K75 fuel pump

[36532]

I had to have a friend come pick me up on the side of the road in order to get back home from Wyoming. My 87 would not run in the heat of the day or when it got hot. It just started as I neared Gillette and got worse as the weather got hotter. Can I unhook the fuel pump wires and then hook up some small jumper wires to the battery in order to test the pump. When it will not run or start I have good fire to the plugs. Thanks CoachP

[Gilly]

Hope the bike is home now.
Unsure why heat makes a difference. You should hear the pump run if you "bump" the starter, like a rattlesnake noise from the tank. You might want to press your ear against the tank to hear it. When was the filter last replaced?

Gilly

[motoedde]

Not sure if you experienced vapor lock...if the process persists at colder temps, that might be the most likely culprit.

There is much more diagnosis left before pinning down the problem...speaking of which, check the tank connector pins on the right side of the tank for contact flexibility, corrosion, etc. These connectors, when faulty, interrupt the power supply leading to intermittent pump operation.

[deilenberger]

I had to have a friend come pick me up on the side of the road in order to get back home from Wyoming. My 87 would not run in the heat of the day or when it got hot. It just started as I neared Gillette and got worse as the weather got hotter. Can I unhook the fuel pump wires and then hook up some small jumper wires to the battery in order to test the pump. When it will not run or start I have good fire to the plugs. Thanks CoachPThe quick answer - yes. Do it with a bit of care since the fuel-level sensor is on the same plug.. make sure you get the right wires (don't have a diagram in front of me..)

[breyfogle]

The quick answer - yes.

Even easier: jumper the battery (+) terminal to fuse slot #6. No need to disconnect the 4 pin connector.

[deilenberger]

Even easier: jumper the battery (+) terminal to fuse slot #6. No need to disconnect the 4 pin connector.Good idea assuming the circut from the fuse to the pump connector is OK. Going directly to the connector eliminates that variable. I always like to start at one end of a circuit and work towards the other end, rather than in the middle.

[breyfogle]

Good idea assuming the circut from the fuse to the pump connector is OK. Going directly to the connector eliminates that variable. I always like to start at one end of a circuit and work towards the other end, rather than in the middle.

There is not much circuit to speak of other than 18 inches or so of wire. The fuse sits between the fuel pump relay output and the 4-pin tank connector. In effect, jumpering to the fuse is applying power to the wiring harness side of the 4-pin connector.

[Gilly]

Wouldn't the fact that it's a high pressure pump system pretty much rule out the possiblity of a vapor lock? I thought that was only on ol' buggy-whip carbureted vehicles.
Gilly

[Acejones]

Thats what I'd like to know. I haven't seen a vapor lock since I was 10 up on Mt. Washington. I'm 60 now. Damn, it's been a long time.

[Kayseventyfive]

My 87 would not run in the heat of the day or when it got hot. It just started as I neared Gillette and got worse as the weather got hotter.

How does it do with a fuller tank? Some submerged automotive pumps need a fair quantity of fuel to take away the operating heat. It is possible that your pump is just getting cranky about heat. There is a theory that a tank should not be run too low to avoid pump overheating.

Can I unhook the fuel pump wires and then hook up some small jumper wires to the battery in order to test the pump.

Just a caution----If you are working with the connections inside the tank, be sure the connections are tight so there is no possibility of a spark. Then apply power from the battery terminal or somewhere outside the tank.

[36532]

I have checked the pin connectors, the fuse is good, there is not a vapor lock. I took the filler cap apart and tapped on the pump itself while hitting the start button and no fuel being pumped. But now once in a while when its cool enough the pump will work for a few miles. I know not to get a spark in the tank! If the pump does not go and I hook up some jumper wires to the battery itself then I know to replace the pump ot look elsewhere. Don't ride your old K bike with the fuel lever low as the pump is not being lubed. thanks for all the help.

[motoedde]

There are three points to jump the pump from...

1. The connector you disconnect to remove the tank that leads to the fuel pump.
2. At the pump itself.
3. Under/inside tank connector itself.

Make sure the tank is fairly full when you're jumping the pump. Sparks in liquid fuel aren't the worry...ignition occurs with the vapors!

[motoedde]

Wouldn't the fact that it's a high pressure pump system pretty much rule out the possiblity of a vapor lock? I thought that was only on ol' buggy-whip carbureted vehicles.
Gilly

It is possible...and my only explanation for the following.

May of 2007, I ran my tank down to about .5-1 gallon of fuel. I was running the bike around 6k+ RPMs for hours on end mid day in 2/3rd gear. Outside temps were 110-120F...bike just stopped...fuel pump wouldn't go.

