Oil fill cap leaks after hard running
Posted: Mon Oct 02, 2006 11:00 pm
I got PM'd by a new member with a question about an oil leak. One thing troubling to me was that I thought the OilHead rider was unnecessarily reving the engine too high steady cruising in the lower gears with sustained 4,000+ engine RPM's in 92F (33.3C) summer heat. Obviously, without an oil temp gauge. Here's the (condensed) PM:
The combination of stop-n-go traffic, high engine speeds and high summer heat can easily get the engine and it's oil hotter than 300F (148.9C) - - - especially reving the engine wih the gearbox mostly in the lower 3 gears. That means the windspeed is marginal for the extra heat being generated by a high reving big twin. That heat can cause the plastic oil fill cap and mating base to stretch/distort while the overheated O ring softens enough to allow the crankcase's pulsating pressure waves to force out a small amount of oil. Also possible is excessive piston blow-by from an overheated, heavily loaded engine overwhelming the timed breather's nominal flow capacity. Most likely a combination of the two. Normally there is a slight negative crankcase pressure maintained by the timed breather passageways machined in the crankshaft when the engine is running. You can observe/confirm the negative pressure by unscrewing the oil cap while the engine idles. It will be pulsating violently and held in place by suction.
Since stop-n-go traffic really overheats the engine in summer, running it over 4,000 RPMs in the lower gears while cruising steady in the lower 4 gears will make it much hotter still. With conventionl oils you have a recipe for disaster when oil temps go over 300F (148.9C) for an extended period. Not so troublesome with synthetics.
Sometimes the riding pace doesn't allow taking it easy on an overheated engine, but you must not make things worse by using high steady state RPMs in the lower 4 gears when lower RPM's will do. When faced with those conditions described above, my tach needle is 2,200----2,600 steady cruising in the lower gears until I see the crankcase temperature go below 210F (98.9C).
BTW, My oil cap has never leaked, but after it popped out while riding normally -and- reading of everyone else's leaky caps, it promped me to examine it very closely early in my bike's ownership. I noticed very minute casting flash crossing under the O-ring's sealing surfaces on the black plastic base, the part the cap screws into that's pressed into the valvecover. Check both the inner and outer O-ring's channels. Careful scraping with a knife removed the barely noticeable flash. Re-installing it required a little oil, a large socket serving as a special tool and a lot of hand pressure. Do not attempt to remove yours if you have minimal wrenching talents as you'll likely break it trying to re-install it.
I'm one of the few weird one's that likes the lightweight, heavily finned, cool looking, leak-free, stock plastic fillercap.
Well !I went on a 180 mile ride today in about 92 degree weather (I don't know if that makes a difference, but thought I would throw it in there just in case). When I stopped for lunch I noticed oil around the oil filler cap and some on the sparkplug cover. Not a whole lot, but enough for me to notice it. I could run my finger over it and pick up some on my finger. I took a look at the stock oil filler cap's O ring. It still looks brand new. I wiped off all the oil and put everything back together. I don't see any leaks from anywhere else on the bike. If it matters what type of riding I did - I rode about 45 min in stop and go traffic, then rode 80% of the rest of the trip above 70 mph, hitting 105 a couple of times. Shifting usually around 4800-5000 rpms and riding around 4000-4200 RPM's.
Is there a chance that I have an ongoing problem with the (crankcase) pressure building up and not being released the right way? It seems like the normal pressure build up should have a place to be released and I don't think it would be through the filler cap.
The combination of stop-n-go traffic, high engine speeds and high summer heat can easily get the engine and it's oil hotter than 300F (148.9C) - - - especially reving the engine wih the gearbox mostly in the lower 3 gears. That means the windspeed is marginal for the extra heat being generated by a high reving big twin. That heat can cause the plastic oil fill cap and mating base to stretch/distort while the overheated O ring softens enough to allow the crankcase's pulsating pressure waves to force out a small amount of oil. Also possible is excessive piston blow-by from an overheated, heavily loaded engine overwhelming the timed breather's nominal flow capacity. Most likely a combination of the two. Normally there is a slight negative crankcase pressure maintained by the timed breather passageways machined in the crankshaft when the engine is running. You can observe/confirm the negative pressure by unscrewing the oil cap while the engine idles. It will be pulsating violently and held in place by suction.
Since stop-n-go traffic really overheats the engine in summer, running it over 4,000 RPMs in the lower gears while cruising steady in the lower 4 gears will make it much hotter still. With conventionl oils you have a recipe for disaster when oil temps go over 300F (148.9C) for an extended period. Not so troublesome with synthetics.
Sometimes the riding pace doesn't allow taking it easy on an overheated engine, but you must not make things worse by using high steady state RPMs in the lower 4 gears when lower RPM's will do. When faced with those conditions described above, my tach needle is 2,200----2,600 steady cruising in the lower gears until I see the crankcase temperature go below 210F (98.9C).
BTW, My oil cap has never leaked, but after it popped out while riding normally -and- reading of everyone else's leaky caps, it promped me to examine it very closely early in my bike's ownership. I noticed very minute casting flash crossing under the O-ring's sealing surfaces on the black plastic base, the part the cap screws into that's pressed into the valvecover. Check both the inner and outer O-ring's channels. Careful scraping with a knife removed the barely noticeable flash. Re-installing it required a little oil, a large socket serving as a special tool and a lot of hand pressure. Do not attempt to remove yours if you have minimal wrenching talents as you'll likely break it trying to re-install it.
I'm one of the few weird one's that likes the lightweight, heavily finned, cool looking, leak-free, stock plastic fillercap.
Had a BMW mechanic tell me today that running synthetic oil could be my problem, also said synthetic was not necessary in my bike since I change it every 5K anyway.
