Revisiting 911SC Idle bounce - stall
 

So I want to post exactly what is happening. It should narrow the cause down.

Start up in the morning and it idles high, like 1500-2000. Because the idle is so high I have no stalling or idle-bouncing. by the time I hit my 1st red light, about 2 miles away, I push in my clutch and the idle drops off to either stall or 200 Rpm's. and raises up to 1500, back and forth until it either stalls, or I manage to tap the gas enough to hold it steady at 1000, Which is very hard because the slightest gas pedal touch revs it up to 2000.

After about 5 more minutes the bounce starts to go away and the idle seems to stabilize.

Warm up regulator? I am thinking if this was anything to do with fuel mixture it would be consistent and not go away?

I plan on taking it to the shop here soon, but its difficult as I always need a ride to drop it off, and the WIFE is never happy about taking me...

Thoughts?

Sounds like a rich condition to me. Next time you start cold and once the idle comes down from 2000 RPM, take off the air filter cover, pull down on the air meter arm slightly and see if it improves the idle.

Could be rich. That is usually what causes the dip and then recovery (sometimes it won't recover and will stall).

Where is the idle when warm?

If you search, you will find a lot of info on this. It is quite common.

The high idle @ cold start is correct. The idle hunting during warmup is usually a sign of a too low cold control pressure causing a too rich mixture. Or sometimes it is merely a too rich mixture setting. As the engine temp rises, the control pressure has to change to lean the mixture. Because of the cold and warm settings of the WUR, the mix could be within spec @ 1 point but not @ another, which is why you could get that symptom only during warmup.

You should check your CPs (WUR) and your mixture CO%. Also make sure your sensor plate in the mixture control unit is correctly positioned @ rest.

I would start w/ those.

I just went through this thoroughly. I know some of the engine management components are different between the SC and the Carrera but here's what I verified or replaced...
1) Get a wife that will put up with your *****.
2) Check cylinder head temperature sensor.
3) Check for vacuum leaks.
4) Verify ICV operation (I don't think the SC has one of these?)
5) Adjust throttle so that it makes contact w/ the idle position switch correctly.
6) Bench test AFM.
7) Replace O2 sensor.
8) Adjust AFR for correct ratio.

My car was running TOO rich!!!
This most likely was the cause of the idle bounce.

This morning, the weather is actually very warm today. I started the 911 and it idled at 2,000 RPM for 65 seconds. then it started with the idle bouncing from 200 to 2000 rpm's for a about a minute more. It leveled out at 950 and then I headed out, when I hit my 1st light 2 miles away it almost stalled and then iit idled pretty solid at 800-900 range the rest of the way. No problems - warm day equals a lot better running for sure?

"warm day equals a lot better running for sure?"

Correct.

Sounds like your mixture is still too rich during the warmup cycle of the WUR, though.

Let me clarify this. When it's cold and you get idle bounce, but then on a warm day it behaves better, your problem is still there, definitely, it's one of the symptoms.
You are doing good by asking questions, but have you dug in and performed any diagnostics yet? What have you done to attempt to correct the problem to this point?
Like paulporsche states above, sounds like you're AFR is too rich.

If the mixture was rich to begin with, warmer weather should make it even richer, not the other way around.

Is CIS really this finicky?

I would think that minor AFR adjustments might be necessary on a seasonal basis if one was really trying to dial in for optimal performance.

It seems to me that a properly running CIS should be able to maintain the same AFRs year round (within a small margin).

Agreed. Especially w/ an SC w/ lambda, which he may or may not have. We don't know the year.

That's why I'm thinking (and as you said) it's probably too rich--either just set too rich or the ccp is set too low.

For those of us w/o lambda, like Souk and I, do go through some of that seasonal tweaking, richer in fall, leaner in spring--just a little.

Even with lambda CIS runs on open loop for at least a few minutes after start until the thermo switch on chain cover clicks. It will not help at all with too rich a condition immediately after start up.

the year is a 1979 SC. I have not performed any diagnostics, sorry I have no equipment. I am hesitant to touch that mixture screw. My shop does all that, and they did mess with the mixture after dropping the engine out for a Oil cooler replacement. It idled solid til that point. I swung it up for another adjustment which was done on diagnostics equip, while I waited, bt now they are asking i go drop it off overnight. Perhaps when they attempt to start it in the AM they will better be able to adjust mixture to correct the bounce?
Again, today is really warm and I pretty much had no problem with it.

mine does the exact same thing during the cold season. today is 45. it did it. after about 5 miles of getting engine up to temp....its smooth and perfect the rest of the day. in the summer time, i have no such problem at cold start....except for the slight idle increase for a few seconds...which is normal

I dont know what the problem is and i dont care at this point. its a little annoying but ive just come to believe my car hates the cold weather as much as i do.

one thing i would not do is mess with the mixture screw unless you have the proper gas analyzer tools and understand what your looking at

anyway, your not alone

Your first order of business is to have the shop check your pressures. Forget about mixture for now.

If pressures are OK then have them properly set the mixture when the engine is warm. Also have them check for vacuum leaks and make sure your timing is OK.

Based on your post you don't have a cold start problem. It is normal to idle high for a bit when you first crank it up. It sounds like you start having issues once the car is warmed up.

My bet is that everything is perfectly fine but you are running a tad too rich and/or your idle bypass needs to be adjusted.

i have a 78 which should be the same as your 79. Its not real hard to adjust the A/F screw at all and if you remember "about" how far you turned the screw you should be able to get it back where you are now. I have had good luck with the "lift the plate" method which can get you pretty close to where you want to be. for now, you could just start the car, let it warm up until it starts to hunt (probably rich) then just turn the screw counter clockwise about 1/8th of a turn to lean it just a bit. Adjust idle to 950 if it changes. it could be that you just need to raise the idle up a bit to stop it from falling off. I cant get my car to idle any lower than 900 without problems. If yours is solid at 800 just raise the idle up to 950. Sounds to me like you could tune this out yourself. Timing is probably ok if they adjusted the A/F.

I just got my 82 SC and it had a high idle when warm (stays @ 2000 rpm) thanks to this forum I went to my Titleist golf bag and "fixed" it. It does however when cold starting search up and down as I lightly hold my foot in it for a bit then the warm up takes over and holds at approx 1500 then almost falls to a perf idle getting closer to "launch time":D. My question to this link is if the decel valve is out (heard it's the inner diaphram) and you give it that 1mm squeeze, does that..will that fix a existing valve or 205.00 later, then squeeze it ????? Easy to remove ????

Thanks this site rocks and maybe see the San Juan boys this Sat...................;)


Scott

boogie man...what is the Titleist fix??? inquiring mind wants to know! ;)

I think he's referring to a golf tee.

DING-DING-DING !!!!!! After all it's Masters week and no I'm not a golf nut. It was pretty cool to start the car, pull that line and stuff the "tee" in it and watch the idle settle to a cool growl.......Now, it seems a bit more work to cold start it as my 79 would fire to approx 1500 and settle into a nice idle when getting warmer.

That's one of my favorite parts of the car is cold starting it and letting it do it's thing....What can I do to get that back ????

Again thanks guys, this site is awesome.

Scott