Fuel pump refusing to prime/pump

Afternoon all

I've got a problem with the fueling on my '92 ST1100 - for one reason or another the fuel pump isn't priming or pumping. It died last week, four or five uses after a 2 week spell off the road while I was on holiday. There were no warning signs - it worked in the morning and didn't in the afternoon.

Since then, I've tried a different fuel pump, changed the relay and checked all the fuses but it's not getting me anywhere. I've also had a go at testing the vacuum pump but that didn't tell me anything new. For the sake of completeness, I've checked the handlebar cut out switch, and that seems to work fine (no power to the engine when off; engine turns over when on). There's half a tank of fuel and the electrics to the float/gauge are fine.

Any suggestions where I need to be looking?

Thanks all


Neil

Can you hear it whir when turning the ignition ON? Removed the mains hose from the fuel filter and held it over a (fuel resistant) container/bowl/else to check if its actually (not) discharging? Voltage at the pump terminals (brown/red - green)? If not measure your way back across the fuel cut relay. I'd also check the fuse box as well as all accessible ground connectors for corrosion. Faulty ignition lock switch-block/wiring/connectors also come to mind...

Maybe it isn't the pump. As a test have you tried to bypass the vac fuel cutoff? Just throwing it out there as it is easy to do, a known point of failure and could cause an issue like you are seeing.

Hey ST

Nope - no whirring when ignition is on. No sound at all and no fuel being pushed through the hoses. That's what made me think it was the pump or relay was the problem. Fuses are all fine and everything they run works no problem. I can see the fuel pump relay has some crossovers with the lighting circuit and everything there checks out. I can't see any breaks in the wiring and I'm assuming that if the ground connections were knackered, there would be other electrical troubles.

I've tried the vac fuel cutoff with a vacuum pump and got nowhere, but I have to confess I haven't tried bypassing it completely. Having said that, I wasn't sure whether the pump power was dependent on the vacuum. I kinda assumed the pump would work regardless of the vacuum (hence the test where you put the end of the hose from the pump into a jar to collect pumped fuel).

Meantime, I've just discovered that I've swapped out the three pin/red plug fuel relay, which is different from the four pin/white plug fuel cut out relay. I'm assuming that it's possible that's gone sideways and is causing the trouble . . .

Hmmm.

the-chauffeur said:

Hey ST

Nope - no whirring when ignition is on. No sound at all and no fuel being pushed through the hoses. That's what made me think it was the pump or relay was the problem. Fuses are all fine and everything they run works no problem. I can see the fuel pump relay has some crossovers with the lighting circuit and everything there checks out. I can't see any breaks in the wiring and I'm assuming that if the ground connections were knackered, there would be other electrical troubles.

I've tried the vac fuel cutoff with a vacuum pump and got nowhere, but I have to confess I haven't tried bypassing it completely. Having said that, I wasn't sure whether the pump power was dependent on the vacuum. I kinda assumed the pump would work regardless of the vacuum (hence the test where you put the end of the hose from the pump into a jar to collect pumped fuel).

Meantime, I've just discovered that I've swapped out the three pin/red plug fuel relay, which is different from the four pin/white plug fuel cut out relay. I'm assuming that it's possible that's gone sideways and is causing the trouble . . .

Hmmm.

Click to expand...

Yes, you are right that the fuel pump will work with the vac cut off working or not. So if you disconnect the fuel filter and are not getting any gas there then it would be a fuel pump issue. Check for proper voltage at the fuel pump or do a flow test as outlined in the Service Manual. There is also a fuel pump relay that could be flaking out. If memory serves me it is located up under the cowling behind the headlight. Remove the windscreen and black plastic bit below it and you should be able to access the relay. There are several in that area.

For the vac cutoff a vac pump wouldn't really do much in terms of testing. The cutoff has a diaphragm in it. The vac from the engine pulls the diaphragm open and that action opens the cut off allowing fuel to flow through it. With age this diaphragm will rupture and the engine vac can't open up the cutoff anymore and you will have fuel starvation issues. Applying vac from another source (hand pump) wouldn't do any good as the vac would only open the valve.

EDIT: I went back and reread your first post. I see you messed with the relay already. I would look for voltage and verify the flow rate.

Given the way it stopped - as in, it's a sudden failure (as opposed to intermittent fault) and I haven't messed with anything recently - I'm guessing it's the relay I haven't replaced. I get the feeling that had I buggered about with something or loosened/split wires, the system wouldn't have just failed like it did - there would have been a period of decline/intermittence.

I'll wait 'til the other relay turns up - it was only $10 - and try that before I start wielding the voltage tester. The slight worry for me is that it could be something to do with the CDI, but even then I can pick one of those up for not a huge amount.

It's all a bit odd.

Also if you have the service manual I want to say the process for testing involves removing the relay and doing a jumper across the socket. This would eliminate the relay as the problem.

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From my manual.



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