removing oil cap whilst engine is running

cyclonebri1

Gloomendoom wrote: »

I disagree. Removing the oil cap can allow air to bypass the mass airflow sensor. This will mess up the fuel/air mixture and the engine will run poorly even if it is otherwise in perfect condition.

I do agree that oil fumes belching out of the oil filler is a bad sign.

Sorry but it doesn't. Any air or pressurised air in this case has automatically bypassed the airflow meter due to the piping arrangement.

There should be no or neglegible pressure within the rocker of a healthy engine, certainly not enough to mess up the engines ecu fuel system, no offence of course intended, ;)

skiddlydiddly

Lum wrote: »

I don't have one. Have an RB20DET, and a CA18DET in the GFs car.

For some reason we both prefer the older design. I also have a philosophical objection to pushrods :P

Was yours the rare old awd nissan linked a month or 2 back back concerning stone chippings?I presume your GF drives an S13(or did the old cherry turbo have that engine too?)

Gloomendoom

cyclonebri1 wrote: »

Sorry but it doesn't. Any air or pressurised air in this case has automatically bypassed the airflow meter due to the piping arrangement.

There should be no or neglegible pressure within the rocker of a healthy engine, certainly not enough to mess up the engines ecu fuel system, no offence of course intended, ;)

I still disagree. The air is sucked into the induction system via the crankcase ventilation system. The crankcase is normally sealed to keep the nasty oily vapours in. Any positive pressure generated in it is relieved by the crankcase ventilation system which sucks the crankcase air into the induction system (normally at a point between the MAF and the throttle or any supercharger). If you take the oil cap off then the crankcase is open to atmosphere and fresh(ish) air is allowed in. From there it gets sucked into the induction system. The MAF is bypassed and as a result more air enters the cylinders than the ECU knows about. The engine will then run lean.

Where have I gone wrong?

PsiDOC

So Gloomendoom, are you saying that whoever has taken the filler cap off the OP's car whilst the engine was running has wrecked the engine by making it run lean then?

Gloomendoom

PsiDOC wrote: »

So Gloomendoom, are you saying that whoever has taken the filler cap off the OP's car whilst the engine was running has wrecked the engine by making it run lean then?

No! Look back at post 5.

And 6.

PsiDOC

OK so now can we get back on topic please as this is simply spiralling out of control.

Gloomendoom

PsiDOC wrote: »

OK so now can we get back on topic please as this is simply spiralling out of control.

With respect, the second question the OP asked was whether a whirring and wheezing engine as a result of the oil cap being removed was a sign of a poorly engine.

That is what the rest of us have been discussing.

Lum

skiddlydiddly wrote: »

Was yours the rare old awd nissan linked a month or 2 back back concerning stone chippings?I presume your GF drives an S13(or did the old cherry turbo have that engine too?)

Correct on both points.

Wig

Gloomendoom wrote: »

I still disagree. The air is sucked into the induction system via the crankcase ventilation system. The crankcase is normally sealed to keep the nasty oily vapours in. Any positive pressure generated in it is relieved by the crankcase ventilation system which sucks the crankcase air into the induction system (normally at a point between the MAF and the throttle or any supercharger). If you take the oil cap off then the crankcase is open to atmosphere and fresh(ish) air is allowed in. From there it gets sucked into the induction system. The MAF is bypassed and as a result more air enters the cylinders than the ECU knows about. The engine will then run lean.

Where have I gone wrong?

This is an interesting thread :T

I was going to disagree with gloomendoom and agree with cyclonebri, because I too thought of the rocker cover as having no excess pressure. so removing the cap should have no effect, but I didn't account for the breather tube in to the intake -which should theoretically create a negative pressure.

But IMHO gloomendoom is right, the cap seals (almost) the rocker cover, and there is a pipe from the rocker cover to the air intake usually after the MAF (never seen one before it). This pipe logically causes a negative pressure in the rocker cover and not much air will travel down it under normal use because a vacuum will be created in the rocker cover. Remove the cap and you let loads of fresh air suddenly travel down that pipe, not to mention causing any gunge in the pipe to suddenly enter the intake not a good idea IMHO. Which in turn means the MAF is not reading correctly the amount of air into the intake, which will cause an error in the fueling, which could be noticeable to the observer.

In short, remove the cap - I would expect to notice a difference in a good engine. No difference could mean the rocker cover breather tube is blocked.

My answers to the OP's Qu's

Possibly. (dirt into intake from breather tube)
No. He's not right.

However,........ Food for thought......I just had a thought.....
The crankcase breather also goes into the rocker cover, BUT it is my understanding that the crankcase is not sealed, if you were to turn the car through 90° it's my understanding that the oil will flow out past the flywheel if so then it is not a sealed unit and there should be no negative pressure in the rocker cover so removing the oil cap should have no effect.

discuss

cyclonebri1

Even in a brand new engine there will allways be some positive pressure caused by slight blowby at the piston rings, just consider the pressure created at the bang stage rather than just the 200psi or so under commpression. In a worn engine this is obviously magnified and the normal ventilation system cannot utilise enough vacuum, (max vacuum only exists when the throttle is closed),to negate the pressure.

With older cars it was possible to tune out this issue, but removing the filler would allow the engine to breath properly and wreck the present tune of it.

I'm not talking damage op just rough running as you experienced.