Kuroki924

I'm nearly at my wits end here... My 82 924 N/A is still idling like it has a big cam swap, and I can not for the life of me figure out why. New spark plugs, ignition system is foolproof, no vacuum leaks, engine/CIS is in tune, new and still spotless distributor cap and rotor. Where do I go from here? I've been trying to fix this for weeks now with no luck.

Kuroki924

Confident that everything else was in check, I decided to start from scratch with the A/F mixture and idle speed. I read on another forum to turn the idle speed screw all the way in, and then out 2 turns, then to adjust the mixture from there. I tried that, and got the car to idle decently smoothly at ~950 rpms, though the engine would be hesitant to rev up under acceleration at first, kind of sputtering. I read that that means the mixture was too lean, but richening it from there means dropping idle speed and choppier idling. If the idle speed screw is supposed to be set at 2 turns out, with the mixture adjusted accordingly to have the idle speed sit around 950 rpms, why is it behaving as though it's too lean? And if I richen the mixture, should I in response turn the idle speed screw in or out? Idle stabilization unit has been bypassed, and adjustments were only made once the engine was at operating temperature. Would bad oil have anything to do with this as well? It is a little overdue for a change but I can't see worn oil having this drastic of an effect on the engine's performance.

morghen

I had the same situation on my 1980 931. The AF screw is soooo sensitive.
You would need a wideband sensor at least in the tail of the car if not directly on the manifold.
Your trouble is the mixture i'm 99% sure.

If you don't have a wideband sensor and gauge then its trial and error.
Like you've read on that other forum, turn the idle screw (on the throttle body) all the way shut, then two(I say three complete 360° turns) turns out.
Then adjust the mixture so the engine idles smoothly, stop at the smoothest spot don't care about the idle RPM at this point.
Then the RPMS should be too high...just tighten the idle screw until you get it at around 1000 RPM.
Then give it some quick rev ups by hand from the TB, just 2500-3000RPM don't WOT it...it shoud snap from idle to 3000RPM quickly and return just as quickly if the mixture is right...that's your sign that the mixture is about right...the way it revs up and down.

Let it properly warm up, take it for a test drive, take the flathead screwdriver and AF mixture tool with you in case you need to adjust on the road.
If needed do very fine adjustments to the idle screw or AF but at this point try to keep the adjustments to the idle screw.

thomasmryan

Its been a few decades since CIS for me.

See if the injector holders that screw into the head are free of cracks.

Are you adjusting the air bleed or the Allen between the plunger and the air flap on the fuel distributor? Didn't the specs run fat/rich like 1.5% CO?

thomasmryan

These little things.

morghen

He has a 2L NA, the injector sockets you posted are for the 2L turbo.
Nevertheless its valid for the NA as well...the NA head has some sockets machined and uses O-rings to hold the injectors in...check and if needed replace those O-rings...careful to get the right size and temp rating otherwise they will perish quickly.

Kuroki924

Well I started at three turns out, the car started and idled very low until I blipped the throttle just a bit, then the car spit out a black cloud and began steadily but chunkily idling at 950. The mixture seems very rich but leaning it out isn't helping the idle at all, causing popping and the engine almost choking out. Should I try richening it a little just to see what happens?

Kuroki924

Status report:
Curious on the condition of the o rings since you mention it, I had my dad spray carb cleaner around the injectors to see if it affected the idle speed at all while I observed the tach from the driver's seat, and around the second cylinder injector (this was the only one that did it) the idle speed fell significantly. So now I'm assuming I should change the o ring, or should I take out that injector holder as well and check it? The holders all seemed unharmed but the second cylinder was definitely sucking in that carb cleaner, making the idle speed fall almost to the point of stalling, so I'm thinking the o ring might be bad. Which makes sense to me, because knowing this very finicky CIS system, if there's any underlying problems with the system at all, it isn't going to run right no matter how you tune it. What do you guys think? And don't worry, I'm just going to replace all the o rings. Also, do you have any recommendations for specific rings? I'm open to suggestions.

thomasmryan

Always OEM on parts and do one thing at a time so see if there is an improvement. Do the o-rings and start with the initial settings on the screws.

You might seal the screw bore and use a dab of thread sealant or loctite on the screws to keep them in place. (My CIS experience is from VW rabbits from the 70s so a long time ago. The WUR valve was problematic and the accumulators where increased from 20cc to 40cc at that time)

Also check the seal on the throttle pivot(s) for leaks. The Rabbits had progressive butterflies which we would tie together just for fun.

Kuroki924

Alright, so I got the o rings changed and I know they're good now because spraying carb cleaner around the injectors no longer has an effect on idle speed. Now the only problem I'm having is, I set the idle speed screw three turns out and I'm adjusting the cis accordingly, I got it to idle at about 950, but leaning it out to attempt to bring the idle speed up only causes the engine to start popping and sputtering. Richening it only drops the idle speed, so where do I go from here? I thought I was supposed to be able to get the engine to idle smoothly but too high?

morghen

With a warm engine:
Idle screw all the way in, 3 turns back.
Adjust the CIS in 10° increments if you can until the idle is smooth and throttle response is the best...idle should then be between 700 and 1500 rpm.
Then use the TB idle screw to fine tune the idle speed back to 1000RPM.

Kuroki924

Any idea on where the cis adjustment would be? Like in terms of two or three turns out typically? I can't seem to get a clean, calm idle right now.

morghen

Well you could get a gas analyzer to help you out...it would make things much easier.

I cant remember how many turns the sweet spot of the CIS is from any ends..but certainly you should not reach the ends of the mixture screw or you'd be in trouble.

If you lost your reference set the TB idle screw to very loose so the idle is 1500-2000RPM.
Then adjust the mixture until the idle is smooth and the throttle response is best.
Then turn the TB screw (TB air bypass) closer to shut to bring the idle down gradually(take like 3-4 steps), but tap the throttle after each step lowering the idle rpm to get a sense of the mixture and response.

I recently did this procedure myself and it takes a few tries.
The key is very fine turning of the mixture screw and no pressing on it while screwing...and removing the tool after turning.

Kuroki924

I've wanted to get a gas analyzer for some time now to make this easy, but the problem is the last owner cut the cat off, and as a result also deleted the O2 sensor, so wouldn't that defeat the purpose of trying to use one?

Kuroki924

Maybe this will add to insight as well.. So the car is currently idling at about 950, a little choppily but nothing ridiculous. Throttle response is a little hesitant at first, and same with idling down. It wants to hover at about 1400 for maybe a quarter of a second, then it falls back to idle. Exhaust is a little dark on throttle blipping, and for whatever reason when the fans kick on the idle drops and becomes fairly shaky for a few seconds.