85 KRT100 not starting, problem with fuel system

[pinhead]

I have a 1985 KRT100 that turns over but does not try to start.  With a little starting ether in air intake fires up very briefly so its not getting any fuel.  I have looked a little on fuel pump replacement and have seen a number of you tubers replacing fuel pump which was not the issue.  Can any one give me a way I can diagnose this with out replacing or tearing down anything before I know what is wrong?

[frankenduck]

#1 free thing to do: Pull the fuel filter from the tank and try to blow through it in the direction of the arrow. It should be easy to blow through it.

Replace:
https://www.amazon.com/gp/product/B000C9WJBM

[pinhead]

I doubt its a problem with clogged fuel filter but thanks anyway

[frankenduck]

I doubt its a problem with clogged fuel filter but thanks anyway

How do you know without trying?

My most sincere apologies for suggesting a free way to start troubleshooting. I will ignore this thread henceforth.

[pinhead]

I checked fuse for pump and it appears good.  But when I turn ignition on there is voltage to every fuse but no voltage to the fuse for the fuel pump.  Where is the fuel pump relay on this bike?

[Laitch]

If the fuel filter is clogged, there will not be sufficient pressure at the injectors to deliver sufficient fuel to the cylinders for ignition. If you already have installed a new fuel filter oriented in the correct direction, and you have checked that the fuel tank's interior hoses aren't leaking and they are fastened tightly, say so and let us know what other new fuel and ignition components you might have installed.

The fuel injection relay that supplies power to the fuel pump's wire when the engine is cranking or running is in the relay box beneath the tank. You need to check for voltage on the yellow wire that connects to the fuel gauge under the tank while the engine is being cranked. First, verify the battery is fully charged.

[Laitch]

The fuse can be good but its terminals/receiving sockets can be corroded and cause pump failure

[pinhead]

What is really strange is that when I first checked fuses and put them back, the bike started and ran normal.  I thought it was a loose fuse.  I shut off the bike and came back a few hours it had the same problem again, no start at all!  And it isn't a clogged fuel filter for sure other wise it wouldn't act this way.  Either an electrical component between the fuse and the fuel pump which supplies electricity to the fuel pump or connections.  And I dont think there is anything wrong with the fuel pump, filter or injectors since how could they work if there is not electricity supplying the fuel pump when the ignition is turned on?   And according to the wiring diagram, the fuel injection relay supplies electricity to the fuel pump fuse.  They didnt make this bike easy to service, requiring removal of fuel tank just to check a relay.  My wiring diagram shows a mini fuse under there too! 

[Laitch]

Crank the engine while checking for current upstream and downstream of fuse #6. The fuel tank doesn't need to be removed; it needs to be tipped and braced so the lid can be removed from the relay box. Fuses with good bodies but bad connections can fail intermittently.

[pinhead]

I will try that first.  I have a spare bike with a relay I can salvage from it and see if that is the problem.  Thanks!   When it does start I can rev it up with no problem, so I cant believe its a clogged strainer.  It is an intermittent electrical problem, either wiring, relay,  fuse connectivity, or failing fuel pump, at least thats what i think.  If replacing the relay makes no change, I will take the fuel pump and filter out, probably replace them and also inspect all the wire connections.  And of course, replace the fuse.   

[Laitch]

When it does start I can rev it up with no problem, so I cant believe its a clogged strainer. 

You can rev it up, but can you ride it any distance?


When a fuel filter is not completely clogged, fuel pump residual pressure can slowly force fuel through the partial clog when the engine isn't running, filling the fuel hose and the rail. When the engine is started, there is enough fuel to run and rev the engine, then fuel entering the filter under pressure becomes turbulent inside the filter and turbulent fuel can't pass through the filter easily, so the engine stalls from fuel starvation. As frankenduck suggested, blowing through the filter in the direction of its flow should verify the filter's condition.

The fuel injection relay only supplies current to the fuel pump when the engine is cranking or running. Testing on both sides of the fuse while cranking the engine will determine if there is an electrical fault in that region.

[Past-my-Prime]

I checked fuse for pump and it appears good.  But when I turn ignition on there is voltage to every fuse but no voltage to the fuse for the fuel pump.  Where is the fuel pump relay on this bike?

I may have missed a critical part of the discussion so will hazard a question: is there voltage to the fuel pump fuse when you are actually cranking the bike? If there is, then my best guess would be the four pin connector under the fuel tank, that sends the power onward to the pump itself. These can be finicky!

Best of luck and keep us posted.

