the lights came up on the dash. As I had momentum I bump started it again but it was struggling to rev, it cleared itself for a short while but any attempts to increase revs caused it to stall out.
I coasted into a layby and it restarted first time. I held it at a fast idle for a couple of minutes until it seemed OK and attempted to get going again. During the idle there was some evidence of 'missing' which cleared itself as the car warmed up. After this it behaved perfectly all the way home.
The only consistency with the previous weekend's performance was that it cut out during the warming up period. It starts perfectly from cold and drives without any issues when warm. Is there something (electrical or fuel supply) that could be triggering this during the warm-up process - I assume there is some enrichment device that is gradully reduced as the car warms up. Also both occasions where it cut out were when accelerating up through the gears.
Any more ideas would be gratefully received. I am trying to use the car as much as possible but need to get this sorted first.
Thanks Davidoli, I assume the fuel pump relay lives in the glovebox with the ECU? My Bentley manual (for 'up to' the 535i) shows it on the side of the underbonnet fusebox but I guess the M5 is different in this respect. Mine has no relays on the engine-side of the fusebox. Looking at the pin numbers it appears to be the first relay facing you at the top of the glovebox.
I would consider a couple of things. Fuel filter, fuel pressure regulator (end of fuel rail), and of course the fuel pump relay. The next time it dies try turning the key to "on" and see if you can hear the fuel pump pressurize. You can also swap the fuel pump relay with another relay (some are identical) in the fuse box and see if it fires up. Relays go bad and can cause the symptoms you describe. You can also disconnect the fuel line and put it in a bucket and get someone to turn over the engine and see if it is pumping (being cautious of course).
Relay should be in the fuse box under the hood (the square things are relays)...if my memory is correct! Also, those weiner type fuses get corroded easily and lose connections, so you may want to check and clean them up also.
Last edited by emccallum; 10th November 2009 at 22:41.
I changed the fuel pump relay at the weekend (assuming it is the first one you come to within the top of the glovebox). The car performed perfectly on Saturday although I did let it warm up before testing it. However I did several trips with it starting from warm and each time no problem.
However, today starting from cold the old problem was back. It starts perfectly and performs OK for a minute or two and then just cuts out completely. The temp gauge needle is near the top of the blue. The cutting out seems to occur on a trailing throttle, either when you feather the throttle to change gear or when you back off completely. The car would not restart immediately but did after I had left it for a couple of minutes, then it was fine.
Any other ideas? I still don't really know if it is fuel supply or electrical. The suddeness of the cutting-out is suggesting it is more electrical as has been pointed out above.
When it cuts out, I would check fuel pressure. There should be a "how to" in a Haynes or Bentley manual. If the fuel pressure is ok, then check for spark at the plugs. There is also a relay that is called the main relay, that gives power to the ecu. Check the Bentley for its location. They are known to go bad. Did you say you have changed the fuel filter? You need to do some diagnosing to find out whether it is fuel or spark related.
From another site, this may help. Main relay: Remove it and look under it. You should see 85 the power side of the relay coil, 86 the coil ground side, 30 the main power side from the battery and 87 or 87a/b the output pin. Compare the relay prongs with the socket slots and find 85 in the socket. Test it for voltage with your meter, with meter ground to 86 or the the car body. When the key is off there should be no power. Key on there should be about 12 volts. This relay supplies power to the Motronic Control and to the fuel pump relay when it is activated
Last edited by emccallum; 16th November 2009 at 20:45.
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