Author Topic: BMW K75 - Won't start with fuel pump connected (Read 7831 times)

Johnny K75 · « **on:** June 21, 2020, 01:12:09 PM »

Hello,

I am wondering if anyone can help me.

I have a 1987 K75 which I am turning into a cafe racer. I have built this all back up from the ground up, with new parts.

When the tank is all plumbed in and the electric connector connected. I press the starter and here to fuel pump prime the system. The bike just turns over and will not start. However, if I disconnect the 4 pin electrical connector the bike will start up and runs a dream until all the fuel is used etc.

I can repeat this process over and over, plugging the 4 pin back in priming the system, disconnecting the 4 pin and the bike starts up and runs fine until the fuel in the pipe and rail is used.

Please can someone assist as I am lost as to why this is as I am at my wits end.

-- 4 pin connected - Won't start --

-- 4 pin disconnected - Started --

Regards,
Johnny

volador · « **Reply #1 on:** June 21, 2020, 01:48:20 PM »

Is the fuel pressure regulator connected properly?

check hoses to the Fuel Pressure Regulator found mounted behind the throttle bodies
1-vacuum hose from throttle bodies to the bottom FPR
2-middle hose from petrol fuel injection fuel rail
3-top hose to petrol tank return line

Johnny K75 · « **Reply #2 on:** June 21, 2020, 03:40:54 PM »

Hi,

Thanks for the reply. I will double check that this week and let you know.

Thank you,
Johnny

Laitch · « **Reply #3 on:** June 21, 2020, 03:53:15 PM »

Quote from: Johnny K75 on June 21, 2020, 01:12:09 PM

When the tank is all plumbed in and the electric connector connected. I press the starter and here to fuel pump prime the system. The bike just turns over and will not start. However, if I disconnect the 4 pin electrical connector the bike will start up and runs a dream until all the fuel is used etc.

On a K75, the fuel pump really isn't heard until the starter button is released. Then it whines for a couple of seconds. Some of these motos will run on gravity-fed fuel. Read this. You should establish whether your fuel pump operates and whether it is receiving current from the four-pin connector.

Johnny K75 · « **Reply #4 on:** June 21, 2020, 04:17:31 PM »

Hi,

Fuel pump working, I put a video up showing this to help me diagnose the issue. You can hear it every time I press and release the starter

Regards,
Johnny

Laitch · « **Reply #5 on:** June 21, 2020, 04:21:16 PM »

Quote from: Johnny K75 on June 21, 2020, 04:17:31 PM

Fuel pump working, I put a video up showing this to help me diagnose the issue. You can hear it every time I press and release the starter

Is it pumping fuel?

Johnny K75 · « **Reply #6 on:** June 21, 2020, 04:28:09 PM »

Hi,

Yes, it seems to be. The pump is brand new and the fuel rail & pipe seems to fill and when disconnected you can see the fuel inside the pipe. Also, the excess pressure of the fuel going back into the tank.

If I then disconnect the 4 pin to stop the pump working, the bike starts straight up (second video) it revs and ticks over nicely for around 20/30 seconds until all the fuel in the pipe and rail has been used.

Regards,
Johnny

Laitch · « **Reply #7 on:** June 21, 2020, 04:32:16 PM »

Quote from: Johnny K75 on June 21, 2020, 04:28:09 PM

Yes, it seems to be. The pump is brand new and the fuel rail & pipe seems to fill and when disconnected you can see the fuel inside the

Let's get back to this after you've verified the correct connections indicated in Reply #1.

Johnny K75 · « **Reply #8 on:** June 21, 2020, 04:38:50 PM »

Will do, thanks for your help. I'll post back soon, once stripped down and double checked.

daveson · « **Reply #9 on:** June 21, 2020, 08:49:21 PM »

A quick check which might point to a cause.

With a screw driver on an injector, and its handle to your ear, you should hear a "tick, tick, tick" while idelling. That's the injector turning on and off. If not, since fuel is returning to the tank, that points to constantly open injectors, causing overfueling. That points to constant earth to the injectors, that's a problem. Other symptoms might be a strong smell of fuel, or fuel dripping from the headers/muffler joint, after repeated failed attempts with the pump connected. That would also explain the delayed start in your second video, it takes time to clear out the excess fuel.

Often, overfueling is caused by a faulty coolant temperature signal.

Bon Jon Bovi · « **Reply #10 on:** June 25, 2020, 09:39:00 AM »

In addition to what volador and Laitch suggest, you could also unplug the pump to get the bike started, then plug the fuel pump connector back in. Will it continue to run smoothly, run rough, die immediately?

Johnny K75 · « **Reply #11 on:** June 28, 2020, 10:14:10 AM »

Hi all, thanks for all the assistance.

I've checked the fuel pressure regular and everything is fine that side.

I've checked the injectors and they spray but looks quite a bit of fuel, so not sure if they're stuck open. I'm going to send them away for testing even though they where brand new but do look slightly different to the originals. Does anyone have a picture of the spray to show how much should come out on priming the system?

I've heard about the coolant sensor behind the radiator, so I will be purchasing a new one of those too after checking these injectors.

