91 accord - air blowing out oil filler cap

So I have this old honda that I drive occasionally. It has been parked longer than usual - about 10 weeks or so.

I started it up and let it idle, then popped the hood and started checking fluids and things. The oil was a bit low so I opened the oil filler cap and there was a significant amount of air (exhaust I am assuming but I did not lean over to smell it) blowing out of the oil filler cap. Like lots. Enough that when I stupidly started to pour the oil it wouldn’t pour against the air flow and blew all over the valve cover etc.

So I know this has not done this before. My questions are these:

What is wrong? I keep thinking some sort of valve shaft seal but that is just a guess.

Will the extra pressure under the valve cover cause the valve cover gasket to rupture?

Can I still drive it anyway?

This car has 240k miles and has never needed any serious engine/transmission work and I hate to start now.

Thanks in advance

A clogged PCV valve and/or worn rings will contribute to crankcase pressure. Why do you check the engine oil level with the engine running? It should be checked with the engine off. With the engine running it will read low and you’ll end up overfilling it.

That is what is called “blow-by”. It will be less after the engine warms up.
Yes, it is safe to drive. And, no, there is nothing you can do about it.

You should check the oil level, and add if necessary, only with the engine OFF. You will get an incorrect dipstick reading with the engine running. You may not have needed to add any oil at all.

Check the oil again, with the car on a flat surface and the engine OFF. Then add or drain oil as necessary to achieve the correct dipstick reading.

There is nothing to worry about as long as you don’t overfill the engine with oil. Drive it and don’t worry.

I am not familiar with the plumbing on that car, but there should be a couple of places for blowby to go such that it would not blow out the oil fill hole strongly enough to blow a stream of oil away. It is possible that while it was parked, the PVC valve stuck shut. However, at idle, the PVC is pretty much shut anyway, so that is not the main problem. It would not hurt to replace it or at least clear it with carb cleaner solvent.

There should be a passage from the crankcase to the air inlet duct between the mass airflow sensor and the throttle valve. It may have a mist eliminator and/or a flame trap in it. My guess is that it is plugged up and needs to be cleaned. Disconnect the tube (should be about the thickness of your thumb) that connects the air duct to the crankcase and try to blow down it to see if it is plugged. The easy way to clear it (sort of) is to just pour some SeaFoam solvent down the tube and let it soak overnight. That will probably help but is no substitute for dismantling the whole system and cleaning it. If you pour in solvent, change your oil immediately after as you will be dilluting your oil with solvent.

One way to see if it’s rings or something else - do a compression test. Make sure you follow the instructions, or have a mechanic do it.

You have excessive blow by. If there is enough positive pressure you can start blowing gaskets/seals and it sounds like that is the case here. If your car has a PCV, you should check/replace it to minimize the effects of the pressure or you’re going to have a serious oil leaker in short order. Likely, the only “cure” is an engine rebuild.

You are NOT supposed to check the engine oil level or ad oil to the engine while the engine is running.

The pressure you feel is the normal pressure that is created inside the engine as the pistons travel up and down in their cylinder bores.

Turn the engine off when you are checking the oil or adding oil to the engine.


Most engines employ an even number of cylinders. When one is going up, another is going down so they tend to counteract each other. There should be a slight NEGATIVE pressure at the oil fill hole when the engine is running if the PCV is working correctly.

Total displacement inside the crankcase remains the same no matter what position the crank is in. Therefore, the pistons moving up and down won’t cause any air flow. There is no mystery here. It’s excessive blow-by due to the rings being worn. This is not unusual for an engine with a lot of miles on it. Ignore, rebuild, or replace.

You have excessive blowby, as others have stated, due to wear in the cylinders, wear on the rings, and loss of spring tension of the rings, all normal for a 17 year old 4-banger with 240,000 miles.

Even though all this can be considered normal, I do recommend that you check your PCV valve and the breathing passage referred to by Manolito (they’re both part of your closed-loop Positive Crankcase Ventilation system) because excessive crankcase pressure can also push oil past tired old crank seals. To be honest, other than doing that I’d just keep on driving it.

You can’t usually put oil into a running engine. If you even start it up right after putting oil into it, the rocker arms will throw some out of the filler. If something is wrong with your car, you probably wouldn’t want to spend any money on it anyway. It runs like a car with 240,000 miles on it.

Thanks for all the responses. I will check the pcv valve this weekend. I can’t remember when it was replaced last - 2005 maybe?

I am going to drive this car until it does not move anymore. I bought it in 2003 for $1000 at 140k. I have already gotten my money’s worth.

All of the automotive repair manuals that I’ve read say to check the oil while running and warm.

In 25 years, I’ve never had a car that said that.

As most of the other posters mentioned, why would you be putting oil in a running engine??

If there is a large volume of air blowing out the oil filler, your engine is worn out, it’s as simple as that. Instead of the rings sealing the combustion chamber, combustion pressure blows by them into the crankcase. The PVC system must deal with this blow-by and it it can’t, the excess will blow back into your air cleaner via the crankcase INTAKE filter, which now has become the crankcase supplemental vent…(Hey! How come my air filter housing is full of oil!!)

Yes, for checking some kinds of oily fluids, on some automatic transmissions, you do do the check with the engine running; but, this doesn’t apply to Honda automatic transmissions (nor, to some other cars). Do I hear those fateful words, “Repair manual”?
The engine oil level is never checked with the engine running (just to clarify the servicing).

Transmission fluid is often checked with the engine running and warm. I haven’t heard of a car yet that recommends the oil be checked that way. It usually is impossible to put oil in a running engine. If the blowby doesn’t get you the rocker arms will or the engine fan will come on.

You are mistaking the motor oil for the transmission fluid. On most cars, transmission fluid should be checked while hot and while the engine is running. Motor oil should be checked when cold and when the engine is turned OFF.

That being said, it sounds like your PCV is stuck, which is certainly plausible with over 200k on the odometer. I suggest that you invest the money (under $5.00) in a new PCV. Even if I am wrong about you needing a new PCV, you will not have spent very much money.

Thats only for automatic transmissions. You do not check engine oil with the engine running.