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Discussion Starter · #1 ·
Hope to get inputs on where I should start looking-

My m5 does some hard starting when the engine is cold. If it does a hard start (3-4 seconds of starting it max), I turn to position1 then start it again, always works. never missed.(so two clicks max)

Just to expound on some history:

1.) car is tuned
2.) have an exhaust installed
3.) just replaced fuel pump

Originally, the previous owner mentioned that there is a fueling problem: When the gas tank goes to 1/4, The car dies because it senses it has ran out of fuel. upon checking TEST 6, I see that the fuel from the left tank still has fuel, but the right tank has 0. I immediately changed the fuel pump and saw that they initially put in an e39 525 fuel pump (only 3.5 bar *dumb) I honestly believe that this car was tuned to overcompensate the wrong and underpowered fuel pump. Immediately, I felt better idling. I have hoped that the hard starting would be gone.

Unfortunately, the car is still hardstarting.

Any leads to where i should look next? Should I be looking at fuel pressure leaks?

11,285 Posts
Should I be looking at fuel pressure leaks?
Yes more than likely you have a few issues. Your pump that transfers the left tank to the right is shot. There have been many instances of that reported here. I have not had that issue so only know that a good term to do a search under would be "transfer pump"or "siphon pump".

Hope you have your old filter left you will need part of it to look for pressure leaks. There have been other causes of delayed starts but since you are asking fuel I will give you fuel and it is still the best first choice to rule out. In this thread I gave a description with pics of how to see if the injectors are leaking. You just need to take it a couple steps further. Do two more pinch offs to determine which of the three places that are suppose to not let the fuel escape is not doing its job. Injectors could drip, the pressure regulator could leak, and the primary regulator on the fuel pump could leak. It is all simple just pinch off lines without doing damage to the line. That should get you started.

906 Posts
The motive force that drives the jet siphon pump system (to move fuel from the left to right sides) is the "return" fuel pressure.
This pressure, in turn, is controlled by the fuel pressure regulator. The reference pressure this regulator uses is the engine's intake
manifold vacuum from it's source is at the #7 throttle body. The vacuum line connection to the throttle body is prone to deteriorate
from the surrounding area temps. If vacuum supply to the regulator is not present, the regulator will provide the full 5 bar pressure
(wide-open-throttle mode) to the engine fuel feed, which then closes the regulator's port to the return line, resulting in a loss of
pressure to operate the jet siphon pump system.

In addition, with your having the fuel pump replaced, it may well be the interlock line connection (from fuel transfer line going over
to the left side) to the fuel baffle located under fuel pump was not re-connected properly, which will also result in loss of transfer
to the right side.

Ideally, (if you have access to one) you can hook up a pressure gauge to the Schroeder valve test port, located at the fuel filter/regulator
assembly. With the engine at idle, pull off the vacuum line from the pressure regulator and you should then see pressure go to the full,
5 bar pressure. Obviously, checking for presence of full manifold vacuum while at idle should also be noted (reference above).

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