Oil filler neck rough idle and knocking

Hello Rennlisters, 2001 996 C2 So I broke my oil filler neck removing the air cleaner housing.... unfortunately this was about a year ago and I used the "duck tape method" as a temporary fix until I finally had time for some other maintenance. I ordered a filler neck and gasket a few days ago. The swap was straightforward and the redesigned neck went right in without any issues. I put everything back together and started the car. Car would not idle and cut out immediately. I restarted with a little gas and the car revs right up. Idle hovered around 4-600 RPM awfully. Idle was loud, knocked, and started to smoke. I removed the air cleaner, throttle body, and intake plenum and reinstalled thinking that I had a vacuum leak somewhere or had the rubber couplers on the intake plenum incorrectly seated. Removed and checked everything, put back together, three times and every time super rough idle, knocking, and smoke from the exhaust. I ran a search here and found mainly AOS's causing the same issue along with oil overfills. My AOS is new and the car has been at the proper oil level for over a month. My theory is that after running the car with a broken filler neck for so long the crankcase ventilation was not working right or not working at all. I never noticed any effect with the engine because the ECU compensated for the lack of vacuum. This allowed excessive oil to build up in the intake or upper part of the crankcase. I went back outside and allowed the engine to run until heating up burning off all of the excess oil. Idle kept getting better and better. I cleared the codes and took it for a drive. Idle checks completely good. Does this make sense or am I out to lunch? I'd like to get a second opinion before finding something else out on the way to work Monday morning. Thanks, Aaron 142,000 miles and counting

These engines I believe compensate for just about anything to get the idle and running correct. That's one of the reasons why they are so expensive. So I think you are 'good to go' that's only in MHO. Time will tell but I oulwd take a gamble and drive it like you stole it again on the way to work.

How about disconnecting the battery and resetting the ECU?

Are you sure the new filler neck is fully seated correctly? It took a bit of effort to get mine correct, and I remember thinking, man this sure is easy to put back together wrong.

OK i still have a problem. I did clear the codes after troubleshooting the problem with a cable. Codes listed were a bunch of misfire codes. The car continues to knock, rough idle, and smell like oil burning but only on cold start. I took the car for a long drive today. It literally takes only about 2 minutes of warm up for this condition to go away. Car runs, drives, and idles as it should. If i restart the car while it is hot it idles normally. This knocking and rough idle hovers between 4-600 RPM, is when cold. Normal idle is 800 RPM when its working properly. Someone asked if I had the new filler neck seated properly. I did this kind of blind and left the alternator in for the swap. I was able to tighten both 10mm bolts easily and everything felt snug. I tested the new neck by turning on the car at a warm idle, removing the oil fill cap. The RPM changes slightly as the pressure in the crank case has changed. This is how I can tell that the seal at the base of the neck is good. I can also put my palm against the open filler neck and can feel the vacuum build. I used this as a test also to verify that the vacuum wasn't going too high which would be a sign that the AOS is going bad. I am completely baffled. The only thing I can think of is maybe a vacuum leak somewhere underneath the intake plenum. I verified that the plenum boots and little vacuum lines were reattached on each side. I'm baffled?!

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What codes did you get? Are the ignition coils original?

I got codes P0300, P0301,P0302, P0303, P0304, P0305, and P0306 misfire on all cylinders. CEL came on this morning at cold startup. It's like whatever manages the idle at cold start is not working. Engine ran fine after 2 minutes of warmup. The coil packs are all new. The old ones were cracked and would misfire after the rain or washing the car. Thanks for the help!

I had essentially exactly the same problem after doing the same work. Likewise, I checked the plenum and throttle body connections twice...but the problem was still there. In my case I was tired and annoyed and I didn't take everything back apart. I felt along the sleeves and everything felt fine. It wasn't. It's annoying, but I suggest you take the throttle body and plenum back out and reassemble it to make sure it's correct. The positioning of the rubber sleeves is tricky (look for the marks on the tubes indicating the alignment). It's easy for one side (on the back) to have an opening that you can't see or feel. I'd bet money this is the problem. -shawn

While you are at it, cleaning and checking the throttle body and connections, a couple more things to check: throttle body to plenum housing o-ring, clean and check the idle control valve, clean and check the mass air flow sensor. Check and clean the electrical connectors to the idle control valve and mass air flow sensor. Since you are getting codes on all cylinders, it is logical to assume something that affects all cylinders, i.e., intake leak, vacuum leak, throttle body leak...etc. My WAG is along those lines... You also say that it is better once the engine warms up...so either something is affecting cold start, idle control valve, lean fuel mix...thus pointing to a air/vacuum leak, and heat expansion is closing up the gap after the engine is warm. One other thing you don't mention is if the secondary air injection pump is lighting off when you start cold. Is it? I know it just pumps air into the exhaust cats, but it also provides feedback to the ECU regarding air/fuel mix on cold start. Check the electrical connectors to the SAI pump, and the O2 sensors as well (4). Something is wrong with the cold start air/fuel mix and the ECU is confused as to what to do.

You didn't put back something that you should of or its not sealed properly. Your "compensation" theory doesn't make sense.

Thanks to all for your help especially m3bimmer who told me nicely to stop eating crayons and go check my work again. After pulling everything back out again the rubber rings on the intake plenum looked seated correctly but i found they werent only after pulling the whole assembly back out. I broke the vacuum lines on both sides of the plenum... bummer. Managed to temp repair the secondary air injection side but now I have no vacuum line to the fuel pressure regulator. I plugged the little port with a q tip as a temporary fix to get me to autozone to get some replacment 1/8'' tubing. I should be able to fix it tomorrow when it gets light again. One thing.. the car feels like it gained 50hp. Smooth idle on cold start. Runs and drives great now. Thanks again to all who helped. Aaron

Glad you got it resolved. I even took the alternator out twice to confirm the tube was seated....that damn plenum/tubes is tricky!