Rough Idle when Oil Filler cap removed

[DMan328]

I've been shopping for a 328. I've looked at a few that when I remove the Oil Filler cap, the idle drops and becomes somewhat rough. When I drive the cars they feel fine.

I am just curious as to why this would happen. I wouldn't purchase a car without a PPI, but I'd like to know if this is a big problem before I shell out the cash to have the car checked out.

Any advice would be appreciated.

Thanks
Dave

[El Presidente]

Im confused.

Why would a wierd idle concern you that only occurs whn you remove a part that isn't supposed to be driven without? As to why that happens, it's probably disrupting the vaccum.

[MrM3]

Originally posted by El Presidente@Mar 28 2004, 02:05 AM
Im confused.

Why would a wierd idle concern you that only occurs whn you remove a part that isn't supposed to be driven without? As to why that happens, it's probably disrupting the vaccum.

exactly, removing the oil cap releases the pressure under the valve cover. why would you remove it with the car running anyway?

[DMan328]

Checking for blow-by... Listening for strange noises... etc....

[Eggrolled211]

Hmmmmmm....its does the same with all car doesn't it? I changed oil etc. 4got to install oil cap on a perfectly running used and new car, does the same thing.....keep the oilcap on its not suppose to be off. I get strange noises when I drive with 1 of the sparkplug wires removed.....heehheeehehe. Definately hear strange clicking! eheehhehe

[DMan328]

I put the cap back on before I drive the car....

I take it off while under the hood...

[roguesniper1]

Originally posted by DMan328@Mar 28 2004, 01:16 AM
I've been shopping for a 328. I've looked at a few that when I remove the Oil Filler cap, the idle drops and becomes somewhat rough. When I drive the cars they feel fine.

I am just curious as to why this would happen. I wouldn't purchase a car without a PPI, but I'd like to know if this is a big problem before I shell out the cash to have the car checked out.

Any advice would be appreciated.

Thanks
Dave

You gotta be kiddin, right?

[Autotechnica]

This thread is confusing. I don't understand what you were trying to test by removing the oil cap? It's like removing my O2 sensor and asking why my idle is so rough?

Can you tell us what you were trying to test/accomplish by removing the cap? Why on earth would you drive with the cap off when the hood is closed?

Bry

[DMan328]

Don't drive with it that way, only take it off for a few seconds when I'm under the hood. I've never had a car do this before.

I take it off to check for piston ring blow-by.

I don't see how anyone purchasing a car doesn't take a peek inside the oil filler. On some cars you can see quite a bit.

I never said that I drove with the cap off. I don't know of anyone selling a car that would allow that kind of tomfoolery.

[bimmerfan620]

agree with DMan328.. When my stock engine from my vr4 was misfiring and was sounding really bad at idle, rough puffs from the exhaust I knew somethign was wrong, I also had the car started and took the oil filler cap off to check for piston ring blow by caused by broken rings or melted pistons. When I did this, the air blew out in puffs at my hand, and it was pretty bad. I ripped my whole motor apart to find the #3 cylinder on the front bank had melted a huge peice right between the top and the middle compression rings. barely scared the cylinder wall though. But yes, I also have always checked this as well as it's easy for an inexperienced person to pass up certain things such as exhaust puffs at minor levels.

[TECH]

Despite whatever reasons for removing the cap, here's why it idles bad when you do so.......

Ever heard of PCV? (Positive crankcase ventilation) The vapors from the crankcase, including small amounts of blowby gasses, are sucked into the intake manifold by the pcv system. This keeps the crankcase depressurized, and vents the harmful vapors. They are sucked into the intake for reburning.
When you open the oil cap, the pcv system sucks in a bunch of outside air which is full of oxygen. The engine sucks the air into the intake, and causes it to run lean, and idle bad.
This is especially evident on an engine that uses an airflow meter, or mass air flow system. The mass airflow sensor cannot meter that extra air as it does not pass through it. It's the same as a vacuum leak.
Engines that use a MAP sensor instead of a Mass Air Flow, CAN compensate immediately, since they measure the pressure in the manifold, not the actual airflow into the intake. If a vacuum leak is induced to a MAP equiped engine, the idle will probably just go high, but still run fine.

[dutchbmw]

Is there no splashing out of oil when removing the oil cap and engine is turning in idle?

A better idea to control the engine condition, is to do a compression test of each cilinder, or check the ignition tension of each cilinder. I've heard from this last method lately, and never see of heard this before, so correct me if i'm wrong!