Oil Fill Cap Questions

[natenkiki2004]

I'm not really looking to diagnose anything with this post, just discuss something I'm thinking about.

The oil fill cap, first off, does yours tighten and stop or does it constantly spin once it's cranked down? Is it meant to ratchet like the gas cap?

Being that the oil goes into a valve cover, there's a tube that runs from the fill neck into the intake tube after the MAF sensor. Wouldn't that mean that if the cap was faulty and not sealing, it would let excess air into the intake? Wouldn't that also mean that a PCV is un-necessary since there can't be any excess pressure in the crankcase due to the tube on the fill neck?

 

[vq5speed]

My oil cap ratchets too that is the way it is.

The vacuum at the PCV valve is sucking air from the crankcase to relieve blowby pressure. That vacuum is really strong and will suck more air faster then the amount that is getting past the rings. So its also drawing air from that tube connected to the air intake going to the filler neck. The main purpose of vacuum through PCV is to relieve that blowby pressure. But the PCV is there to seperate oil from the vacuum. So if you took out the PCV valve and plugged that vacuum line then the air that normally would draw from the intake in through the filler neck would go the other direction and suck crankcase air into the intake and draw possible oil with it. You would almost need to put some sort of catch jar there to catch oil. I think its just better to leave it the way it is. I know on early hot rods they would just have two valve cover breathers and just let blowby blow out into the atmosphere. But they say that is polluting the air.

 

[natenkiki2004]

One would think that with the PCV in place, you wouldn't need the hose on the filler neck. Maybe that's why the intake gets so dirty, it's sucking oil mist in from the filler neck.

Maybe the engineers found that with the bigger displacement engine, they needed more than what the PCV could handle, being that it has small hose on it.

I would think that any blowby would take the path of least resistance and go through the big hose on the filler neck, rather than the restrictive PCV.

 

[Wolfie_85]

Mine ratchets too. I think the tube going to the filler neck from the intake works together with the PCV valve. I've never questioned it. If the engineers thought that it needed to breathe more on an engine that has no problems putting several hundred thousand miles on the clock, they must have been right.

 

[vq5speed]

One would think that with the PCV in place, you wouldn't need the hose on the filler neck. Maybe that's why the intake gets so dirty, it's sucking oil mist in from the filler neck.

Maybe the engineers found that with the bigger displacement engine, they needed more than what the PCV could handle, being that it has small hose on it.

I would think that any blowby would take the path of least resistance and go through the big hose on the filler neck, rather than the restrictive PCV.

The vacuum is stronger closer to the combustion chamber. Air will move in the path where vacuum is strongest. Try removing the hose that is connected to the PCV when the engine is running. You will see how strong that vacuum is there then you will understand why air travels in that direction. Pull the hose off the intake while the engine is running you will feel the air sucking into the filler neck.

But all the air that enters the engine goes past the MAF sensor first. So it all balances out. If you took that hose off the intake and just put a breather and let outside air get sucked in then your adding air that was not read by the MAF sensor.

 

[natenkiki2004]

If you took that hose off the intake and just put a breather and let outside air get sucked in then your adding air that was not read by the MAF sensor.

That's exactly what I was wondering. Maybe a faulty oil fill cap (mine just spins and spins, no ratcheting) would leak air and then that would get sucked in AFTER the MAF sensor and would be unmetered.

I wonder if that's a hidden item that people overlook.

Good info on the vacuum being stronger closer to the combustion chamber. Actually, thinking about it more, the PCV hose would have a much higher vacuum since it's on the other side of the throttle plate which is what creates most of the vacuum in the first place. So, that makes sense that air would get sucked through the PCV, through the driver's side valve cover, through the crank case, through the passenger side valve cover, through the filler neck and then the hose going to the intake tube right after the MAF.

So, not having a tight or properly working oil fill cap could have negative effects?

 

[vq5speed]

The cap is 2-pc the thread part is separate from the top part you twist. Your cap should have a rubber gasket that seals the threaded halve. So I don't think you would be getting any vacuum leak there unless the gasket is missing or the cap is cracked. Hold the bottom part of the threads and you should be able to twist the top part that ratchets. guess you'd have to inspect your cap to see if its cracked on the threaded halve then I think you could be getting a vacuum leak. How do you get your cap on and off if it just spins..

 

[2stroke]

Ratcheting oil cap on a first gen? Mine doesn't, and I've never seen one. Mine just screws down tight. That breather tube is for exactly what it sounds like, for the engine to breathe. The PCV valve is kind of a one way deal to keep oil in. The problem with that is then you would be recirculating the same nasty gases into your crank case. The breather tube allows fresh air in. Air can come in and out of your oil dipstick too.

 

[natenkiki2004]

The problem with that is then you would be recirculating the same nasty gases into your crank case. The breather tube allows fresh air in. Air can come in and out of your oil dipstick too.

That makes a lot of sense. The air flowing through the crankcase to make sure any nasty gases are sent into the combustion chamber and burned.

My theory still holds true then, if the oil fill cap was malfunctioning, loose or missing entirely, that would mean that unmetered air could get in or metered air could get out.

I'll check my cap tomorrow. If I recall right, I press down and twist it off and it comes off just fine. Tightening though, it goes down a bit and then the top part just spins. There's no way to ensure it's tight. Caps are cheap, I might see about getting an aftermarket one at the auto parts store and see how it behaves, if any different.

 

[vq5speed]

I was trying to figure out why they ratchet. I think so you don't over tighten? it I don't know...

My dip stick has a rubber O-ring on it that keeps it sealed from air entering.

 

[natenkiki2004]

I pulled my cap off today and looked at it. It locks in when you go to loosen it (no spin) and when you go to tighten, I can feel what I can only describe as worn out teeth in the ratchet mechanism. Got a new aftermarket cap from STANT and it's a solid piece of plastic, no ratchet. But it does have a rubber gasket. I don't know that it's quite enough as the filler neck edges are bent upward slightly and the plastic edge of the cap seems to meet that first, before the gasket seats.

I did notice that with the stock cap, since it didn't crank down anymore, it had a slight wobble to it as it was 'tightened' all the way. I don't think it sealed. So I'd say, if your cap isn't ratcheting, get a new one.