[pinhead]

OK, I have been on this blog for a number of weeks, maybe months now, trying to diagnose why a 1985 K100 won't start except when you use some starting ether.   Most if not all of suggestions dealt with the fuel pump and one even suggested a clogged fuel filter.  That didnt sound logical at all to me since if I got it started it would run like a champ until I shut it off.  But oh well.  I took the fuel supply line off that goes to the injector rail and it pumps fuel when I hit the starter like it should do.  I have taken the fuel filter out and I can blow through it easily.  I know, or at least i think i have known from the start that it is an electrical problem relating to the FI.  Hence the strange intermittency.   I have switched out the fuel pump relays and no change.  I might do a final hail mary and replace the fuel pump anyway, cause that s what dumb people do when they dont know what's wrong.  Like changing a fuel filter when  vehicle won't start.  But I can tell you after that, without any other clue of what's going on, this baby is to the boneyard!  I got my money out of it long ago. 

[degoe]

Did you try this using this guide?

https://bmwmotorcycletech.info/K100%20troubleshooting%20Starting.pdf

[daveson]

Before you get a pump, check if the injectors are opening and closing. Using a screwdriver as a stethoscope on the injectors you should be able to hear them clicking.

[pinhead]

Did you try this using this guide?

https://bmwmotorcycletech.info/K100%20troubleshooting%20Starting.pdf

Wow!  That is a great diagnostic schematic and something I wish I would have known 3 months ago before starting my tinkering.  Thanks!   Its got some really great suggestions on diagnosis rather than guessing its this part, etc. 

[pinhead]

Before you get a pump, check if the injectors are opening and closing. Using a screwdriver as a stethoscope on the injectors you should be able to hear them clicking.

  I presume you didnt read my ongoing trauma with this thing.  I can get it started with ether and once I keep it going,  (I assume a little warm up necessary) it runs like a raped ape until I shut it down and its a cold start again.  To me from the start, it seems like an electrical problem, either a connector or a component failing. 

[Laitch]

I presume you didnt read my ongoing trauma with this thing.  I can get it started with ether and once I keep it going,  (I assume a little warm up necessary) it runs like a raped ape until I shut it down and its a cold start again.  To me from the start, it seems like an electrical problem, either a connector or a component failing. 

It's likely to be both an electrical and a fueling problem. If the coolant temperature sensor gives a signal to the the fuel injection control unit that the engine is warmed up when it isn't warmed up, then the engine will be hard to start. If the signal doesn't indicate that the engine is completely cold, the engine will be difficult to start but eventually can be started, then as it warms up it runs better.
In summation, the coolant temperature sensor is an electrical component activating a fueling component. A coolant temperature sensor can be partially defective and produce this outcome. It can be tested while it's in position on the Brick, or it can be removed and tested.

[daveson]

  I presume you didnt read my ongoing trauma with this thing.  I can get it started with ether and once I keep it going,  (I assume a little warm up necessary) it runs like a raped ape until I shut it down and its a cold start again.  To me from the start, it seems like an electrical problem, either a connector or a component failing.

True, missed it, I think I found the thread you're referring to, I'll post some tips on it that might help.

I agree it seems like it might be an electrical problem, that's why I asked if the injectors are working.

[daveson]

I'm a bit late to this thread.

It seems like this thread has stalled. There have been a few questions to try to narrow down the problem, but you haven't answered them. So as mentioned the fuel filter might be partially blocked, is it? Only you can answer that.

I'll ask the last question in a different way, when you press the starter button do you hear the pump? That would confirm you have power on both sides of the fuse for the pump, plus a few other things.

[daveson]

For how long do you have to spray ether before it runs without help?

Some unlikely suggestions, but quick checks. If you open the fuel cap, does the problem disappear? After starting attempts, if you remove the vacuum hose from the fuel pressure regulator to throttle body four at the throttle body, is there dampness in the hose?

This thread might be getting to the point where you want to put on a gauge so we can find out what the fuel pressure is.

[pinhead]

the tank has been cleaned and filter replaced.  Took tank off and cleaned all electric connections under tank.  Fuel pump is gushing out fuel when I test at fuel rail.  It is some where in the operation of the fuel injectors, they are not operating.  And I have already replaced coolant temperature sensor.   Its in the complex of the fuel injector controls some where?

[degoe]

Hall sensor?
Airflow mass sensor?
Connectors clean?

[daveson]

It is some where in the operation of the fuel injectors, they are not operating.  And I have already replaced coolant temperature sensor.   Its in the complex of the fuel injector controls some where?

I'm also thinking that could be it, that's why I asked the above questions, which might quickly point to where the problem is.

I like the suggestion from degoe, cleaning connections sometimes fixes all kinds of things, and is good to do regardless, including the fuel injection computer under the seat, sounds like you've cleaned the computer plug under the tank.

[PitchlerBrick]

Check the wires inside the tank. Mine had been replaced by someone in the past and the insulation was missing in many areas. The bike would run great until something slightly moved inside the tank and then blown fuse and no go.