'91 K75S starts but won't keep running, or restart?

This bike sat for many years. Last spring I cleaned and replaced the fuel pump, pump holder, fuel filter etc and cleaned out the tank. At that point it started right up. As I am still working on the bike off and on to get it roadworthy, I only start it every month or so, and it usually starts and runs fine.

That is not so anymore. For the last 3-4 times this is what happens: The throttle lever is set fully on, all dash lights come on. I hit the button and it starts up. It then runs for only 3-4 seconds and then quits. When it stops, I hear a short whirring sound for a second or two. After that, it just cranks and cranks but won't fire up. The plugs are new, as is the air cleaner. It only has about 20,000 miles showing on the odometer. The battery, (I was told) is fairly new, and I keep it on a trickle charger all the time. I'm afraid I may burn out the starter if I keep cranking it. And it normally starts instantly anyway. Any ideas on how to fix this will be greatly appreciated. Thanks.

As a follow up, the bike will no longer start at all. But it is now around 30 degrees. I think it may be a cold weather start problem. I'm guessing possibly the water temperature sensor, or the "cold start switch" in the fuel injection system. Checking the MAX BMW catalog I see the temperature sensor is no longer available. I have looked hard and can not find the cold start switch listed anywhere. So, if the temp sensor is bad, and you can't buy one, what do you do? Also, does anyone know where to find the cold start switch on the parts listing? Incidentally, this switch screws into the front of the throttle body assembly, with a two pin lead that is atatched to a connector under the tank.

There is no cold start switch. Are you refering to the choke light switch, which is at the right end of the throttle bodies between the #3 throttle body and the throttle position switch. It is the same style switch as you clutch switch and your front brake light switch.

Are your plugs properly gapped? Are they the correct NON-resistor plugs? Do you have spark? Are the plugs fuel fouled or wet?

When you replaced the air filter, did you happen to tear or dislodge the 2" diameter 90 degree hose that connects the air cleaner box to the air plenum over the throttlebodies?

Are you sure you are getting fuel pressure to the fuel rail?

I really doubt that your problem is the temp sender. Look at the top of this forum under
K-BIKE DIY/TECH LIBRARY to see how to test it. If you really need one, PM me.

I believe your problem is either no spark, no fuel, or a large air leak.

Have a Happy Thanksgiving and get back to it on Friday.

Lee,
Thanks for the reply. I'm just looking at the Clymer 85-95 K75/100 manual on page 322, where is tells how to remove/install the "cold start (choke) switch". It shows it screwed into the throttle valve assembly. No where in the book does it describe how it actually works, but I just assumed that it enriched the mixture somehow for starting. It has some electrical connection to the bike by way of a two pin connector under the seat. Maybe you can explain just what it does. The main reason I am considering that it is a cold start problem is that when it was warmer, the bike started quickly with all the same plug, air cleaner set-up that it has now, it has just been sitting in my garage all the time. Plus, when I first posted, it would actually start for a few seconds, but as the cold temps have gone down slightly, now It won't start at all. I'll check the spark, as you suggest. I need to figure out how to test the fuel pressure at the (rail)? Thanks for the link on testing the temp sensor. That will be my other test, after I figure out where it is located. Thanks.

The choke switch is only there to turn on a light that indicates that the lever is engaged. It was not installed on later bikes.

as others have said..there is no cold start switch in the ignition/fuel injection system. there is just light switch on the REAR end of the butterfly arm that turns on a yellow light on the dash to remind you the fast idle lever is engaged. i think they stopped installing this switch on 90's, although the light is still in the cluster and the wire is still under the tank. although it is labeled a "choke" on the bar, it is not a choke and does not do any of the enrichment functions a choke would normally do. all that lever on the bar does is bump the idle speed up a tad.

your problem could be many things, but cold start issue it is not.....

fuel pressure at the rail is easy. just "T" in a fuel pressure gauge to either end of the rail. gauge available most autoparts stores. must be for fuel injection systems (37psi).

As I thought about it, I was beginning to suspect what you all have confirmed: the "cold start switch" is nothing but a switch to turn a light on. I don't have the bike near me at the moment but as you say, being a 1992 it may not even be there. Back to the basics I guess, spark, fuel, compression. At least after this I should have a real good knowledge of the fuel injection/ ignition system on this bike. Thanks.

In one of your previous posts, you said the bike was a '91??? If you tell me the last four numbers of your VIN, I can confirm which year it is.

In the Flying Brick section of the forum, we request that you always include the YEAR and MODEL of the bike in the thread title. It helps get more precise answers to your questions and it helps future users to search for the most pertenant info for their bikes.

Lee,
Sorry about the discrepancy on the year. I just checked the title, and it says 1991. Last four Vin # 2588.

To check the fuel supply PSI I am assuming I will first disconnect the rubber hose on the left side of the fuel rail from the rail. Then hook a short piece of fuel line to the rail and put a gauge between the two lines. Then crank the engine. Does that sound like the right procedure? I am hoping to find a loaner gauge at AutoZone. If not, should I just disconnect the hose from the rail and put the end into a gas container to see that it is flowing out with some force?

On an S, it is easier to put the tee at the right (rear) end of the fuel rail. It must be "T'ed" in so that the fuel pressure regulator is still in the circuit. When adding the piece of hose between the rail and the T, Use about a foot of hose to make things more flexible. If you use an actual Fuel injection gauge, you should see about 38psi.

You can get away with using a cheap standard gauge (water) from the hardware store, but they are usually not dampened. This will cause the needle to swing back and forth because of the pulsing of the pump. You can dampen the pulses by squeezing down on the hose with a pair of pliers until you can smooth out the reading. A proper Fuel Injection gauge is best if you can get one.




