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Oil tank cap pop off - anybody seen this?

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Old 03-17-2006, 11:31 AM
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Default Oil tank cap pop off - anybody seen this?

I will give you a history so that maybe you can give me some pointers about where to begin…

Dealer did the 1.000 mile service on my new Night Train in January, I think it was.
I rode the bike about 300 miles after the service, bringing the total miles to something like 1500 or so.
I left for Daytona, visually inspecting the bike but I didn’t really touch the bike.
I rode in a variety of conditions, and spent about 5 hours on I-95 at about 75-80 mph on the way to Daytona.
I stopped in Brunswick, GA to fill up for the final run into Daytona.
I stopped at the Florida welcome center about 60 miles later to call my friend and tell him that I would be arriving in Daytona that night, after riding straight through. While at the welcome center I smelled burnt oil thinking it was coming from the car next to my bike. When the car backed out his lights hit the right side of my bike which I then noticed was glistening with some kind of moisture. After taking a closer look, I noticed that the oil filler cap had popped up and that oil had been streaming down the oil tank, on to my pipes, rear saddle bags, rear tire, and rear brake caliper and pads.
For the first time, I pulled the oil filler cap and checked the oil level. It was slightly above the oil fill line. I rode another 15 miles, stopped and bought some 20-50W oil, and brought the oil back up to the proper level with about ½ quart of oil.
Since I was wearing leathers and loaded down with luggage, I thought there was a possibility that by dragging my boot just over the seat to get off the bike that I had somehow kicked the oil filler cap loose.
When arriving in Daytona, I took the filler cap off, cleaned the rubber seal, cleaned the filler tube, and put the cap back in making sure it was secure and that the oil level was correct.
I spent some time hunting down some Harley execs while I was there. They were unaware of any specific issue regarding the oil filler cap. An employee at the Daytona Harley shop, however, said that he had heard someone talking about this kind of problem.
I rode the bike for 8 days around Daytona at varying speeds.
I rode back to Roanoke, stopping in Charlotte, NC this time, on business. I rode straight through to Charlotte. When I make my last stop for gas just south of Charlotte, I noted that the cap had popped up again. I thought, AHAH, I have kicked the cap loose again, but I couldn’t imagine that I could hit it with such a precise strike that it would pull the cap out of the tank. I pressed the cap back in and rode to Roanoke the next evening without event.
I rode the bike to the RVHOG meeting Tuesday night.
Tonight, I rode about 5 minutes from my house. I went in to a restaurant for about 1-1/2 hours. When I came out, I noted that the cap had popped off and that there was oil down the side of my bike again. The weather has been reasonably warm on the way back and since I returned and I have worn my chaps and was able to get off the bike with no threat of hitting the cap. I have deduced, therefore, that it is not me that is making the cap pop off.

This is driving me crazy. I haven’t found, on first look, anything on the internet about this kind of problem. There are two crankcase vent tubes, one from each cylinder, that hook up to the EFI system. There is a vent tube from the tranny, too. I seriously doubt that all three of these could be plugged in such a way that the cap would be popping off from crankcase pressure. I would hate to think that I need to continually check to see that the oil filler cap is seated properly… and that something like vibration is causing it to work it’s way loose, but that is all that I can come up with. I am thinking about getting a flat faced oil ring to put over the rubber stopper to tighten up the filler cap.

Do you have any ideas?


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Old 03-17-2006, 11:49 AM
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Default RE: Oil tank cap pop off - anybody seen this?

I was over in the special project site last night and read about the same problem. Here is the link to it:


Good luck on figuring this out.
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Old 03-17-2006, 11:53 AM
itsahog itsahog is offline
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Default RE: Oil tank cap pop off - anybody seen this?

ORIGINAL: itsahog

When you pull the dipstick is is tight? On mine I have to twist it and work it out. I have never heard of any other problems (other then Whiskers above information)and it has never happened on my 05 DLX. I would have the dealer install a diffrent dipstick or buy yourself one you like better then stock and see if it still happens. Mine is not the stock one, I have a HD emblem dipstick. Looks good - works good. Good luck and let us know what the outcome is. [:-]
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Old 03-17-2006, 12:33 PM
bancquo bancquo is offline
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Default RE: Oil tank cap pop off - anybody seen this?

Had exactly the same with my FXST 2003, but only once.
Rode about 30 miles, lot of high rpm and low gear work.
When I arrived, I had leakage of oil from the air cleaner cover, which I've put down to oil getting blown out through the breather.
When I started it up again, the oil filler cap popped right out, and landed on the ground beside me.
Oil level was normal - not excessive.
I've put it down to excess crankcase pressure.
Hasn't happened again since.
i have factory fitted Stage 1 with K&N filter.
I believe there is a change to the breather arrangement with this, but I'm not really sure.
I do get blow by of oil after a lot of high rpm riding - none the rest of the time.
I don't think the breather is particularly effective on the TC88....
Others might know better.
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Old 03-17-2006, 02:28 PM
LRonHoover LRonHoover is offline
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Default RE: Oil tank cap pop off - anybody seen this?

Are you using one of those new ultra micron oil filters? What are they down to, 5 microns?

Just a thought.
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Old 03-17-2006, 05:49 PM
went_postal went_postal is offline
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Default RE: Oil tank cap pop off - anybody seen this?

weird... the seat on my fatboy actually sits on top of the cap... It is not going anywhere.
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Old 03-17-2006, 05:57 PM
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Default RE: Oil tank cap pop off - anybody seen this?


You could always lash that puppy down and see what else pops. [:-]

I don't know of any combo cap / tank pressure guage.


Blowby in one of the rings?

Water mixed with oil? It would not take much to build steam pressure.

Too high oil level?

Loose cap fit? Like others, I have to work mine a bit to get it off.
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Old 03-18-2006, 10:28 PM
91springer 91springer is offline
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Default RE: Oil tank cap pop off - anybody seen this?

i had that happen once on a 91 custom sportster. a long day of fast driving and high rpms. i got home at night and realized what had happened when i took off my shirt and had oils specs all over the back. i just thought it was due to the new (aftermarket) oil tank i put on. i bought a new cap the next day at ;the harley shop ($40) and never had an issue again. although i got rid of it a year later
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aftermarket, blew, blown, cap, davidson, deuce, dipstick, filler, filters, harley, oil, pop, popping, tank, tc88

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