Fuel system system mystery

[rbm]

- the bike is running normally one minute and dies suddenly the next.
- it's possible the bike died during an intake cycle with all three injectors open.  However, with power removed, the injectors should return to closed position via the spring inside and not remain open.
- bike idles with no fuel supply or fuel pressure

Recommend:
1. Disconnect injectors from the harness as The Mighty Gryphon suggests. If it runs, possible one or more defective injectors.
2. When the bike is idling with the 4-pin tank plug disconnected, listen to each injector using a long screwdriver touching the injector body with the handle pressed against your ear (like a stethoscope).  You should hear clicks.  No clicks, possible bad ECU or poor connection or possible bad injector.

The suddenness of the failure sure points to electrical. I can't think of a mechanical failure that supports the observed behaviour of the engine.

[cnoon]

Here's the latest.  With the tank removed (and obviously the 4-pin connector disconnected), I made sure there was fuel in the rail.  As before, I set the choke on full and it started up and idled nicely (but will die immediately if attempt to open the throttle).  I turned it off and then unplugged the electrical connectors to all three injectors.  Tried to start it up and no firing at all.  Plugged the injector electrical connectors back in and it started up and idled as before. 

This seems to be supporting the theory that something is telling the injectors to stay open for longer periods of time (either a faulty airflow meter or the Motronic).  When the tank is on and the 4-pin connector is plugged in, the fuel pump is generating enough fuel pressure to immediately flood the bike.   In absence of fuel pressure, the extended open periods are allowing enough fuel to keep it at idle but not enough to throttle up.  The three injectors are not stuck open or it would have kept the idle thing going when the injectors were unplugged. 

So now, I guess my next step is to check out the air flow meter (which seems like a pain to get to, is that correct?).  Clymer says there's no bench test for it, but can anyone support or dispute that?  - thanks, Chuck

[rbm]

Some helpful videos:

[Freelancer]

Quick question.

You said Motronic. Is it really a Motronic or an
L-Jetronic?

I ask because all of the parts microfiche I looked at said L-Jetronic. Just trying to get clarification.

As to where to look from here?.....

Did some reading and there are 2 likely culprits.

First is the ECU (l-jetronic) ground. When checking, be very careful as to have ignition off and disconnect battery ground first.
Seems that one can easily damage these with a very small amount of misplaced current.

From what I read the bike will run even if there is no ground to the L-Jetronic type ECU. The effect that a bad ground has can be similar to what you've described.

It is also similar to the next possible/probable culprit.....


The CTS(Coolant Temp Switch)- While these almost never fail mechanically....., corrosion/electrolysis within the coolant system can lead to a failure. Also a broken/faulty wire to the switch can cause an issue.

What the CTS does is that it tells the ECU when your having a cold start. This results in longer injector openings and reduced airflow until the bike is warm.

When it goes bad, it has caused issues similar to what you are experiencing.


Tldr;
1) Check/clean ground to ECU
2) Check/clean ground to chassis
3) Check/clean ECU contacts
4) Check/clean wires and plug to the CTS
5) Test CTS by idling bike or warming radiator/coolant.
Then plug fuel tank 4 pin connector back in to see if warm motor fixes the problem.

Not promissing these are the solutions, just suggesting based upon a quick read of common L-Jetronic problems. Really sounds like a ground or cold idle issue.

Sorry for the wall of text but hope it helps.

Later,
Freelancer

Ps, The cracked tube you mentioned could cause some issues, might want to go ahead and fix it.  :2thumbup:

[Wombat]

Did you end up solving this problem? I seem to be experiencing something very similar. Injectors are new but seem to be letting way too much fuel through and flooding the motor after initial start up. It fires up, runs for a second or so and then floods.
New fuel delivery from pump to injectors.
New temp sensor.
I'm thinking something is telling the injectors to stay open.
I'm very interested to hear the results of this mystery....

[The Dude]

Hey wombat.yeh frustrating,not knowing....I'm also experiencing sudden issues around very poor engine performance from a smooth idle to farting and yawning when the throttles opened.Black spark plugs.
Im slowly discounting things.yawn.If I figure it out I will post up the long road I went down.
How did your issue turn out?

[wally.fisher]

You might have a faulty FPR.


Sent from my iPad using Tapatalk

[Wombat]

Did you end up solving this problem? I seem to be experiencing something very similar. Injectors are new but seem to be letting way too much fuel through and flooding the motor after initial start up. It fires up, runs for a second or so and then floods.
New fuel delivery from pump to injectors.
New temp sensor.
I'm thinking something is telling the injectors to stay open.
I'm very interested to hear the results of this mystery....

Yeah I did solve my problem. It was a faulty ECU. I pulled the injectors out but still attached to the fuel rail and hit the starter so I could see how they are behaving. Considering the amount of fuel flooding the system I had a hunch that the injectors were staying in the open position and not pulsing. Odds are that the ECU was stuffed. My hunch was correct. The injectors stayed in the open position and did not pulse for the 1.5 seconds or so that the fuel pump stays pumping once the starter is hit. LOTS of fuel was pumping through. I borrowed an ECU from my mates bike, connected it and she started straight up. Happy Days!!

[The Dude]

Cheers for that,Wombat.Im saving that one up till last.(ECU)?I do have a mate with a doner bike but it's his ride.


Hey Wally,after plug lead,plug swaps and bunging the hole into the chamber from the cracked z breather AND finding the vacuum hose "blown"off the nipple for the FPR on first experiencing my bike's problem it is going to be my next investigation?There are very few accounts of the FPR failing but after 30 years...my pump occasionally moans about something,plenty of return flow but...squeals like a pig every so often.
Yes,happy days indeed.

[Laitch]

Yeah I did solve my problem. It was a faulty ECU.

Because you have not put the year of your bike in your profile, Wombat, please verify that it was an L-Jetronic fuel injection control unit under the seat that you replaced rather than a Motronic.

[riots100]

There are very few accounts of the FPR failing but after 30 years

My FPR failed letting additional fuel into the number 3 cylinder from the vacuum line.

[Wombat]

Because you have not put the year of your bike in your profile, Wombat, please verify that it was an L-Jetronic fuel injection control unit under the seat that you replaced rather than a Motronic.

Build date of my K100rs is April 1986. 2 Valve L-jetronic.