Rough idle with oil cap off

[calimedeiros]

Just curious why the engine runs a bit rough when you take the oil cap off to check oil level as I just bought an 81 targa and according to the owners manual this is normal
Just curious why this happens ??

[Len 911]

just be glad yours is doing that, it is normal, you are indroducing a vacume leak with the cap off.

[JINKATIN]

+1 on vacuum leak, if the idle is rough to almost stalling. You need to find the leak to correct it. Idle will fluctuate with cap off but should not be rough or stalling.

[Bob Kontak]

Originally Posted by JINKATIN

+1 on vacuum leak, if the idle is rough to almost stalling. You need to find the leak to correct it. Idle will fluctuate with cap off but should not be rough or stalling.

He's gonna get to that like we all have.

His running rough and your running rough may have different interpretations. I think an RPM drop of about 150-ish from normal is about what you will see if things are pretty tight. Normal being 900-950 after warm-up.

OP - Not sure how much you know about CIS but any air that travels through the air sensor meters fuel to the injectors. No outside air should enter the system after this point. If it does, the air sensor does not lift as high and the fuel metered does not keep up with the amount of air that the engine receives - lean situation.

There are ways to tweak the fuel though and sometimes folks tweak the fuel without fixing the air leaks. Runs ok until the next guy fixes the air leaks and you have a really rich mixture.

Here's a pic (below) and it shows the oil hose entering the system AFTER the air sensor. You take the cap off, the engine sucks in outside "false" air and car is now lean.

If you take this hose off the oil tank there is a restrictor/reducer in the line that shrinks the hole size to about 1/4" so the car does not stall from too much false air.

Search on CIS Primer if you want to get into the system mechanics.