I checked connectivity and applied a voltage across the various parts of the circuit from fuse to pump. And juice was flowing and the pump tried but couldn't.

I let the bike cool down for about an hour...and it was all good...and then 15k miles later, the pump died on me suddenly on my way to Marin BMW!

What luck...I was 2 miles away!

Bottom line: VL is possible because of the proximity of our pump to heat sources(engine, cooling fan blow direction, etc.) on our bikes and the fact that gas boils/vaporises at a much lower temperature than water. Fuel in a tank acts as a coolant when the pump is submersed in it.

[Motor31]

I had a K75RT for about 14 years and rode it during Southern AZ. summers in heat over 105 degrees. Never had a vapor lock. I did have 3 pumps fail but that was due to gasahol. The pump didn't like alcohol in the gas and neither did the rubber components in the tank. It turned the rubber to tar. If the impellor is hitting a rough spot or has some excess slop in it then the pump can bind up especially when hot.

Jumpering the pump is simple. It's just 2 wires. I tested my pumps when they failed by jumpering them. I salvaged one pump when the impellor got a bit of debris lodged in it. I reversed the polarity of the jumpers and ran it backwards just a bit to clear the debris out. All of the jumpering was done with the pump out of the tank and using the bike battery.

[breyfogle]

IMHO, vapor lock is VERY unlikely on any fuel injected motor.

The temperature dependency of the problem sounds more like a classic HALL sensor failure. Also possible, but almost as unlikely as VL, could be a temperature related failure of the Ignition Control Unit which lives under the fuel tank (=> hot area). One of the things the ICU normally does is control when the fuel pump relay energizes the fuel pump. If it fails, or misbehaves, the FP will stop.

[deilenberger]

IMHO, vapor lock is VERY unlikely on any fuel injected motor.

The temperature dependency of the problem sounds more like a classic HALL sensor failure. Also possible, but almost as unlikely as VL, could be a temperature related failure of the Ignition Control Unit which lives under the fuel tank (=> hot area). One of the things the ICU normally does is control when the fuel pump relay energizes the fuel pump. If it fails, or misbehaves, the FP will stop.It might be unlikely, even VERY, but the earliest K bikes did experience the problem. Boiling in the fuel lines. The original bikes didn't have the aluminum pad on the bottom of the fuel tank, that was one of the items "fixed" on the later '85 K100's.

I do agree it would be worth checking the HES for temperature related failure. To do so (Jeff Dunkle method, I take no credit except remembering it):


Take the T shaped cover off the HES (front right side of the engine
Steal wifes hair-dryer if you don't have a heat gun
Run engine at idle.
Heat up the back of the HES plate with the heat-gun/dryer (don't overdo it..)
If engine starts missing or stops - the HES is suspect
If it does - then turn the heat-gun/hair-dryer to blow unheated air and cool it off, see if bike restarts.
If it restarts after cooling - it's likely the thermal failure the HES is known for.

[Kayseventyfive]

Gasoline boils.

If it boils in the fuel line, it is traditionally called vapor lock. If it boils in the tank, it is called boiling in the tank. This can happen starting at 102 F. Different components of gasoline start to boil at that temp, although altitude and tank pressure enter into this. In the Army, we had to keep our OD gas cans in the shade to prevent boiling in the desert. You could hear them boiling, and if a curious person released the pressure by unscrewing the cap, the boiling became much more violent.

If gasoline boils in a gas tank, it, by definition, produces vapor, which can be sucked up by the fuel pump. This results in vapor in the fuel line, which, if it is not the classical definition of vapor lock, it certainly mimics it. It is less likely to happen in a 38 psi system than a carb system with 6 psi, but it could be possible. I have experienced it in a carbureted car where an exhaust leak heated the gas tank. No experience on a FI vehicle, but it could happen.

Some motorcyclists have experienced gasoline boiling in the tank.

[motoedde]

It might be unlikely, even VERY, but the earliest K bikes did experience the problem. Boiling in the fuel lines. The original bikes didn't have the aluminum pad on the bottom of the fuel tank, that was one of the items "fixed" on the later '85 K100's.

I do agree it would be worth checking the HES for temperature related failure. To do so (Jeff Dunkle method, I take no credit except remembering it):

<SNIP>



Yup...fuel boilingis what caused my issue I'm pretty sure...as after an hour or so later, the bike started up no problem and didn't have an issue(except for the connector) until the pump finally died near the end of my trip.

BUT i did not check the HES then for thermal related failure...I could do it know as its the same one I have.