Air pressure in the cam cover?

[Hokiefyd]

Hey guys,

When I open the oil fill cap on our MDX with the engine at idle, there is no air pressure in the cam cover, and I can look straight in and watch the valves move, etc.

But when I open the oil fill cap on our CR-V with the engine at idle, air puffs out the top of the cam cover with a "puff-puff-puff" as the engine idles, and sounds like you're holding a rag over the exhaust pipe.

One of the engines is a V-6 and one is an I-4, but which behavior is normal? Or maybe they're both normal for each respective engine architecture? As I understand it, air will flow into the crankcase through the breather and out through the PCV valve? Would air pressure under the cam cover indicate a stopped PCV valve?

 

[Bob Woods]

My grandsons 2000 four banger Accord puffs out the oil fill port like you describe, it also does it if you unplug the pcv valve with it running. The oil doesn't get dirty so maybe it is normal I don't know. Hopefully some one will give us the answer. Cheers Bob

 

[Tegger]

Originally Posted By: Hokiefyd
Hey guys,

When I open the oil fill cap on our MDX with the engine at idle, there is no air pressure in the cam cover, and I can look straight in and watch the valves move, etc.

But when I open the oil fill cap on our CR-V with the engine at idle, air puffs out the top of the cam cover with a "puff-puff-puff" as the engine idles, and sounds like you're holding a rag over the exhaust pipe.

One of the engines is a V-6 and one is an I-4, but which behavior is normal? Or maybe they're both normal for each respective engine architecture? As I understand it, air will flow into the crankcase through the breather and out through the PCV valve? Would air pressure under the cam cover indicate a stopped PCV valve?

Both behaviors are normal. Which one you end up seeing on your MDX's V6 depends on the exact configuration of the emissions system of that vehicle.

If the PCV system were plugged, you'd see LOTS of puffing from the oil-filler cap, regardless of anything else.

How many miles on the CR-V and the MDX?

 

[Hokiefyd]

Thanks guys. The Acura has a shade over 70,000 miles and the Honda is closing-in on 50,000 miles.

 

[mechtech2]

Air fluctuation are not the test. Some do, some don't.
Vacuum is.
There are sensitive gauges which will tell you what is up, They go over the intake breather or it's hole.
Or, a piece of paper over the fill hole when it's idling.

 

[Tegger]

Originally Posted By: Hokiefyd
Thanks guys. The Acura has a shade over 70,000 miles and the Honda is closing-in on 50,000 miles.

Then don't worry about it.

If there was something really wrong, you'd have a Check Engine light and/or driveability issues.

 

[Hokiefyd]

Originally Posted By: mechtech2
Air fluctuation are not the test. Some do, some don't.
Vacuum is.
There are sensitive gauges which will tell you what is up, They go over the intake breather or it's hole.
Or, a piece of paper over the fill hole when it's idling.


What would the piece-of-paper test do? My CR-V would blow the piece of paper up like strips of paper on a fan.

 

[OVERKILL]

What are you seeing may be blow-by.

 

[mechtech2]

If the paper is not sucked flat, and is being blown up, you failed the test.
There is improper vacuum in the system.
Blow by is overcoming your PCV system.

The intake vacuum should be more than the blowby except art full throttle.

 

[onion]

Originally Posted By: Hokiefyd
Hey guys,

When I open the oil fill cap on our MDX with the engine at idle, there is no air pressure in the cam cover, and I can look straight in and watch the valves move, etc.

But when I open the oil fill cap on our CR-V with the engine at idle, air puffs out the top of the cam cover with a "puff-puff-puff" as the engine idles, and sounds like you're holding a rag over the exhaust pipe.

One of the engines is a V-6 and one is an I-4, but which behavior is normal? Or maybe they're both normal for each respective engine architecture? As I understand it, air will flow into the crankcase through the breather and out through the PCV valve? Would air pressure under the cam cover indicate a stopped PCV valve?


My 2004 Accord's 2.4L I-4 engine does exactly the same thing (is this the same engine as you CR-V?). It concerned me a little as MOST engines will not do this... and excessive pressure in the crankcase is often a good indication of excessive cylinder wear. But I saw no other signs of problems, and I know from experience that some Mercedes medium-duty diesel engines (the 900-series, specifically) do exactly the same thing even when new. So I went ahead and bought the car.

I replaced the PCV valve just in case- it made no difference. Now that I've had the car for almost a year, I can say that it uses absolutely no oil, and I've seen no reason to believe that there's anything wrong with the engine. I'm convinced that this is normal for this engine. I suspect that the pulsation is a result of how the crankcase is vented, along with relatively small crankcase volume and relatively large air movement within it due to the 4-cylinder's unwieldy firing order (paired cylinders on 1 and 4, and paired cylinders on 2 and 3).

Now on most American engines that I've dealt with- mostly V6, V8, and I-6 engines, this pulsation while running with the oil cap removed would be very concerning. With most of these engines you'll feel very little if any air movement at an open oil cap. But I suspect that the difference is that these engines have a larger crankcase volume, less crankcase pressure variation (due to having more cylinders), and possible differences in crankcase ventilation.

