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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: cnoon on September 06, 2015, 06:31:07 PM
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Hi everyone. I thought I'd start a first post with something that's got me stumped. It's a 1991 K75s and here's the background:
1. Riding on a warm day 5 months ago and the bike just quits. It starts for just short bursts and is getting spark so I suspect a fuel issue.
2. Months later, I finally get time to deal with it and test the fuel pump via the fuse box and it's dead.
3. Replace the fuel pump and filter (and bench test the prior one and confirm truly dead).
4. Start the bike and it idles for a few seconds and then dies. Won't restart.
5. Confirm that the new fuel pump is working via the fuse box and also confirm that it pumps fuel into the front of the FI rail (also to confirm I didn't put the pos/neg on backwards on pump).
6. Now here's where it gets weird. If I disconnect the 4-wire plug between the bike and the tank (with the yellow, white, brown, green&green/white). The bike will start and idle fine if on full choke. This means the fuel pump is not on and it's just relying on gravity feed(?). If I try to rev it at all, it will starve out. If it is idling in this fashion and I connect the plug, it immediately dies and will not start (until I disconnect the plug). If I try to start it with the plug connected, I can hear the fuel pump engaging on the start button and continuing for a second or two after start button release.
7. I've confirmed that the incoming green/white wire is delivering power when the starter is engaged but I'm not getting any power to the yellow or white wires when the key is on or when the starter is engaged.
So, any ideas on what to check next? I'm thinking of idling it unplugged and then running the fuel pump by jumping it at the connector (essentially bypassing the yellow and white connections). If that kills it, then there's something related to fuel pressure I guess. If it doesn't kill it and even allows it to be revved, then it's something electrical related to the yellow and white wires (fuel level indicators). I suppose I could always just snip those but that's a kluge.
- thanks, Chuck
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That does sound interesting......
I suppose it's possible that something could be breaking down in the wiring pass thru where the wiring enters the tank. (ie the sending unit assy)
There have been reported issues(other owners) with the brown ground wire connection failing where it connects to the tank. Other riders have had fuel pump related issues with this, generally the pump doesn't run at all.
The repair was to replace the sending unit assy.
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I'm thinking that it could be a faulty sending unit. But does the sending unit have the ability to tell the bike to quit running. I thought it would just be a passive sensor.
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greetings...
have you removed the motronic connector and cleaned up the connections...
what also stops working when you plug in that 4 pin... head light... dash lights... neutral light... clutch switch...
j o
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I'd recommend you check the ground path continuity from the 4-wire connector(brown wire) to the lead connected to the fuel pump.
If your ground path is interrupted, or not clean and solid, all kinds of funky stuff can happen.
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You won't get power at the white or yellow wires because they are sensor outputs. Furthermore, the yellow terminates in the aux. instruments connector, which is essentially open circuit.
The sudden nature of the failure indicates electrical. Check all grounds as Scott says. Did you investigate how the original pump died? Was it a mechanically frozen rotor or did the motor burn out? This might help diagnosis.
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I'll definitely remove the motronic and clean the connections although there's nothing else that seems to go offline when the connection is made. It is worth noting that before all of this went down, the bike had a slight short which required me to disconnect the battery or it would discharge in a couple of days. There's no evidence of that short now after changing out the fuel pump.
Not sure what exactly caused the fuel pump to fail. The strainer was still intact but I think the hose connected to it was missing or broken. The fuel pump vibration damper was in bad shape so it could be conceivable that the pump got gummed up. When bench testing the old pump, it sort of thrusts when you apply power as if it is mechanically jammed.
The brown ground wire path must be clear to the fuel pump since the pump will run either when the connector is connected or if I just run positive and negative into the tank side of the connector. Now, whether it's only partial or not clean, I don't know but can check.
The way it immediately dies when connected seems to imply something electrical, but that would imply there's some sort of signal being sent upstream and that's what's perplexing me. Thanks!
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It could be a bad FI relay. That is the device that controls the pump. If the contacts were bad on the relay, power to the pump could be interrupted when the load is placed on the circuit. Power to the injectors is supplied through a separate set of contacts so it's conceivable that the injector power circuit works and the fuel pump circuit does not. Do you have a spare relay to test? If not, maybe taking the existing relay out of the bike and bench testing it is a possibility, before forking out money for a new one.
