1997 BMW R850R Sudden No Start - Not Just the Hall Sensor!

[Solved] 1997 BMW R850R Sudden No Start - Not Just the Hall Sensor!

Hey folks, I'm new to BMW ownership and I'm stumped by my Oilhead. I'm a fairly experienced home mechanic and have resurrected at least three previous motorcycles from the dead, but I don't know these models as well as most of you do and I'm out of ideas. Here's the story with probably a lot of unnecessary details.

I recently purchased my first fuel-injected bike and first European bike, a 1997 BMW R850R, non-ABS, built 08/96, with 34,500 miles. This bike has lived a hard life the past few years: complete neglect in some dude's yard in Georgia, and the owner before that did other unkind things to it like installing "custom" lighting that I've since removed and stripping back the tape on the wiring harness front and rear for gods know what reasons. I've been trying to take much better care of it than they did.

I got it running about two months ago, and have put a few over 50 miles on it since then after making it safe. Lots of new or used parts to fix stuff that was broken. New fuel pump and filter, new fuel lines, replacement fuel sending unit/pump mounting plate, new spark plugs, new tires, and lots of other stuff.

Truthfully it never ran quite right. It was rough to start from cold in any temperature even with the "choke" lever and would usually run briefly, die a few times with restarts, and then eventually stay running. Only time it wouldn't do this is if it was at or near operating temperature. It didn't run particularly smoothly and the engine vibrated quite a bit, which was very noticeable at idle and got worse the higher it revved up. Not atrocious but definitely more than I expected from what I had read about Oilheads. I assumed it needed a throttle body synchronization and probably a valve adjustment and had those on the long list of stuff to do.

Last week, after putting on new tires and changing the alternator belt, I figured out how to adjust the "choke" lever cable correctly and realized it had been misadjusted such that it always had tension. After I fixed it to have just a touch of free play and lubricated the cable (with silicone oil so as to not eat the lining) and the lever mechanism, the bike then started easier than it ever had before. It died once after running for a few seconds, then kept running after the restart. I was pleased and figured I had completed one more step in getting it all sorted out.

I then did my first interstate ride on it, about 15 miles of mostly highway between 55mph and 80. It ran fine, and seemed to be a bit smoother than before. The road was a bit wet as it was very slightly raining and had been doing so all day. It didn't stutter at any point. I had also installed a fender extender so I assumed spray wouldn't get to the engine or electrics. I got to where I was going, parked the bike on the sidestand around 1AM, and then went to sleep.

The next day around 2pm, it wouldn't start. Crank and crank and crank but nothing happened. I could clearly hear the fuel pump prime and run with engine rotation. I had to go run errands so I put it back under the cover and took a car. The day after that, I got to diagnosing it. I cranked it over and it had spark on both cylinders but the spark plugs were dry. I figured that it was likely the typical failure of the original Hall Effect Sensor wiring harness and pulled that out for inspection. I did mark the position of the alternator pulley and the alignment sensor before removing them and immobilized the engine.

I opened that up yesterday and it was typical; the original wiring insulation inside the harness was totally falling to pieces. I'm pretty handy with a soldering iron and have a full toolkit for repairing electronics and wiring, so I fixed it myself -- one wire at a time, I soldered in silicone jacketed wiring and wrapped it all with automotive high-temperature foil tape. Today I installed the repaired sensor and made sure to align the parts perfectly with the timing marks I had made.

It still won't start. However, I do now have exhaust that smells like gasoline after cranking the bike, and I have strong spark on both cylinders, so I believe my repair of the Hall sensor wiring was successful. The fuel pump primes and on a freshly charged battery the bike cranks over strongly. Compression doesn't sound abnormal.

Is it possible the failing Hall sensor wiring shorted out this poor bike's Motronic brain? Could it be a bad oil temperature sensor?

I'd like to hear your thoughts.

I know GS Addict re-wires these all the time and I haven't seen any complaints of his work.

I do know for 100% certainty that when you solder close to the actual sensor, BMW Hall Effect or others, you can't have any heat transfer back to the actual sensor.

I assume the sensor is working because before, I was getting no fuel spray, and after my repair I've got good spark on both cylinders as well as unburnt fuel vapors coming out of the exhaust.

1) Is the timing cup (rotor) referenced to the crank?
If the pulley is not correctly torqued or the cup has slipped the bike will exhibit the symptoms you describe.
See below.

