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Oil Tank Explodes!

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rfranz1952

Member
Riding home from work tonight, and about 5 minutes into the ride, I heard this POP!, almost like a backfire. When I got home, I notice my oil tank cover was half off, and the temp gauge cap looked askew. On further investigation, the top to the tank is separated from the body right along the seam around the top edge.

Apparently, what I heard was the tank exploding. What would cause this? Anyone else with this experience?

Bike was running fine, before and after. No missing, it seemed to pull strong, as usual. I could touch the side of the rocker box covers when I got home, and they were warm, but not unusually so.

It sounded like a random backfire. I was slowing down for a stop sign and turn. Based on where I heard the "backfire", and my normal commute time, I did ride it for about 7 minutes after it happened. The only thing that was a little "odd" was that it was a bit chilly, about 15, but I have ridden in those conditions many times--down to 10 or so. I did have to pump the throttle a bit to get it started and running for the first minute or so, but that is typical when it is this cold.

The tank separated at the top seam, and popped the cover partially off. It did not lose any oil. I run AMSOIL 20-50. When I drained the oil out of the tank, I did notice some light color with it, like there was a bit of moisture in it at first, but it was pretty limited.

If the breather hose that drains from the heads became frozen shut would that create backpressure through the oiling system?

By the way, this top end has about 600 miles on it. I suppose a compression check is in order.
 
Isnt the oil system vented? If tank exploded then it would appear there was pressure in the tank. Something is not breathing. Or as Glider said possibly gas is getting into the oil.
 
I think I figured it out.

I pulled the oil tank yesterday. In order to get the tank out, you have to, at least on an 05, remove the rear fender, which is a job in itself!

Anyway, not only was the seam split, there was about a four inch rip through the top of the tank, so there is no question of being able to repair the split seam. The plastic from which the tank is constructed is approximately three-eighths of an inch thick at the point of the rupture. There was obviously a great deal of pressure applied.

I have also studied the schematics of the system in my service manual.

1. Oil feeds via gravity flow from the bottom of the oil tank to the oil pump. It is gravity feed, because the geroter’s in the oil pump do not provide suction, they simply cavetate (run empty) when oil is not flowing into the pump.
2. The oil pump moves the oil, under pressure, which varies with the temperature of the oil and engine rpm, through the filter and into the oil passages of the engine. The oil pressure sensor is located in the entry cavity to the oil filter; if there is insufficient pressure at this point, the oil pressure light will illuminate. There is a check valve at the exit to the filter, which permits one way flow only.
3. After passing through the engine, oil gather in the sump, and is returned to the tank by the scavenging pump. The oil enters the tank from the bottom of the tank. The return line is quite long.
4. A “vent” line passes from the bottom of the oil tank back to the cam case. When there is excess pressure in the oil tank, this vents to the cam case, and since it attaches to the bottom of the oil tank, carries oil with it, lubricating the cam area. This oil also finds its way to the sump and the scavenging for return via the return line.

At this point, we have a closed system—there is no vent to the outside, and any pressure variations will be contained within the system—until the weakest point breaks. This could be a line, the tank, or seals and gaskets within the system.

Pressure within the system can come from two sources. The first is temperature variation. As the engine (and the liquids and gases within) heats and cools pressure will fluctuate gradually throughout the oiling system.

The second source of pressure variation comes from the downward stroke of the piston. This motion will create compression on the underside of the piston, just as the upward stroke does on the upper side. The volume of the chamber is much larger, but every time a piston is in a downward stroke, a pressure wave is created. This is unacceptable, because pressure underneath the piston is detrimental to performance. It also will result in pressure pulses to the oil tank because the tank is connected to the cam case via the “vent” tube. Therefore, the engine has a “breather” system, by which an outlet for this pressure is provided.

In the Sportster, the pressure wave from the downward stroke of the piston is routed from the crankcase to the cam case. It then travels upwards through the pushrod tubes to the breathers in the heads. In the stock configuration, this is routed to the engine intake through the air cleaner. However, a common after market modification is to route the breather into a catch can via a tube or tubes from the head breathers. . In addition to pressure, this system carries water vapor created by condensation along with a small amount of oil droplets or vapor out of the system. The result is that our catch cans normally accumulate water, and a white sludge that is a mix of oil and water.

The key is that both of these open the system, and relieve pressure variations within the system. If the breather is blocked for any reason, the oil tank, as well as other components of the system will be repeatedly hammered with pressure waves resulting from the action of the pistons.

I now checked my catch can. It is 2/3rds full of water-sludge mix. The drain line has some sludge lining it. Remember, it was on the order of 15 degrees (F) when this happened. My bike had been sitting outside the entire afternoon, while the outside temperature fell from the mid thirties (F).

I now believe that what happened in my case, and perhaps the other cases of exploding oil tanks, is that water/sludge in my head breather drains froze to the point where the system was blocked or nearly blocked when I started riding, resulting in the oil tank (and other components) being hammered with repeated pressure waves, and it eventually gave way.

As I said before, I have ridden in this kind of cold before—but it has always been early morning after the bike was garaged all night. The difference this time is that the bike sat in extreme cold with a catch can that was nearly full of water for four hours.

Lessons learned:

1. It was probably not a defect in the tank.
2. It was related to the cold, but not because the cold made the tank brittle (although that may have contributed to the situation, it was not a root cause).
3. Check and empty the catch can—and check to make sure the drain tube appears to be clear. Run a wire of something up the tube to verify that it is open in extreme cold.
4. A second arrangement for depressurizing or breathing the cam case may be prudent. However, if that is done, the head breather system should not be blocked off, because that eliminates the system redundancy.
 
Where do the head breathers fit into your theory? They vent the crank case to eliminate the pressure build up.
 
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