obtusegoose
Face 63954
Need help diagnosing an intermittent engine running issue on a Marrakesh red 1988 BMW K75S that has ~ 110,000 touring miles on her. Bike is stock (other than a Corbin seat, run-n-lights and motolights running lights) and has been cared for regularly with most called for regular services carried out according to schedule either by me or an authorized BMW dealer here in the Northeast US. Bike has performed flawlessly up until now except for a fuel pump electrical connection issue (spot-weld breaking on fuel tank level sender flange on underside of fuel tank) which occurred around ~ 60,000 miles ago. At the time, the fuel pump relay (OEM) was replaced as well as re-spot welding the connection.
This intermittent running issue just started in the last couple of weeks and occurs whether the engine is warm/hot or cold/recently started (which seems to eliminate the Hall effect ignition sensor as the culprit). Daytime outside temps have been in the 80s and humidity in the 70% range when the issues have occurred.
Symptoms:
This intermittent running issue just started in the last couple of weeks and occurs whether the engine is warm/hot or cold/recently started (which seems to eliminate the Hall effect ignition sensor as the culprit). Daytime outside temps have been in the 80s and humidity in the 70% range when the issues have occurred.
Symptoms:
- With the engine at temp the engine completely dies while riding down the street with a turn of the throttle giving no response. Then all of a sudden, while coasting to a stop due to the engine quitting, the engine will catch and run normally. Sometimes after an engine stoppage and coasting to a complete stop and turning the key off, sitting for ~ 5 seconds and then turning the key back on, the engine will immediately start up again after pushing the start button. However, when giving her some throttle in neutral (where the engine has been started), the engine will die only to be easily started again by pushing the start button whether the key has been turned off and then on or just left on. Sometimes after one of these restarts I can get the engine to rev up a bit and actually get it to move forward but it feels like it’s running on less than 3 cylinders. Sometimes it’ll then start running fine again only to exhibit the aforementioned symptoms and stoppage again. I should note that all other electrical systems (lights, horn, blinkers, etc.) come on and appear to operate as expected.
- Sometimes with the engine at running temp or first starting the ride, the engine will stutter and not come to a complete stop but keep going and feel like it’s running on 1 or maybe 2 cylinders. Then, the bike will “wake up” and appear to run normally (on 3 cylinders). It has never quit completely and stranded me.
- So far, when she does exhibit symptoms, I’ve been able to “limp” home, even ~ 15 miles by restarting the engine and hoping a turn of the throttle will get me some engine response which it will.
- Since the original fuel lever sender had failed ~60,000 miles ago and the electrical connectors on the inside of tank looked corroded, it was replaced with a new one I had as a spare. All electrical connections on the new sender checked out fine (continuity to inside connections and fuel pump) and appeared to work as intended (low fuel level light worked, fuel pump runs, etc.).
- Fuel tank filler cap was opened and the bike was started (seems to always start and at least idle) and the original in-tank fuel pump can clearly be heard running
- Multi-connector Motronic unit under seat has been disconnected, cleaned (deoxit) and reconnected with an audible “click” heard indicating it has connected and should remain secure even on bumpy roads
- Battery is a few years old but gives ~ 12.6 – 12.8 V standing voltage and bike starter easily turns over and bike always starts and idles.
- Battery connections were cleaned and re-tightened
- Key/ignition switch was wiggled extensively thinking that maybe a bad connection in the switch could be causing the intermittent running issues but no change was observed in solving the problem. Key switch was dis-assembled and cleaned according to those sources posted on-line and appears not to have solved intermittent problem.
- Fuel pump/sender 4-wire connector on right side of bike was cleaned (deoxit) and appears nice and tight when reconnected
- Kill switch lever was also pushed from side to middle to other side and back to middle again in hope of fixing a bad electrical connection but problem persisted. Kill switch/start button was cleaned according to those procedures posted on-line and after reassembly, bike started and ran fine but has again exhibited problems.
- Spark plugs renewed (Bosch stock recommended plugs). (Original spark plug wires still installed and appear to be fine as evidenced by the normal operation of the bike when problem doesn’t present itself.) They were disconnected at coil and at spark plugs and then reconnected.
- Valves shimmed (no shims were actually needed) at ~ 85k miles.
- BMW stock air filter installed at ~ 90k miles.
- Crankcase breather hose (Z-hose) replaced at ~ 105k miles.
- Fresh 91 octane ethanol free gas fill from a reputable service station is in the gas tank. (Bike had most of its ~85k miles operating on 87 octane 10% ethanol gas.)
- ~ 30 mL of isopropanol gas additive added in hopes of “absorbing” any potential water that could be in the fresh gas fill up but no benefit was observed
- BMW fuel filter replaced at ~ 100k miles. At filter install, all original in-tank gas hoses appeared fine and rubber fuel pump shim was still intact and not a “gooey” mess as seen on other K bikes using ethanol laced gas.
- All fuel lines outside of gas tank were replaced at ~ 80k miles.
- Fuel delivery pressure has not been checked but is assumed to be operating normally, again evidenced during the normal engine operation episodes listed above.
- Electrical connection to 3 fuel injectors were removed and reconnected to no avail
- Electrical connections of 3 coils were removed, deoxit sprayed and reconnected.
- Ignition unit connector under gas tank was removed, sprayed with deoxit and replaced.
- Bike was started at night and arcing of the coils and spark plug wires was checked but was absent.
- “Ground” junction underneath fuel tank was cleaned at ~85k miles.
- Ground for battery negative on transmission housing cleaned. Battery ground wire, which has been a problem on some K bikes, has not been replaced but appears/checks out fine.
- Fuel pump relay replaced with OEM unit at ~ 50k miles during failed spot-weld episode listed above.
- It is hard to believe that the Hall Effect Sensor (HES) is failing as they are supposedly very robust and rarely intermittently fail. If they do, it occurs when the engine is warm but I’ve experience the symptom when the engine is cool seemingly ruling this out. Before investing a bit of money in the purchase of a new HES, is there an easy procedure to check it to rule it out? I see the BMW parts outfit in the UK has reproduction units for ~$150 dollars. Anyone have experience with these and their installation?
- Another thought is a gas vacuum lock preventing fuel getting to the rail when hot (doesn’t explain the cold start issues described above, however). Any way to check/fix a problem like this? (Original foil underneath the gas tank is still present.)
- Should the gas injectors be removed and sent out to be professionally cleaned? (Hard to believe these could be the cause as the bike will run great at times.)
- Should this original fuel pump be replaced? (Can the fuel pump intermittently be failing leaving some pressure in the delivery line but not at adequate pressure?)
- Could a faulty fuel pressure regulator be responsible?
- I believe the issue is fuel related but open to all ideas