I did try my best to index the timing rotor tab properly to the cutout on the crank and I'm pretty sure I got it spot on -- but I didn't have a torque wrench on hand when I reinstalled it. I squeezed the timing rotor and pulley back together and pushed them both onto the crank at once, and if I snagged them anywhere or it didn't feel right I tried again. When I did get the location right it felt snug and could only rotate just a tidbit with no free play in any other direction. I held it there until I got the bolt in and snug by hand, then tightened it with the engine in 1st gear and standing on the rear brake.

This is one of those "it broke 20 miles away from all my tools and I don't want to drive 40 miles for one tool" scenarios.

It's quite possible it slipped rotationally when cranking the bike or when I tightened the bolt. I could only make it "pretty tight" with a ratchet yesterday (likely I didn't even get close to 37 lb-ft). Thankfully I didn't hear any snapping or grinding noises so I think it didn't destroy anything. I'll get hold of a torque wrench today, remove and reinstall it and be more careful about the plate rotating, and update on what happens.

Thanks for your advice, GSAddict.

GSAddict said:

1) Is the timing cup (rotor) referenced to the crank?
If the pulley is not correctly torqued or the cup has slipped the bike will exhibit the symptoms you describe.
See below.

Click to expand...

You and a Motorrad-wise buddy I know in person were completely right -- I got the bike running today!

It was exactly that. The rotor must have slipped when I first attached it because I wasn't able to torque the bolt correctly. I used a bit of super glue to stick the timing rotor and the pulley back together, got a torque wrench, immobilized the engine at TDC the right way, and torqued it down. That worked. The bike starts, but still is a little difficult to get running stably just like it was before the sensor failure. Requires some throttle input to cold start. So the Hall sensor wiring repair didn't fix the rough starting and running, but at least it runs again.

I plan to go through and clean the throttle bodies, replace most of the associated rubber parts and O-rings on them and the injectors, and do a throttle sync, as well as test and possibly replace my oil temperature sensor. Hopefully those solve my issues.

Check this on the right throttle body

GSAddict said:

Check this on the right throttle body

Click to expand...

I wish it was that easy -- no issue with throttle cable seating on either side.

Today I figured out what is most likely the issue. The bike will start, and run (but not well) under its own power when it's cooler out...but as soon as the ambient temperature gets above 80 degrees, especially if the bike's been in the sun, it simply refuses to start without starting fluid sprayed in the airbox.

I'm going to hit both the air intake temperature sensor and the oil temperature sensor with a multimeter. Likely one or both of them is busted and the engine is trying to start with a terrible lean condition because it thinks it's at full operating temperature all the time, or maybe it thinks the air is 150 degrees.

No prior mention of this, so - Have you pulled the big brass air screws and cleaned off their tips, and cleaned out the passageways in there? Also look for a crapitation buildup in the bores of the throttle bodies.

Pauls1150 said:

No prior mention of this, so - Have you pulled the big brass air screws and cleaned off their tips, and cleaned out the passageways in there? Also look for a crapitation buildup in the bores of the throttle bodies.

Click to expand...

I haven't done it -- yet. The plan was to address that soon, when I fully clean out the throttle bodies and injectors and replace most of the rubber in and around them. Do you think that kind of buildup would be suddenly preventing it from running?

"Running", no, but they'd be a major factor from idle (including starting and fast-idle lever effect) up to about 1/4 throttle.

htomsett said:

I'm going to hit both the air intake temperature sensor and the oil temperature sensor with a multimeter. Likely one or both of them is busted and the engine is trying to start with a terrible lean condition because it thinks it's at full operating temperature all the time, or maybe it thinks the air is 150 degrees.

Click to expand...

I'll venture that neither of these is your problem. While I'm sure its happened, I've never heard of either one of these failing. As Pauls1150 says, get to work cleaning up your intake side and see what happens. Check for leaks around the intake boots as well. While you're at it, set the valves if you haven't already. Simple stuff first.

Just cleaned out both brass screws and their passages; they definitely had a lot of carbon gunk on the needles and the first few threads. That didn't get the bike running for more than a second though, even at a cooler temperature.

I also tested both temperature sensors with a multimeter. I got 1930 ohms from the air temperature sensor, and 1970 from the oil temperature sensor, around 78 degrees F. From the info I found that's the correct range for both. Better to know and waste 5 minutes with a multimeter than not, at least in my mind.