 

[Hokiefyd]

Originally Posted By: onion
My 2004 Accord's 2.4L I-4 engine does exactly the same thing (is this the same engine as you CR-V?). It concerned me a little as MOST engines will not do this... and excessive pressure in the crankcase is often a good indication of excessive cylinder wear. But I saw no other signs of problems, and I know from experience that some Mercedes medium-duty diesel engines (the 900-series, specifically) do exactly the same thing even when new. So I went ahead and bought the car.


Yes, it's the "same" K24 engine. Mine's a K24Z1 and I think yours should be a K24A4, but they're largely the same. Mine uses no measurable oil either, doesn't dirty the oil substantially, and there doesn't appear to be anything amiss with the engine. It runs out smooth.

 

[Hokiefyd]

Originally Posted By: mechtech2
If the paper is not sucked flat, and is being blown up, you failed the test.
There is improper vacuum in the system.
Blow by is overcoming your PCV system.

The intake vacuum should be more than the blowby except art full throttle.


Hmm... Let me think out loud here.

In this vehicle, there is a hose connecting the intake tube from just upstream of the throttle body to the cam cover. I assume this is the breather hose, and air should be flowing through it from the intake tube to the cam cover (right?). Then in theory, air would travel down into the block and out the side of the block (where the PCV valve is) and through the hose that connects to the intake manifold (right?).

With the oil cap CLOSED, should I check the breather hose and see if air is getting pushed out through it? Because if so, that would be reverse of what it's designed for, right? I did look into the breather hose (with the engine off) and there's no real oil in it, so I don't think air is getting pushed back out.

Could it be that with the oil cap OPEN, that's just the path of least resistance, so crankcase pressure is venting through that INSTEAD of the PCV system, bypassing the PCV entirely?

So far, about 50% of the responses say this is normal, and 50% say it's too much blow-by.

 

[KrisZ]

My Mazda 3 does the same thing. But the engine has a breather hose going into the intake pipe before the throttle body and a PCV that connects to the intake manifold. No oil consumption.

If the car in question has similar arrangement, I'd say it's normal.

 

[OVERKILL]

Originally Posted By: KrisZ
My Mazda 3 does the same thing. But the engine has a breather hose going into the intake pipe before the throttle body and a PCV that connects to the intake manifold. No oil consumption.

If the car in question has similar arrangement, I'd say it's normal.


That's how my Expedition is setup, and most vehicles (except my BMW which has a bizarre setup due to not having an intake manifold).

I think crankcase size being mentioned is important. With the fill cap off the Expedition, you don't feel anything.

 

[jaj]

As these engines run, the actual volume of the crankcase changes slightly as the pistons move up and down in the cylinders. The changing volume acts like a pump that pumps air in and out of the open cover. The more cylinders, the smaller the effect and the more precisely it cancels out.

 

[OVERKILL]

Originally Posted By: jaj
As these engines run, the actual volume of the crankcase changes slightly as the pistons move up and down in the cylinders. The changing volume acts like a pump that pumps air in and out of the open cover. The more cylinders, the smaller the effect and the more precisely it cancels out.


This is quite right. And usually why one feels positive and negative "pulses" when checking for crankcase pressure.

However, from what the OP is describing, he only has positive pressure which strikes me as being possible blow-by......

 

[mechtech2]

Cars vary in the air flow out of the filler cap at idle.
Big time. Some pump and fluctuate a lot.

But there should be a negative pressure from the PCV [intake vacuum] when it is 'sealed' by the paper test.
Different PCV inlet breather sizes will modify this, to some extent.
But be happy with vacuum there.

 

[Hokiefyd]

I did note in my original post that it will "puff-puff-puff" out the oil fill hole. I would characterize this as "pulsing". I wouldn't argue against there being vacuum at the hole between the "puffs".

And if the paper test means if you can get the paper to seal, would it stay sealed? I don't know, that is possible. I haven't tried to force a piece of paper to seal and then see if it would stay sealed.

For what it's worth, when I go to screw in the oil fill cap, it doesn't try to blow it out of my hand...it doesn't seem to take much effort to "plug" the hole, so it sounds like it has a normal "pressure-vacuum-pressure-vacuum" pulsing thing going on.

Thanks again everyone.

 

[mechanicx]

I think 4-cylinders do this because they have a secondary imbalance (pistons at the top of their stroke move faster or further than the pistons at the bottom of the stroke) which leads to a slight crankcase volume imbalance. Six and 8 cylinder engines have secondary balance and you don't notice this pulsing.

 

[mechtech2]

Originally Posted By: Hokiefyd
I did note in my original post that it will "puff-puff-puff" out the oil fill hole. I would characterize this as "pulsing". I wouldn't argue against there being vacuum at the hole between the "puffs".

And if the paper test means if you can get the paper to seal, would it stay sealed? I don't know, that is possible. I haven't tried to force a piece of paper to seal and then see if it would stay sealed.

For what it's worth, when I go to screw in the oil fill cap, it doesn't try to blow it out of my hand...it doesn't seem to take much effort to "plug" the hole, so it sounds like it has a normal "pressure-vacuum-pressure-vacuum" pulsing thing going on.

Thanks again everyone.


Puff puff puff is normal with the filler cap off - some more, some less.
But when it is sealed [like the paper test]vacuum should be present.
I realize that there are oddball PCV systems like on BMWs and others. But they are not normal.