2588; a '91 K75S built in Sept.1990.

Well, back at it, looking for ideas.
I checked several things on this bike, as suggested.

1) Fuel pressure at rail= 37 PSI measured with a fuel pressure gauge, T'd in.
2) All three plugs pulled out and grounded to frame. Each on has a hot blue spark.
3) Compression test on front and middle cylinders= 150 PSI. I didn't bother doing the last one.
4) Spark plugs are new Bosch Super RO 385

Bike starts, stops, will not restart.

I will try to test the coolant temp sensor using the directions I got here. My problem is that I have no idea how you would "locate pin # 10". I am assuming that the FI computer is the one under the seat connected to the storage tray. I enclose a picture of the connector. I don't see any numbers on either of the rows. And there are two rows of pin holes. How do I find number ten?

But it's not that cold out at this point, so I don't know why it won't keep running, since it does start for a few seconds.

I'm out of ideas.

I checked the three plugs before I put them, and when I looked closely, I see that they are all covered in a fluffy black material. To me that would indicate that the bike is running too rich. It might be why it will start, and then flood out, not allowing it to re-start. What would be causing the too rich signal to be coming from the FI computer?

Stuck injectors
ECU
Bad system ground
Injector wiring harness
Coolant temp sensor
Mixture adjust
FPR

And more...

mlytle said:

Stuck injectors
ECU
Bad system ground
Injector wiring harness
Coolant temp sensor
Mixture adjust
FPR

And more...

Click to expand...

Yes, it looks like this "incredibly reliable" k75 will need some incredibly expensive diagnostics and parts.

Meanwhile, my 1994 Honda Pacific Coast starts every time. Now why is that?

Now why is that?

jconway607 said:

This bike sat for many years. Last spring I cleaned and replaced the fuel pump, pump holder, fuel filter etc and cleaned out the tank. At that point it started right up. As I am still working on the bike off and on to get it roadworthy, I only start it every month or so, and it usually starts and runs.

Click to expand...

I think the answer is in your first paragraph.

Was the "sitting" for "many" years all during your ownership?

How many is "many"?

When was the last time you rode it for over 50 miles at one time?



Diagnostics don't necessarily have to be expensive. You need as much background info as possible on the bike and the symptoms history and then do methodical troubleshooting.

There used to be a link to an excelent troubleshooting guide in the DIY/Tech Library sub forum of this forum. I just checked it and the link is no longer working. I have a hard copy of it and will repost it to the Tech Library in the next few days if my wife can get our scanner working again.

First I would throw new plugs in it just because I'd want everything to be as perfect as possible. It has helped SO many times that it's not worth thinking about whether or not it "should."

Then check the S-shaped hose and general condition of the rest of the intake stuff.

How old is the fuel?

Lastly, I had a problem with a K75 one time where the bike would fire up and immediately die, every time, but it was like half a second rather than three or four. The details are a bit fuzzy but it turned out to be a bad ignition module that would drop power on one pin (to the fuel pump I think) as soon as the engine caught. I think I fixed it by bypassing whatever pin it was. It was a very interesting problem to find. I don't suspect this is your problem but if it turns out that you are losing spark after some engine cranking then you could have something similar going on.

Anton's reply raised a few questions in my mind.
You checked the plugs for spark at one point. Presumably the bike was in the "will start then quit" mode at that point.
When the bike is in the "quit and won't restart" mode, do you still have spark?

I'd give it a test under those conditions, if you haven't already, as lack of spark under those conditions gives you a direction to pursue.

I hadn't looked at the thread for a while, but thought I would check back in to update the situation.

First off, I would like to apologize for my somewhat cranky last post. As all of you can imagine, after putting in all these new parts, all the money it cost, and then have this bike start then stop, well, I guess you can imagine my frustration. I blame myself for getting into this situation in the first place. The bike had sat for years, I thought I could save the little guy, and it was pretty cheap. All bad ideas. Live and learn...again. I do not foresee rebuilding another neglected BMW, (my third now.) My R1100RT was not cheap, and was very well maintained over the years by an MOA member. Of course, it has never given me any trouble. I believe the connection has finally been made for me.

Second, I want to sincerely thank those of you who have tried to help. It has restored my faith in being a member of the MOA. I'm not riding 30,000 miles a year, or buying a new GSA every few years. But people here still do their best to help. That's appreciated.

Lastly, many of you were right about the problem. I did some checking on posts and found that the K75 engines have a reputation for flooding. Also, that once flooded, the plugs are often ruined. This seemed contrary to fifty or so years of auto repair, but I had little to loose. As a final attempt at repair before hauling it to a dealer, I replaced the hardly used plugs(Bosch) with a new set of NGKs which had been recommended here on the forum. The bike started instantly. I will now continue to re-do the body work, recover the seat, etc to finish the project. The K75Ss are beautiful bikes, so, I hope it's worth it.

Thanks again to all of you for the patience and assistance.

jconway607 said:

I do not foresee rebuilding another neglected BMW, (my third now.)

Click to expand...

You say that now... but I think you'll have a hard time wiping that smile off your face the first time you go out and exercise the throttle.

jconway607 said:

Yes, it looks like this "incredibly reliable" k75 will need some incredibly expensive diagnostics and parts.

Meanwhile, my 1994 Honda Pacific Coast starts every time. Now why is that?

Click to expand...

See, that wasn't so expensive now, was it?

You probably killed the plugs with the short runs in the garage every month. The plugs never got a chance to run clean. Get that thing ridable and it will put that Honda to shame.