It doesn't make complete sense to me why the bike would be running, rideable at highway speeds, and then suddenly not the next day, if it was a carbon buildup problem in the throttle bodies. I was under the impression that that sort of thing came on gradually.
Maybe something dislodged and clogged my injectors, a rubber seal in the intake system finally cracked wide open somewhere, or the results of the previous owner using a K&N filter (yuck) finally clogged everything up enough to keep it from running.

It never really did run particularly well, so it was probably closer to this than I thought. Guess it's time to do a valve adjustment and tear the intake system apart, which I was hoping to not have to do for a few weeks. Yay. The perils of cheap bikes.

htomsett said:

It never really did run particularly well, so it was probably closer to this than I thought. Guess it's time to do a valve adjustment and tear the intake system apart, which I was hoping to not have to do for a few weeks. Yay. The perils of cheap bikes.

Click to expand...

I had a friend who did a poor job of replacing the alternator belt and took out the hall sensor. That resulted in the Motronic getting screwed up too.

I would consider getting the injectors out and professionally cleaned. I know that there are DIY videos for that, but the pro shop will show you definitively before and after results, and you will know that the injectors are giving you a balanced flow of gas. Not expensive.

Do a compression test, both plugs out, throttle held open.
Report back

m_stock10506 said:

I had a friend who did a poor job of replacing the alternator belt and took out the hall sensor. That resulted in the Motronic getting screwed up too.

I would consider getting the injectors out and professionally cleaned. I know that there are DIY videos for that, but the pro shop will show you definitively before and after results, and you will know that the injectors are giving you a balanced flow of gas. Not expensive.

Click to expand...

Wow. That is a very poor job indeed.
I didn't have any trouble replacing the alternator belt and the bike did run afterwards. This was a day before it stopped running and the injectors stopped squirting gas, so I very much hope my Hall sensor shorting out didn't take out the Motronic with it.

How much does that kind of professional cleaning typically run? I do all this work myself partly because I enjoy it, but primarily because I can almost never afford to pay professionals to do it. If it's more than, say, $50, wouldn't it be more worth it to just get a rebuilt set for $80 on Ebay? https://www.ebay.com/itm/Genuine-Bo...0147?_trksid=p2349526.m4383.l4275.c3#viTabs_0

Though would it be better, if I have to replace the injectors anyways, to buy these? https://www.tills.de/product-47.html Or from here? https://injector-rehab.com/product/injector-upgrade-kit-for-94-02-r850-new-injectors-included/
I have heard these R1200 injectors are a massive upgrade. For around $200, if I want to replace the injectors anyways, it seems worthwhile.


And to GSAddict: I'll (hopefully) post a compression test result within a few days.

htomsett said:

Wow. ....................

How much does that kind of professional cleaning typically run? I do all this work myself partly because I enjoy it, but primarily because I can almost never afford to pay professionals to do it. If it's more than, say, $50, wouldn't it be more worth it to just get a rebuilt set for $80 on Ebay? https://www.ebay.com/itm/Genuine-Bo...0147?_trksid=p2349526.m4383.l4275.c3#viTabs_0

Though would it be better, if I have to replace the injectors anyways, to buy these? https://www.tills.de/product-47.html Or from here? https://injector-rehab.com/product/injector-upgrade-kit-for-94-02-r850-new-injectors-included/
I have heard these R1200 injectors are a massive upgrade. For around $200, if I want to replace the injectors anyways, it seems worthwhile.
.

Click to expand...

Check around your area. There may be a shop locally that does that kind of work.
I used InjectorRx in Houston. They charge $18 per injector, plus $9 for the return shipping, so $45 plus your cost to send to them.

I am NOT trying to turn a sale out of this... I have a new set of the Tills.de injectors for the R11xx. Decided not to install them. $100 plus shipping.

I'll do some brief Internet searching for a local service center. I live in Nashville so I assume there'd be at least one shop that does that around here.

I sent a private message about those Tills injectors, though. That's one heck of a price. That would save me a good amount of work too.

I might have missed it but have the valves been carefully adjusted? If not I would do that next.

PGlaves said:

I might have missed it but have the valves been carefully adjusted? If not I would do that next.

Click to expand...

Not yet, but it was on the Big List Of Things To Do. I just today ordered the gaskets and the measurement shims from Euro Moto Electrics. Definitely going to do that and go through the whole intake system in the next few days, and I'll post the results here.