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So what you are telling us is that the engine will idle without the fuel pump running, and won't run at all when it is. I am having trouble believing that gravity will allow enough flow through the injectors to run the engine, even at idle. I know my bikes won't run that way. I suspect some residual pressure is supplying fuel to the injectors. The question is: how long will it run like this? I would guess not more than a few seconds.
To see if indeed the problem is in the fuel pump wiring in the bike I would try connecting the fuel pump wires in the tank connector to an external power supply so that the pump is running independent of the rest of the engine management electrics.
If the engine runs that would confirm an electrical problem. If not, I would suspect something in the fuel circuit like the fuel pressure regulator, or possible the venting of the tank is interfering with fuel delivery when the pump is running.
If the engine runs, I would connect a meter to the fuel pump wires in the bike's fuel tank connector to see if indeed there is 12v present when the engine is running. If not, the problem is probably in the fuel pump circuitry.
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id hook up one of those drill pumps to the fuel line and see if it will go with the 4 pin disconnected...
drill pumps (https://www.youtube.com/watch?v=xp3Sl9VUcN0)
j o
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6. Now here's where it gets weird. If I disconnect the 4-wire plug between the bike and the tank (with the yellow, white, brown, green&green/white). The bike will start and idle fine if on full choke. This means the fuel pump is not on and it's just relying on gravity feed(?).
Can't be possible! The injectors can't inject without fuel under pressure. Fuel can't be under pressure without power to the fuel pump. This just doesn't add up.
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I took a video for you un-believers:
https://www.youtube.com/edit?o=U&video_id=dsH6NOyYN18
Also, I later used a car booster battery to power up the fuel pump by directing power to the tank side (green and brown) of the plug. It runs the pump when invoked. I then started the bike with the plug disconnected (and it idled fine) and when I turned on the power to the pump, the motor promptly died. Hence, I'm now more convinced it's a fuel pressure (rather than electrical) issue and will check out the regulator, etc.
Hey, if any K75 owners wanted to unplug the tank and see if it idles (with the "choke" on full), that would be helpful information.
- thanks, Chuck
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Putting the theory aside for the moment, empirical data and observation shows the theory to be wrong, if what's written is to be believed. If the FI relay is energizing the injectors and the ECU is actuating the injectors, fuel must be entering the cylinders but not under normal pressure. This could be just dribbling in or there could be residual fuel pressure in the system that just manages to get the bike started and running for a few seconds.
In your video, it certainly seems that the engine is running on residual fuel that is leaking into the cylinders. The fuel would not be under pressure since 32 hours have elapsed since you last started it. I'm guessing vacuum generated during intake cycle is sucking fuel into the cylinders from the injector rail. Once that fuel supply is exhausted, the engine will quit.
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Robert, I just read your message, walked outside, confirmed the tank plug was disconnected, set the stopwatch on my phone, and started the bike and the stopwatch. It ran for 1 full minute and then I turned it off with the key. If pressure is a necessary condition for fuel to enter through the injectors, then there must be some other source of pressure, but from where? If the gravity/vacuum is enough to pull fuel into the cylinders, then we've all been relying on a bad assumption. This is becoming quite interesting (although somewhat frustrating). - best, Chuck
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Just a shot in the dark,
Could be the injectors. If they are shot/ stuck open to the point that fuel can gravity feed then the pump would flood out your bike.
Also, could be something in the control circuit that is locking the injectors in the open position.
I'd suggest looking at your spark plugs after the pump has been on. If they are wet then there is a big clue.
Good luck on finding/fixing the problem,
Freelancer
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Has a previous owner plumbed in a fuel pump somewhere on the bike, maybe to cope with the iffy item you replaced? ".......the hose connected to it was missing or broken" That could just about match all the symptoms.
Sorry if this is a stupid suggestion, but I don't know how familiar you are with all the bits and pieces on your bike. Apologies otherwise.
Because as others have said, if I pull the 4-pin from the tank my bike dies there and then. There's no 'minute' of running on....
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Just a shot in the dark,
Could be the injectors. If they are shot/ stuck open to the point that fuel can gravity feed then the pump would flood out your bike.
Also, could be something in the control circuit that is locking the injectors in the open position.
I'd suggest looking at your spark plugs after the pump has been on. If they are wet then there is a big clue.
