SC Idle Oscillations

[Roadrunner911]

I need some help on this one. My SC developed a significant idle oscillation once it's warmed up. It did the same thing last summer but got advice from the BBS to adjust the CIS mixture screw which I did after I found it. The oscillations went away with about a 1/4 turn. I figured I'd repeat the procedure again this time but I'm not able to turn the hex screw with the allen wrench. What am I missing? The only way I can reach the mixture screw with my allen wrench is with the engine not running, remove the boot to reach the access hole. My 2 1/2" long, 3mm allen wrench reaches the metering plate arm but I'm not able to lock the allen wrench into the mixture screw. Does anyone have experience with this problem? Idle fluctuates from stall to about 1900 rpm every couple of seconds, making the car difficult to drive. Also, please confirm which direction to turn the screw to richen the mixture. It's a stock 82 SC Coupe with about 120,000 miles. It runs great otherwise. I've searched for the problem but didn't find a specific thread for an SC.
Thanks

[Paulporsche]

It's possible the grub screw is turned all the way out and has no further travel. It's also possible the slot is stripped out. More likely is you have junk in the slot. Try blowing it out w/ compressed air, or cleaning it w/ something smaller than the allen wrench, like a coat hanger, nail or knitting needle.

[SeaDweller]

Be very careful futzing with the mixture without the proper measuring device, because you can blow your airbox by doing so.....Don't ask me how I know......

Idle hunting can be caused by improper mixture, vacuum leaks, improper vacuum hose routing, or any of the various components in the vacuum system (warm up regulator, auxilliary air valve, thermovalve, etc.)....

Some "mechanics" evidently like to re-route the vacuum hoses to "corect" other issues, and then jack up the mixture to compensate....

Again, be careful futzing with the mixture, because if improperly set, you can get the dreaded backfire through the airbox (and you don't have a pop-off valve installed), and you be in for more work than I think you care to get into........

[DCred911SC]

I had this problem a couple of years back and it went on for about three or four months. Believe it or not, it was a bad battery. Car started OK from cold, ran OK for a little while, but once it was warmed up it did exactly as you describe. I only found out because one day after it stalled at a junction the battery hadn't enough power to restart. I went to the nearest store, bought a replacement battery, drove away, and low and behold the idle problem went away and hasn't returned since. This problem and cause of it baffled almost everyone whom I talked to, and might not be the cause of your problem, but my advice would be to get the battery checked out ....... it's usually free.

[Paulporsche]

Steve,

You enrichen the mixture by turning the screw clockwise when viewed from above. Counterclockwise = leaner. Too rich usually causes the oscillations. I'm wondering why you would have to repeat this procedure since last summer, since you don't see significant temp changes where you live. You might want to do a search. There may be other contributing factors.

[Noel]

If you unplug your OX sensore, what does it do? Check your mixture as stated above. This is usually the first thing I would try, then check the fuel pressures to make sure all is well there.

[Roadrunner911]

Thanks everyone that made suggestions. I went back and read my 101 Projects book which has an excellent discussion of the problem in Project 31 "Tuning and adjusting CIS fuel injection". I have a game plan now. First, I need to get a longer 3mm Allen wrench that I can fit into the adjustment screw while the engine is running. I will clean out the area first. Then I'm going to install the popoff valve in the airbox.
Another question though, while researching this problem, it peaked my curiousity about using an air fuel meter/analyzer. Any suggestions about getting one of these? It sounds like THE way to tune your engine. I've seen some posts about an LM-1 $350 analyzer. Is there another option?
I'll try the mixture screw route first and let you know how it goes.

Thanks everybody!

[nexium]

Keep us posted.

[gerard vaglio]

One option would be to stop by an independent Porsche mechanics shop and ask them to adjust the mixture.

It only takes 5 minutes for them to adjust it with a meter and I doubt they would charge you. Without the meter you won't get it right, ask me how I know. Once it is adjusted properly you can rule mixture out .

Yes, get a pop off valve, I got one right AFTER my airbox blew up.

If you need to adjust the mixture, after you get a pop off valve, there is actually a tool to make it easy. It's a long allen wrench with a "T" handle that sells for a few bucks on Pelican.

An idle that goes up and down is typically attributed to a too rich setting but as others have correctly suggested there could be lots of reasons.

Good luck.

[klatinn]

Hi,

One possible cause of idle oscillations on CIS has not been mentioned. The throttle switch can be missaligned or having bad contact at it's idle switch portion. This switch tells the lambda brain to disregard the O2 sensor signal at idle because the sensor is too cold and therefore reacts too sluggish. If the switch does not close at idle, the lambda brain tries to maintain 14.7 AFR using a sluggish sensor. This causes overshoots in the regulation and oscillating idle. Can be tested by disconnecting the O2 sensor. If the oscillations go away, check the throttle switch.

Regards,
Klaus

[Roadrunner911]

I got my idle oscillations fixed! I did as Gerald suggested and took it to a local Porsche guru, Tim Knight of Knightwerks in Honolulu. I popped the rear deck lid while the car was idling (oscillating), Tim tweaked the mixture screw and adjusted the idle screw before I could close the driver side door. The oscillations smoothed out immediately. I shut off the car and Tim proceeded to give me a 15 minute dissertation on the CIS. I took it around the block and Tim gave it a final tweak. Now it's rock solid on 975 rpms. WOW, I'm impressed, I spent hours researching this. Nothing like seasoned Porsche SC veteran. By the way, he said he was putting a Turbo in his Volkswagen Van. That might be fun.
Thanks everyone, Pelican Parts forever.
Steve

PS: Lessons learned 1) Get a wrench sized to fit the mixture screw while the engine is running. 2) Learn more about the CIS and associated components. 3) Consult a pro when you get stumped. 4) Pop off valve goes in ASAP.

[nexium]

Glad to see you know Tim at Knightwerks he's been a good mech for me through all my 911 adventures.

Your mixture might have been on the "rich" side.