K 75 Won't Run (well) - long

[Gentleman Sawye]

Hello. I'm at my wits end with this bike. Long story short: Bike is an '88 K 75C with 6400 miles. While riding one day it quit. Was running fine and just quit. I determined the fuel pump was not getting power and traced the problem to the fuel level sending unit. I replaced that, started the bike up and it would try to run and idle, but when I opened the throttle, it would quit. Before long, it wouldn't even idle for more than 5 or 10 seconds. I checked the fuel pressure on the feed side of the rail and it would peg the meter (40 lbs.) on startup and then gradually fall off to nothing. I pinched off the return line and got the same results. All this was done with the fuel filter off, since I didn't have a new one at that time; the fuel line went straight from the pump to the metal line inside the tank. I figured my problem had to be the fuel pump, so I ordered a new one along with a new filter. Put them on this evening and the result was about the same, although it will idle a little longer. Does anyone have any ideas what else I can try before I haul this thing off to a Dealer?

[CustomSarge]

You might try:
1> Pull the fuel pump relay & put a jumper wire to power the pump directly, see how it runs. If ok, it is a power issue.
2> Make a set of spade terminal extensions to allow access to the fuel pump relay. Put a meter on the fuel pump side of the fuel pump relay & see if power cuts out. If cuts out, put the meter on the coil side of relay. If This cuts out, the engine computer is suspect (ouch). If not, replace relay. Good hunting & please post progress <<<)))

[deilenberger]

I'll be adding some paragraph breaks...

Originally posted by Gentleman Sawye
Hello. I'm at my wits end with this bike. Long story short: Bike is an '88 K 75C with 6400 miles. While riding one day it quit. Was running fine and just quit. I determined the fuel pump was not getting power and traced the problem to the fuel level sending unit.
Exactly how did you determine the fuel level sending unit was tied to the fuel pump?

And how did you determine the fuel pump wasn't receiving power?

The fuel-level sending unit has no connection to the fuel pump.. understandably - K bike wiring diagrams can be a bit difficult to read.

I replaced that, started the bike up and it would try to run and idle, but when I opened the throttle, it would quit. Before long, it wouldn't even idle for more than 5 or 10 seconds. I checked the fuel pressure on the feed side of the rail and it would peg the meter (40 lbs.) on startup and then gradually fall off to nothing. I pinched off the return line and got the same results.

What sort of meter were you using? The results you saw were more or less normal except the reading are a bit high. The pressure should increase when the starter is running since the fuel pump also runs at that time. If the engine doesn't continue running the fuel injection automatically cuts off the fuel pump. This is NORMAL behavior.

All this was done with the fuel filter off, since I didn't have a new one at that time; the fuel line went straight from the pump to the metal line inside the tank. I figured my problem had to be the fuel pump, so I ordered a new one along with a new filter. Put them on this evening and the result was about the same, although it will idle a little longer. Does anyone have any ideas what else I can try before I haul this thing off to a Dealer?

Well... you should stop flailing.

Flailing is replacing the part that looks like it might be the problem without knowing if it IS the problem. It will eventually work if you replace all the parts on the bike - but that is rather expensive. I'd have to guess you've spent about $300-350 so far?

Some diagnostics are needed:

We'll assume you checked the fuel pump fuse? It's the 2nd one from the bottom. 7.5A.

1. Pull the plugs - and tell us what you see.

o - if the plugs are bone dry after trying to start the bike you probably have an fuel related problem. From the pump readings you give (and we don't know how accurate your gauge is), I don't think you have a fuel pump related problem... you may have another problem related to the injectors.

o - if the plugs are wet - then you have an ignition problem.

2. If they are bone dry - start at the fuel pump and try to measure the voltage.

This is done with a voltmeter and clip on leads at the pump terminals. If they are under gasoline this is a GOOD thing since that means there is very little chance of blowing yourself up. Make the connections before trying to start the bike - then turn the ignition on and press the starter. If you see 12VDC or more on the pump terminals - it's not a power problem to the pump.

If there is no power to the pump (unlikely given your pressure readings) - then you work backwards from the pump. You next go to the tank connector, and using your wiring diagram determine the two feed wires to the fuel pump. Measure the voltage there. If there is voltage there, and no voltage to the pump you've now isolated where the problem is occuring (and is has happened where the wires coming from the feed-through in the tank to the pump have gone bad. RARE, but not unknown.

If there is no power to the connector - then continue working backwards - next check is the fuel injection relay. It's in the electrical box under the tank. Same process - read the diagram, check that voltage is in the correct spot.

3. OK - if we have wet plugs - - that points to an ignition problem.

The test for this is - leavce 2 plugs in the engine and connected up. Take the third plug (doesn't matter which one) and very carefully ground it and connect it to the coil.

Turn the engine over while observing the plug - see any spark?

After doing these diagnostics - get back to us and I'll try and point you in the direction of the problem.

Please don't flail, although I wouldn't mind if you wanted to give me the parts you've replaced.. :) Oh - also - it's lots easier to reply if you use some paragraph breaks, or even better - numbered paragraphs. It makes it easier for us to reply, meaning you may have better luck with the answers.

Best,

[breyfogle]

I had similar symptoms a few years ago. The fuel pump would not run reliably. Sometimes yes, mostly no. Not suspecting the computer, the dealer tried all kinds of normal trouble shooting things to systematically isolate the problem. He thought he had it fixed several times only to have it fail again a few minutes later. Turned out to be the fuel injection computer (under the tank, not under the seat).