quick question

[choppa7]

If i remove the oil filler cap when the car is running, will this effect the running of the engine and can someone explain why

Fank-oo

[bazza93]

it does. think its something to do with the ecu

[Nelly]

The revs will drop due to the lose of compresion.

(Chris pm sent fella)

[tezk]

I was under the impression it was due to introducing unmetered air into the intake system via the crank case breather system?

Wouldn't mind more details about this loss of compression if you don't mind, Nelly?

[sportscarfan]

I was under the impression it was due to introducing unmetered air into the intake system via the crank case breather system?

Wouldn't mind more details about this loss of compression if you don't mind, Nelly?

I think that's pretty much it. You will have slightly less air going into the combustion chamber. (So likely to run a little bit rich.)

[Brianmoooore]

tezk is the only one anywhere near correct! Air gets into the inlet manifold via the crankcase breather. Because this air hasn't passed through the air mass meter the engine ECU doesn't get to know about it, and therefore adds insufficient fuel to make the mixture correct. The mixture will be lean.

[sportscarfan]

True, air gets into the inlet manifold via the crankcase breather, however, that air is under atmospheric pressure. When the crankcase breather is attached, there is higher than atmospheric pressure. Therefore, the pressure difference between intake and crankcase breather is less with the crankcase breather disconnected (hence air going in from atmospheric pressure), so overall you will have slightly less air for the same amount of fuel, hence rich. I think that's how it is, but it's definitely an interesting topic.

[Brianmoooore]

you will have slightly less air for the same amount of fuel, hence rich.

Removing the oil filler cap makes the mixture lean! The same as any post AFM air leak.

[Turbo-Brown]

True, air gets into the inlet manifold via the crankcase breather, however, that air is under atmospheric pressure. When the crankcase breather is attached, there is higher than atmospheric pressure.

At no point in the inlet of an N/A engine at idle will there be more than atmospheric pressure. There's also not likely to be more than atmospheric pressure at any other point of engine operation.

Brian is absolutely correct in stating that the unmetered air causes the engine to run lean.

[sportscarfan]

Sorry for keeping on with this topic, but I find it very interesting! It's always good to learn about these kind of things.
Anyhow, the inlet is definitely under atmospheric pressure under idle, and in fact it is only around atmospheric at full throttle. But, the crankcase is basically always over-pressurised so to speak, mainly due to piston blow-by. There are two main points of crankcase ventilation: to relieve the crankcase of this over-pressure, and to keep these gases/fumes from entering the atmosphere due to pollution reasons.
The air inlet is sealed from the rest of the crankcase, the only connection between them is the crankcase ventilation. So, if you take the oil cap off, then you relieve the crankcase pressure by another means than crankcase ventilation. There will still be air (fumes) going through the crankcase ventilation, because there is suction in the inlet manifold, but the pressure drop between the inlet manifold and the crankcase with the oil cap off is smaller than between the inlet manifold and the crankcase with the oil cap on. Hence it will suck more air/gases through the crankcase ventilation with the cap on. The ecu is programmed to take into account the air entering through the crankcase ventilation, and injects the required amount of fuel accordingly. With the oil cap off, there is less air coming in through the crankcase ventilation, but the ecu usually doesn't know that the oil cap is off, and hence there is less air coming in through the crankcase ventilation. It still sends the same amount of fuel in, so slightly rich.
So do you guys still think it would run lean? I would be very interested in some more detailed reasoning. Is it possible that taking the oil cap off would have a different effect on modern cars and slightly older ones (not talking about 1930s stuff, but like E30s, already fuel injected cars, so on...)? Has anyone seen any data that shows lean running with the fuel cap off? Will be awaiting comments with interest!

[johnl320]

Some worn out engines will blow pressure out of the filler cap when its open but this is a BMW and the crankcase ventilation is very good to the point of drawing in air through the filler cap, via the crankcase and into the inlet manifold AFTER the AFM. As Brian says the ecu doesn't know about the extra air so won't put in the extra fuel. As previously posted, inlet manifold is below atmospheric. If it wasn't how would your vacuum brake servo work??
Not many people argue with Brian. Good luck.

john

[Brianmoooore]

johnl320, Turbo-Brown, 1 : sportscarfan, 0.

[murran]

the crankcase is under a vacuum created by the vacuum in the inlet via the breather system. if it wasnt ud have oil pishing out of all the oil seals and gaskets. even at full throttle the inlet is no where near atmos pressure cus its still having to pull air in thru the filter! yes if you take cap off, unmetered air enters the inlet before the afm so leaning off the mixture, cus the ecu doesnt know bout the extra air.... same as any air leak.
this subject is interesting....... but not that interesting!

[Turbo-Brown]

To be honest, you should find the inlet pressure rising to within a fraction of atmospheric at full throttle. Especially if you've got it in a high gear at low speed and snap the throttle open.

The crank case should be maintained at, by and large, manifold pressure.

With the oil cap off, the engine receives air which the AFM doesn't read. Because the AFM's not reading the amount of air it should be, it shuts slightly.

Think of it this way, if you undid the hose from the AFM to the throttle with the engine running and seperated the AFM from that hose ever so slightly, air would enter the hose which hadn't passed through the AFM. In addition to that the AFM which is now passing slightly less air would shut slightly. So you can see that the engine's seeing unmetered air and the AFM's telling the ECU that there's slightly less air entering the engine.

This combination of a low air reading and greater air consumption causes a lean mixture to reach the engine.

[murran]

if you take atmosferic pressure at 1000 (cant remember what the unit is, pascals???) at tick over its around 300 - 350 and at full throttle its around 900 - 950. i know this from taking live data from cars at work with our bosch dignostics. all petrol engines should be around these readings.