ISL 370 popping the oil filler cap off

[Gary RVRoamer]

This is an ISL 370 with 56k miles. It's a late 2003 engine in a 2004 chassis.

When we stopped at a campground a couple days ago, I notice a few oil splatters on the back of the coach and tow bar. A brief look around found the oil filler cap dangling from its retainer (thanks Freightliner, or I would have lost it). Put it back on, and the same thing happened the next day. It doesn't seem to be blowing oil out continuously - just enough pressure to pop the filler cap off. No sign of excess oil usage, no problems with performance or the engine brake. It just suddenly developed a dislike for the filler cap!

The filler cap is a vinyl plastic with a bayonet lock. However, it's a real sloppy fit, so popping it off doesn't take much. It can turn easily enough to disengage the bayonet, or with a bit more pressure the soft plastic tabs would slip right over the bayonet pins.

Anybody ever seen anything like this? I changed the oil and filter yesterday (it was due anyway) and put some tape on the inside sleeve of the filler cap to reduce the slop. Haven't made a test run yet - that comes on Monday.

[RJay]

Hi Gary,
With the oil cap off and the engine running, if you put your hand over the opening of the filler tube can you feel pressure pulsating on your hand? I know you said you don't have any performance problems but if there is any unusual pressure coming from the crank case this would be one way of detecting it. It's probably a loose fitting cap that just vibrates off.

[Happycarz]

When you twist on the oil cap, it should be snug and secure, not loose. There is an O ring embedded in the cap that compresses when the cap is fully seated. Look carefully at the cap seating area for the O ring. If it is missing, or if it is there and no longer soft and resilient replace it. Or, just buy a new cap.
This is for the cap in the valve cover. If yours is in a different location, the above may not apply.

[J Walker]

Vibration or improperly putting the oil cap on caused mine to be hanging from its cord once. I put it on securely and never had it happen again. I cannot build pressure in my crankcase due to the slobber tube. Your 2004 may have a sealed crankcase. Check the pressure as already described.

[Gary RVRoamer]

The engine has a oil breather pipe (slobber tube) and is not sealed like the new engines with EGR.

The filler tube is a long 1 1/4" tube that runs along side the dipstick tube. You can't get at the valve covers on a rear mounted ISL, so an oil filler there wold be impractical (would have to go in from the bedroom floor). It doesn't have any sort of O ring or seal - just a fairly soft vinyl cap and sleeve that slips over the end of the tube and secures in place with a bayonet-type twist lock.

I'll recheck the pressure at the filler tube today.

[Gary RVRoamer]

The problem seems to be cured, either as a result of the oil & filter change or the duct tape I used on the filler cap sleeve to snug it up a bit. The cap is no longer loose and locks neatly in place. Made a 260 mile round trip and it stayed on during both legs.