H20NOO

So this morning I added .5 quart of oil to the car. I've been really lucky as it's hardly burned any in over 7k miles so this is the first time I've added any.

With the arrival of fall, it's pretty chilly in the a.m. so I started the car and added oil while it was idling.

Now for the odd part. I opened the oil fill tube and the engine nearly stalled. It managed to catch itself and settled into the most unbelievable lumpy, staccato, full-race cam idle I've heard from a factory, emissions legal engine. Of course the sucking vacuum sound from the filler tube was a bit of an unwanted distraction but the engine idled like a full blown Porsche flat-6 race engine.

I don't know the mechanics behind Vario-Cam too intimately but, if I had to guess, it seems the elimination of vacuum in the system somehow advanced the cam followers to full aggressive. It was MAGNIFICENT! It could just be that the car runs like crap with the oil lid off... Regardless, it was a very strange phenomenon and I wonder if any other 997 or 996 owners have had a similar experience or know anything more about removing the oil filler cap with the engine running.

MC

gpjli2

Engine is not intended to be run with cap off. Surprised you didn't get CEL. Someone posted the other day about getting codes from running with cap loose. Not a Good Thing.

Alan Smithee

Why would you add oil with the engine running? They don't have dry-sumps with separate tanks, ya know...

Coochas

I have to agree (and sorry MC),...what were you thinking opening the oil cap with the engine going!?!
Same goes for radiators (I learned that the hard way when I was a teenager).

Likemystoppie?

Ok, looks like we have a well done member here. Where's the flame suit when you need one?

H20NOO

For what it's worth, my 993 can be filled with the engine running, no prob. The oil tank is not under atmospheric pressure or vacuum. Every car I've ever owned (more than 10) have been fine with the filler cap off while the engines running. I briefly perused the owner's manual before filling this morning and didn't read anything about the engine having to be off.

What's the big deal about opening the oil filler with the engine running? I've been hands-on with engines for over 2 decades and the vacuum argument doesn't fly. Vacuum used to operate ignition advance and carburetor functions was always drawn from the intake manifold where vacuum is easily generated by the engines pumping forces working against a restricted carburetor or fuel injection system. Conversely, the crankcase would normally generate positive pressure due in large part to piston ring blow by. That positive pressure would be ventilated to the atmosphere through an ordinary pressure release valve or more recently recirculated through a Positive Crankcase Ventilation (PCV) valve.

It strikes me as very odd that removing the oil filler cap has ANY effect on the engines operation other than to potentially create a bit of a misty petroleum mess.

Comments and opinions always welcomed.

MC

gpjli2

I once did an oil change and started the engine w the filler cap off. 2 hours later I was still mopping up oil from the engine, hood insulation and garage floor. Odd?

OCBen

Good point.

Maybe by opening the cap it was enough to drop the crank pressure for the sensitive motronics to pick it up and try to adjust to what it perceived as a drop in oil pressure???

I would ask your local service tech about that. Btw, that long filler neck is sufficient to prevent any oil from splattering out.

99firehawk

when you take the oil cap off you create a very large vacum leak. the engine runs way lean and the o2 adaptaion goes way postive once it its hits 7% which should take about 4-5 miniutes with the cap off you will get o2 senbsor adaptaion faults.
theres no reason to run your car with the oil cap off

gpjli2

Yes. What was said. CEL and error codes. Thank you.

H20NOO

While I'm still unsure how their pulling vacuum from the crankcase, I'll accept my admonishment.

There's no doubt pulling that seemingly innocuous plastic lid off has a big effect on the way the engine runs and it's obviously a matter of presssure or vacuum being compromised.

Thanks for the comments!

MC

Chris from Cali

It's too bad you couldn't get a sound clip!! The 997 does have a great aural range...

Texas993

As a former aircooled owner, I would have done the same thing in adding with the engine on. In fact, you had to check the oil with the engine running.

So I see how why you added with engine on!

H20NOO

Agreed, it's very different with the 993. It holds 12 quarts of oil and if you change your own as I do, you'll know you can't add more than a few quarts without starting and running the engine.

I've had some very messy cleanups by trying to fill a 993 with the engine off. The oil resevoir holds about 6-8 quarts and the rest needs to be circulated to the oil cooler in the front of the car and 2 filters before you can get it topped off properly.

The first time I changed the 993 oil, I ended up with about a half quart of Mobil 1 on the garage floor and that was after adding only 5-6 quarts out of 11-12 total. I'll admit, I'm still out to lunch on what an "integrated dry-sump" engine is. But one thing it's not is a true, dry-sump arrangement as in the 993's and previous.

MC

dstrimbu

I believe that the 997, like the BMW motors, runs negative crankcase pressure. Opening the oil filler will cause an inrush of air and massive stumbling. My M30 big six will die if I remove the oil cap at idle.