Bike only starts with the fuel pump disconnected

[paperhand]

Hello fellow members,

New member here,

Sometimes ago (about 2 years) I have acquired a 1991 K75 RT as a project bike that was a non runner in order to keep me busy.

The bike only stars in the following scenario: I have everything connected, I crank it to get the fuel in, unplug the fuel pump then the bike starts and runs until it runs out of fuel.
Worth adding that my next door neighbour has a working K75S that's he has dismantled to have it resprayed, etc, which gave me access to some parts to try on mine.

in my troubleshooting I've done the following:
- I've measured the fuel pressure between the fuel tank and the fuel rail, the reading is 3.4bar
- I've checked the compression on each cylinder and its 110-120psi
- I've checked the spark plugs, they all have the spark and also tried with another brand new set. No change
- I've checked the injectors if they are spraying, they visually work fine (I've not checked for the volume), but replaced them with another set from a neighbour then procured a replacement set. No change,
- I've swapped the ECU, ignition module and the entire wiring loom in case there was a short in mine with my neighbour's and no change.
- I've replaced the fuel relay and the start relay with my neighbours and no change
- I can hear the fuel pump working, I also tried connecting the pump directly to a battery to bypass the relay and fuses at the connector.
- When the bike is running and I connect the fuel pump, the bike immediately dies then does not start, unless I unplug the fuel pump connector again
- With the Choke fully on, the bike runs for a few seconds before dying after the fuel pump is reconnected.
- I have also checked for faulty neutral gear and kick stand
- I checked for air leaks, tried another air flow meter but again no change
- I've unplugged the TPS to simulate full throttle and no change

I was suspecting a faulty FPR and engine getting flooded, but the fuel pressure is constant at 3.4bar and stays there for while.
My alternator is currently disconnected. I dont think it it charging, when the bike runs without the fuel pump I only get 12v, but didn't think it would be an issue.

Is anyone able to shed some light on what this might be or have come across anything similar?


Thank you,

[Laitch]

There is excessive fuel pressure. Here's the specification from the BMW K100/K75 2V shop manual downloadable from this site.

[paperhand]

Thank you, I will check that and try a different FPR

[Martin]

Possibly over fueling due to faulty temperature sensor.
Regards Martin.

[The Mighty Gryphon]

Your fuel pressure looks like it is way too high.  It is overfueling the engine by about 30%. 

If the problem isn't in the fuel pressure regulator, then look at the check valve in the return line spigot where the fuel return line connects to the tank.

[paperhand]

Update…

I re measured the fuel pressure and it is 2.3-2.4bar, not 3.4 bar as previously stated. I read the value in PSI which is 34psi. My apologies.

I will check the temp sensor now as suggested.

[daveson]

With the help of a screwdriver as a stethoscope, you should be able to hear the injectors ticking. If they're not ticking, that's a problem.

[Laitch]

Verify that that the air flow meter's electrical contacts are clean and that it is firmly coupled with its electrical connector. You don't need to remove it from its location; you need to check the condition of its connection.