-- Yes, bike runs and soon as the cable is plugged in the bike dies instantly.

Johnny

daveson · « **Reply #12 on:** June 28, 2020, 10:59:40 AM »

Hi Johnny,

I'm hoping you disable the coils before checking the spray pattern, so you don't have a possible fire on your hands. Consider finding out what the problem is before spending money. If it was a constant spray of fuel, that points to constant earth, if it was a series of sprays, maybe it's the temp. sensor.

Your bike runs with the fuel pump disconnected, that's a sign of overfueling. It stops with the fuel pump connected, that's another sign of overfueling.

How about the "tick, tick, tick."

Johnny K75 · « **Reply #13 on:** June 28, 2020, 11:56:41 AM »

Hi Daveson,

Thanks for the reply.

I can confirm it seemed a constant spray. The earth cables coming from the loom for the injectors which sit under the tank, I originally checked them and made sure I cleaned them etc. Can you confirm what would cause the constant earth and advise a bit further?

Regards,
Johnny

daveson · « **Reply #14 on:** June 28, 2020, 12:46:14 PM »

I had a constant earth problem with one of my bikes. I don't know how I fixed it. I promised myself not to repeat this, because it doesn't make sense, but the problem disappeared after I replaced the fuel injection relay. I also removed and reinserted fuse 6 before that. Maybe the yellow grey wire is earthing somewhere, that makes sense but I couldn't find it in my case.

Since you rebuilt it, the temp. sensor is clean. If you used teflon tape (or anti-seize) on the temp. sensor, remove it, that might interfere with a good earth.

Somewhere on this forum is a way to test the sensor.

Laitch · « **Reply #15 on:** June 28, 2020, 01:55:21 PM »

Quote from: daveson on June 28, 2020, 12:46:14 PM

I had a constant earth problem with one of my bikes. I don't know how I fixed it.. . . but the problem disappeared after I replaced the fuel injection relay.

That's how you fixed it.

Johnny K75 · « **Reply #16 on:** June 28, 2020, 02:31:59 PM »

Thanks, I've just ordered a fuel relay. I will keep you updated on how it goes when it gets here.

Laitch · « **Reply #17 on:** June 28, 2020, 02:44:28 PM »

Quote from: Johnny K75 on June 28, 2020, 02:31:59 PM

I've just ordered a fuel relay.

Past performance is no guarantee of future results.

volador · « **Reply #18 on:** June 28, 2020, 02:45:09 PM »

Quote from: Johnny K75 on June 28, 2020, 02:31:59 PM

Thanks, I've just ordered a fuel relay. I will keep you updated on how it goes when it gets here.

Amazon Lime Relay

Scott_ · « **Reply #19 on:** June 28, 2020, 04:28:43 PM »

As an electrician, I can't seem to wrap my head around the concept of how replacing a relay between the battery + terminal and the injectors would correct a fault in the ground line between the other side of the injectors and the controller.
A constant ground is going to be caused my either 1) pinched(ie damaged,skinned) wire between the injector plugs and the controller, or 2) a faulty controller.

So the injectors spray with the pump energized and the injectors connected to the loom.... just by chance, what happens if you disconnect the injectors from the loom and then power the pump, do they still spray? could the injectors be mechanically stuck open?
Just a thought.

Johnny K75 · « **Reply #20 on:** July 03, 2020, 04:16:07 PM »

Hi all,

Just like to say... I have eventually got the K75 running!

I purchased the fuel pump rely and fitted that, which seemed to fix the grounding issue but the motor was then locking up with unburnt fuel. I then read on another post about the ECU as it controls how much fuel should go to the injectors, so I had another one lying around, which I fitted and that resolved the issue and it started straight up.

Thanks for everyone's help and will post some pictures and videos of the bike when finished.

I also have another ECU which I found. It doesn't seem to fit but looks similar, can anyone tell me what it is off?

Cheers,
Johnny

Johnny K75 · « **Reply #21 on:** July 03, 2020, 04:21:56 PM »

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The Mighty Gryphon · « **Reply #22 on:** July 03, 2020, 05:33:33 PM »

If you're talking about the black one, it's an ABS controller.

daveson · « **Reply #23 on:** July 04, 2020, 03:24:21 AM »

Here's another way to test for constant earth if you're not confident with your hearing. I used a computer safe test light ( LED, this one glows green to positive and red to negative, and draws 10mA) Connect to battery positive and probe to the loom plug of an injector. With the ignition on, engine off, there will be a constant green glow at both sockets. At idle the light will be off on one side (positive to positive) and should flash on and off at the other, that is negative (right side in the photo, next post)

Don't be surprised if you hear a bit of a diesel knock develop, it's temporarily unbalanced with one cylinder out of action.

daveson · « **Reply #24 on:** July 04, 2020, 06:13:19 AM »

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With a fuel injected brick it's worth considering a computer safe test light. This one was just $5.50 at the local hardware store.

The test light can also be used to test the hall sensors.

This problem and the few others I've seen, differ from mine in that they idle well. By any chance did you try to ride it with the pump unplugged, or did it just die under load?

News:

Author Topic: BMW K75 - Won't start with fuel pump connected (Read 7831 times)