This sounds logical to me. Combined with intake vacuum to pull fuel through the injectors. Opening throttle leans the mixture to where it won't ignite. Might very well be enough fuel in the rail to run the engine at idle for several minutes.
Turning on the furl pump floods the engine through the open injectors. Whatever you do, I hope you keep us posted on what you find.
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Putting the fuel pump and fuel pressure questions aside for the moment,
Unplug the wires to the >injectors< and see if it will run at all. if the injectors are whack, that will define whether it's mechanical (stuck open), or electronic/electrical (Motronic / FI control brain fart). Either way I find it pretty hard to believe all three injectors would do that at the same time unless they all sucked up some debris (that somehow would have had to get past the fuel filter). I don't know if that could mess up the FPR too. AFAIK, that's only a diaphragm and spring set-up. So I don't think there would be anything 'clog-able' in there, but ...Brings me back to the comment on the missing or broken hose and the gummy fuel pump isolator. (Maybe the PO had a clogged fuel filter and moused it to get home?)
If you could prove out the mechanical side (fuel pump [even hot-wired if necessary] will produce correct pressure [not under spec, not over spec...got a gauge?], hoses are all in the right place, and injectors arent jammed open, then it's probably safe to go after the electrical gremlins.
Just remember to follow the Professor Mayhem's First rule of troubleshooting (anything)... Never overlook the obvious.
Keep us posted...
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Thanks for the ideas, everyone. I plan to do a few more tests this weekend and will report my findings.
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Since you've invited everyone to throw ideas on the wall ( maybe getting tiresome?), let's go back to your initial post "replaced fuel pump": did you use a stock or after-market, and could you test the pressure, should be about 65 lbs. to the pressure regulator, about 35 lb. to the rail. Seem you have a functioning pump, functioning injectors, but under a load greater than idle, the fuel supply is insufficient for the engine to rev.
Do let everyone know the solution to your puzzle!
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I checked out a few things this evening and here's the update.
- I started it up with the fuel pump unplugged at the 4-pin connector and it idled as it has been doing. I then connected it and it promptly died as before. I immediately pulled the #2 plug and it was very wet.
- I disconnected the forward gas line from the fuel injector rail (as pictured) and redirected it through a clear tubing and into a gas can. With the 4-pin disconnected, I started it and it idled as usual. I then plugged in the 4-pin and it kept idling but there was gas now flowing through the clear tubing. Keep in mind that with the fuel line disconnected from the rail, there was absolutely no positive pressure within the rail but the bike was still idling.
Based on the above, I'm beginning to think that all three injectors are wide open and the combination of gravity and cylinder vacuum are pulling any residual fuel from the rail and maintaining the idle. That would also cause immediate flooding when the system is all connected up.
So, what could cause all three injectors to be in the wide open position or perhaps something is telling the control unit to open them up (perhaps an air flow sensor)? Thanks!
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Was gas flow through the clear tube under fuel pump level pressure? How about rate of flow through the hose to the rail?
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The fuel flow was brisk but I don't have a gauge (yet) to test pressure. I put in a brand new Bosch pump just last week.
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I think the no load flow of the fuel pump should be on the order of 1 liter in two minutes.
Next test is to unplug the injector wires and see if the engine still idles. If not, the problem is electrical, not stuck injectors. If it runs, the injectors need to be looked at.
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The replacement fuel pump was a Bosch exact, by the way, not an alternative requiring adaption. Given that fuel flowed in the right direction when the plug is connected suggests I have the pos/neg and fuel lines hooked up correctly. I'm going to check out the air sensor this weekend. I'm wondering if it is sending a signal that results in the injectors opening richer. I also just noticed there appears to a crack in the S-shaped vent hose towards the rear of the engine that seems to connect the air collector with the crankcase but not sure if that's enough to be the culprit. More experimenting this weekend. This has gotten personal.
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- 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.
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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
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Some helpful videos:
BMW Air Flow Meter Testing (https://www.youtube.com/watch?v=hQYRep6K15k)
BMW Air Flow Meter Refurbishment (https://www.youtube.com/watch?v=Z-QlPtsjh7g)
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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:
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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....
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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? :popcorm
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You might have a faulty FPR.
Sent from my iPad using Tapatalk
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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!!
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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.
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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.
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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.
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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.