I’m probably just being stupid, or behind the times.
I opened the oil cap on my car with the engine running, and found strong positive pressure under the valve cover.
Does this mean blowby?
Old days I always used to add oil to my car with the engine running, but it would be impossible in this case.
Honda 2.5L I4, 2011

The pressure should be vented to the air filter housing. Take a close look at the vent plumbing from the air filter housing to the valve cover. But yes. It is blowby.

This car has only 30k on it. I assume that this is an incorrect, and possibly serious, situation.


Just curious . . . why did you add oil with the engine running?

In the old days, I mean

Just quoting you . . .

The pressure is from blowby, but are you using excessive oil? If not, I’d just be sure PCV plumbing and valve (if your vehicle has one) to be sure it’s all flowing freely. The pressure should be getting drawn back into the engine via the PCV system.

Excessive blowby would be exhaust smoke, and heavy blowby would almost certainly mean excess oil passing the rings into the combustion chamber which would result in the exhaust blowby being a very obvious dirty smoke to blow out of the oil fill port. Again, it would be worthwhile to inspect the crankcase vent before jumping to conclusions. And if the crankcase vent becomes plugged and pressure builds in the crankcase it will cause oil consumption and a rapid deterioration of the engine when the catalyst gets toasted.


Another question . . . did you have a particular reason for wanting to remove the oil filler cap with the engine idling?

If everything was fine . . . and I don’t know your car’s history . . . why would you want to do that?

Just being curious . . . ?

In this case, I was putting in an additive (CD2 oil detergent) and wanted to be sure it got well mixed. The car uses no measurable amount of oil (I have 3k on it since I bought it about 6 weeks ago). The gases coming out of the oil fillter hole are clear, and don’t look like smoke.
So, I will check out the gas plumbing.

Over filling the crankcase can result in the oil level reaching a level above the piston skirts at bottom dead center. That might result in a rise in pressure as the pistons reach the bottom.

Was it really positive pressure, or just strong pulsations?
If you try to rest the cap on the hole does it blow straight off or does it just vibrate a lot?

Perhaps there is a perception of a problem, when in reality everything is fine

Pull the rubber tube from the intake pipe to the valve cover. There should be little if any oil inside. If oil drips out, you may have a problem.

do you mean from the air intake (past the air filter) to the valve cover?
And yes, there is positive pressure. Lots of air blowing out of the oil inlet hole.

That’s the one.

You shouldn’t have that much pressure. Normally the only source of pressure to the crankcase is blowby, and you shouldn’t have that much blowby. The crankcase is directly connected to the open spaces under the valvecovers. It is, however, possible that an exhaust valve is leaking and pushing gasses past the valvecover seal and thus pressurizing the space under the valvecover…

A compression test would be a really good idea. Post the results. A leaky valve should also show up on a compression test, but it should also be noisy (a ticking noise) and show up as an unstable vacuum gage (test) too.

Post the results.

THAT HONDA IS IN PERFECT RUNNING ORDER…WE ARE TRYING TO SOLVE A PROBLEM THAT DOES NOT EXIST… The point here is DO NOT REMOVE THE CAP AND TRY TO ADD OIL TO THAT MOTOR WHILE IT IS RUNNING. When you open that cap while running you Break the seal on the PCV SYSTEM…and it will produce a lot of “WINDAGE” from the open oil cap with a running engine.

Just add oil when the engine is off…and MOTOR ON…THERES NOTHING WRONG WITH THAT HONDA


In the old days STP was poured into a warm engine while running so it would have a better chance of mixing properly. Those were the instructions written on the can.


But isn’t STP now known to be snake oil . . . ?!

@Honda Blackbird , I am willing to accept that you may be right, since this car has no other signs of anything wrong with the engine. But answer me a question. Cars of the past did not have this pressure. In fact, the PCV valve modulated a vacuum to suck out of the valve cover area any stray exhaust fumes that got there. There was a device on the engine to ventilate, often with a wire mesh filter. Has everything changed? Do modern engines generate pressure up there? Certainly the amount of air is greater than you would get pulled through a PCV valve.

Given how cheap a PCV valve is…why not just go ahead and replace the PCV valve?
Even if it doesn’t improve the situation, you will only have spent…IIRC…about $3.00 on this experiment.

Yup, everything has changed. Those old mushrooms on the valvecovers with the wire mesh did a great job of ventilating the crankcase, but they allowed hydrocarbon molecules to also get vented along with the fumes. The EPA many years ago declared that a major no-no. EPA regulations have forced manufacturers to go to extreme measures to prevent the release of hydrocarbon molecules, much to the chagrin of many people struggling to stop EVAP system fault codes.