I did the 1st oil change yesterday on the Prius and ran the engine to fill the oil filter before doing the final oil level adjustment. I had left the oil filler cap off becuase I was expecting to add more. With the engine running a lot of gas (not fuel) was coming out of the filler opening. There was more coming out than any other auto engine I have seen, even old worn out ones. Anyone know where the gas pressure is coming from? Is everyones doing this?

Usually this would be called blow-by that is getting passed the rings or the vavle stem into the crankcase and is vented thru the positive crankcase vent (PCV) into the air cleaner and burned during combustion. The amount of blow-by increases as the engine parts wear.

 

I've never run with the oil cap off, Mike. Maybe the explanation relates to the valve timing on this engine? I know that under some conditions, the intake and exhaust valves are both open at the same time.

DAS

 

That would not be the case if the engine is running while parked. But regardless of the engine ops, the compressed air (fuel injectors off) or air/fuel mixture should not be entering the vented crankcase area of the engine.

 

Hi Mike,

This is interesting! Two weeks ago, I pulled the dip-stick and smelled the oil and it had a fairly strong gasoline odor. I was planning to change the oil in the next month and was going to have it tested.

The gas that was coming out, did it smell of gasoline?

I may try to capture some from my car into a balloon and find out if it is flamable.

Thanks for the tip!

Bob Wilson

 

On my older cars I used to perform a "jiggle test" to look for PCV problems. That just involves loosening the oil filler cap and just letting it rest in place while the engine is idling. The vacuum from the PCV system should be enough to hold it in place without rattling around. If the cap "jiggles" or rattles in place you know that there is not sufficient vacuum to clear the blow-by gasses. That might be worth trying in addition to visual observations.

 

No "jiggle test" required. It was BLOWING out.

I did not notice a smell of gasoline from the oil, but I was not looking for that. I have the oil and will check it Fri.

Someone please go and run your ICE with the oil filler cap off and tell me whether yours is blowing like a fan.

 

I'm goint to be troll like now and state the obvious.

There is a lot of stuff churning around inside a crankcase when an engine is running. All that churning of the crank, cam shafts (this car is dual overhead camp), the rocker arms and anything else in there that is moving is bound to stir up the atmosphere inside the crankcase. The churn of the air is what you are noticing, not a pressurized environment. In fact, I'm a little surprised that anything was "blowing out of the crankcase". The whole point of PCV is to reduce/eliminate the free explusion of crank case gasses into the atmosphere. I'm inclined to believe that the crankcase is negatively pressurized, meaning that vacuum from the operating engine is essentially sucking crank case gasses into the intake at some point prior to combustion in order to pass those gasses through combustion and onto the cats. The crankcase receives fresh air from some other ventilation method and cycles that back through the intake; and on and on.

What I'm wondering is why you were running the engine with the oil cap off? If vehicles were designed to do that, why would we have oil caps?

Nope, you are not going to find me in my garage running my ICE with the cap off, not because there is any immenant danger to running the vehicle with the cap off, but because I don't really want to get oil splatter all over my nice clean head and block.

 

JeromeP hit the proverbial nail on the head. That's why observing the filler cap in place but not tightened is probably a more informative test. The seal between engine and atmosphere is retained, allowing the PCV system to operate normally as long as it is not compromised. Just don't forget to tighten it back down when you're done Nearly done that myself.

 

2001 Prius. Removed the oil filler cap after starting the engine, and noted air strongly blowing out. Retightented the cap. Further noted that the engine ran the same with or without the cap sealed. I offer no interpretation of these observations, but I'm pretty sure that nothing is operating incorrectly on this vehicle.

DAS

 

Thanks! That is info I can use.

JeromeP, What can I say, we are just different kind of people. I like to learn all I can about my vehicles, even if that means something might get dirty.